1987_ftp

Review

Ion-Selective Electrode Potentiometry in Environmental AnalysisRoland De Marco,a* Graeme Clarke,a Bobby Pejcicb

a Nanochemistry Research Institute, Department of Applied Chemistry, Curtin University of Technology, GPO Box U1987, Perth,Western Australia, 6845, Australia

*e-mail: [email protected] CSIRO Petroleum, ARRC, PO Box 1130, Bentley, Western Australia, 6102, Australia

Received: May 16, 2007Accepted: May 29, 2007

AbstractThis review will illustrate how it is possible to develop ion-selective electrode (ISE) methodologies that meet thestringent requirements (i.e., high selectivities and very low detection limits) for the analysis of important analytes inthe environment, and will present a variety of examples on the application of ISEs in environmental analysis. Despitethe experimental biases that have limited the analytical performance of ISEs through apparently high detection limitsand modest selectivities, there has been a plethora of research in the application of ISEs in the monitoring ofenvironmentally important trace metals and anions in natural waters and soils. Most popular has been the analysis offree metals in natural waters, as this parameter is known to be a master variable in the uptake and toxicology of tracemetals on aquatic biota reflecting the bioavailability of trace metals in the environment. Furthermore, as copper is amajor trace metal in coastal waters due to its extensive use in antifouling paints on sea vessels and structures, there aremany reports in the literature on the use of the copper ISE in assays of either free copper or the copper complexingcapacity of natural waters and soil peats. Moreover, there have been a variety of studies showing a strong correlationbetween free copper levels and the toxicity of copper on a variety of marine and fresh water organisms. Nevertheless,there are numerous reports in the literature that have used ISEs to monitor important anions such as fluoride,phosphate, sulfate, nitrate, nitrite, chloride, cyanide, etc., as well as other important cations such as ammonium andchromium(VI) in waste and natural waters. In conclusion, this review will present new and interesting perspectives onthe application of ISEs in environmental analysis using approaches such as real-time remote monitoring of waterquality, shipboard monitoring of environmentally important analytes using flow analysis instrumentation, the use ofrobust all-solid-state ISEs in submersible instruments for long-term deployment in the field, and innovative analyticalapproaches such as backside calibration and switchtrodes that avoid standard addition analysis and the concomitantperturbation in analyte speciation in natural samples.

Keywords: Ion-selective electrode, Trace metals, Metal speciation, Natural waters, Field monitoring

DOI: 10.1002/elan.200703916

1. Introduction

Despite the outstanding potential of ion-selective electro-des (ISEs) for the analysis of environmentally importantanalytes such as tracemetals, phosphate, nitrite, nitrate, etc.,it is perceived that ISE devices generally lack the sensitivityand selectivity needed for the analysis of trace analytes incomplex and challenging samples such as seawater, estuar-ine waters, rivers, lakes, soils, etc.Notwithstanding, there were several excellent papers in

the 1970?s and 1980?s [1 – 4] demonstrating that it is possibleto use a crystalline membrane copper ISE in the analysis ofnanomolar levels of copper in natural waters, as long as theISE is handled correctly, so as tominimize the dissolution ofthe ISE and the concomitant carry-over of copper fromsample to sample. Clearly, these seminal papers were aheadof their time, and laid the foundations for the use ofcrystalline membrane ISEs in environmental analysis.With regard to polymeric membrane ISEs, Sokalski et al.

[5] presented a landmark paper in 1997 on the influence of

transmembrane fluxes on the detection limits and selectiv-ities of ionophore-based ISEs and demonstrated that thisuncompensated experimental bias was responsible for theapparently mediocre sensitivity and selectivity of this classof ISEs. Clearly, this research paved the way for the creationof new and improved polymeric ISEs with vastly improveddetection limits and selectivities, enabling the ISE analysisof trace constituents in the environment. Accordingly, therehave been several papers on the application of this new classof polymeric ISE in the analysis of trace metals in theenvironment [6 – 8].Regarding the analysis of tracemetals in the environment,

the authors? view is that the great virtue of ISEs is theirability to sense the free metal ion activity, which is widelyrecognized as a master variable responsible for the uptakeand toxicity of metals by biota [9] (see Fig. 1), therebyproviding an analytical technique capable of monitoring theimpact of trace metal inputs in the environment. Notably,the free metal content of environmental waters is regulatedby the metal buffering ability of the natural water [9], as

1987

Electroanalysis 19, 2007, No. 19-20, 1987 – 2001 D 2007 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim

dictated by the inorganic and organic speciation ofmetals inthe sample, and this may also be estimated using the so-called “metal complexation capacity” that is determined bymonitoring metal titrations of natural waters using ISEs.Clearly, the speciation of a trace metal with the inorganic

(i.e., OH�, HCO3�, CO3

2�, Cl� and SO42�) and organic (i.e.,

HzLn where n denotes the possibility of 1, 2, 3, etc. differentchelating strength organic ligands depending on the sample)ligands in natural waters is crucial in regulating the level offree metal, and the concomitant bioavailability of the tracemetal. The speciation of trace metals may be generalizedsatisfactorily using the following reaction scheme [10]:

HþþOH�! H2O

HþþHCO3�!H2CO3

HþþCO32�!HCO3�

Mg2þþCO32�!MgCO3

Ca2þþCO32�!CaCO3

Mnþþ zCO32�!M(CO3)zn�2z

Mnþþ zHCO3�!M(HCO3)zn�z

yMnþþ zOH�!My(OH)zyn�z

Mnþþ zCl�!MClzn�z

Mnþþ zSO42�!M(SO4)zn�2z

MnþþHzLn!MLnn�zþ zHþ

In essence, the aforesaid speciation equilibria can be probedreliably by using an ISE tomeasure either the concentrationof freemetal under equilibrium conditions, or by elucidating

the metal-ligand formation equilibrium constants and con-centrations of ligands via ISE metal titrations of the naturalmedium.Either way, an ISEprovides a useful analytical toolfor investigating the trace metal speciation of environ-mental waters, which is crucial in determining the bioavail-ability of metals in the environment. Most significantly,these equilibria also demonstrate that acidification of anatural water sample is expected to breakdown the inor-ganic and organic metal complexes in the sample, therebyensuring that an ISE is capable of determining the concen-tration of total metal in a natural sample, if it has beenacidified to a low enough pH (viz., pH< 2). Consequently,ISEs are a powerful research tool in environmental mon-itoring, as they permit a simultaneous monitoring of metalinputs, as reflected by the level of total metal, and thebioavailability of these metal inputs – as ascertained fromthe level of free metal.Like many electrochemical sensors, ISEs are extremely

attractive in environmental science since their simplemeasurement principles and portability make them suitablefor on-site, shipboard and/or in-situ field analyses ofenvironmentally important analytes. Significantly, an ISEpotentiometric sensor only requires a measurement of theelectrode potential at near zero current, thereby ensuringthat simple and inexpensive instrumentation is required infield studies, and this is a highly desirable feature of ISEs inenvironmental analysis. Furthermore, there are ISEs forvirtually every known cation and anion, so it is possible toassemble an array of ISEs that is capable of covering a broadrange of environmentally important trace species, bothcationic and anionic, providing the environmental scientistwith a monitoring dataset that is rich in chemical informa-tion. Notwithstanding, the ISE electroanalysis of complexenvironmental samples like lakes, rivers, seawater, soils, etc.is a challenging task due to a wide range of potentialproblems, as discussed elsewhere by De Marco et al. [11]:

i) electrode foulingby the samplematrix components (e.g.,chloride, hydroxide, organic ligands, other cation oranion interferences, etc.) causing erroneous responsecharacteristics;

ii) electrode drift leading to poor reproducibility as thesensor surface is altered on continuous exposure to realsamples;

iii) electrode dissolution causing a high surface excess ofanalyte that degrades the detection limit of the ISEbeyond the realms of trace analyses in the environment;

iv) electrode carry-over of the analyte which can causecross-contamination of samples with the trace analyte;

v) electrode instability as passivation of the ISE in realsamples causes the response characteristics to becomeerratic and non-representative of the sample assayed.

The problems of electrode drift and electrode dissolution canbe ameliorated by using an ISE that has been conditioned inthe sample matrix, typically overnight, to produce a stable,reproducible and fast responding ISE, also minimizingsample contact times and the concomitant ISE release ofanalyte into the sample. The cross-contamination of samples

Fig. 1. Speciation model for metals in the environment (redrawnfrom Buffle et al. [9]).

1988 R. De Marco et al.

Electroanalysis 19, 2007, No. 19-20, 1987 – 2001 www.electroanalysis.wiley-vch.de D 2007 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim

www.electroanalysis.wiley-vch.de

via electrode carry-over is easily overcome by employing ameasurement protocol in which the electrode is cleansed ina low analyte bearing solution (e.g., a sacrificial calibrationstandard comprising ultratrace levels of the analyte belowthose expected in the natural sample) as well as a samplewashing regime where the ISE is pre-conditioned in multi-ple batches of the sample to be analyzed until a comparableresponse is obtained between wash samples indicating thatthe analyte has not been carried over into the sample.Frequent calibration and consequent rejuvenation of theISE is needed if electrode fouling and electrode passivation isevident upon continuous exposure of the ISE to realsamples, and for crystalline membrane ISEs this wouldcomprise frequent repolishing of the sensor surface, whilefor polymeric ISEs this may necessitate the adoption of afresh ISE membrane. Nevertheless, these electrode foulingand electrode passivation problems may be minimized byusing an ISE in flow-injection analysis (FIA) or continuousflow analysis (CFA) where a shearing of adsorbed speciesfrom the electrode surface under the influence of hydro-dynamic flow can cause the desorption of foulants and theprevention of electrode fouling [12].Despite a preponderance of research on themonitoring of

trace copper in the environment using a crystalline mem-brane ISE, there is a wide range of analytical applications ofISEs in environmental analysis including the analysis ofother important trace metals such as iron(III), mercury(II),cadmium(II), lead(II) and chromium(VI), as well as im-portant anions such as fluoride, phosphate, sulfate, nitrate,nitrite, chloride and cyanide. Table 1 summarizes theanalytical applications of ISEs in environmental science.Notwithstanding, this review will necessarily emphasize theISE electroanalysis of copper in the environment, as thisrepresents about two thirds of the research undertaken inthis field, but other environmental analyses with ISEs willalso be discussed since they demonstrate the general utilityand great potential of ISEs for environmental analysis.

As there are excellent general review articles on crystal-line and polymeric membrane ISEs [13 – 18], the presentreview will focus specifically on the application of ISEs inenvironmental analysis. It is important to note that this topichas not been addressed adequately since the suitability ofISEs for environmental analysis has only been realized inrecent times.

2. Copper in Natural Samples

Copper is a major trace metal in the environment due to itsextensive use in antifouling paints, and poses seriousenvironmental hazards due to its strong toxicity at excessivelevels [19]. Moreover, copper is an essential trace metalnutrient at ambient levels [20, 21], and it has beendemonstrated using a variety of analytical techniques andaquatic organisms [20 – 24] that the inhibition of growth ortoxicity of copper on phytoplankton is related to freecopper(II) or Cu2þ, and not the concentration of totaldissolved copper. This is the single most important featurethat makes the copper ISE so attractive in environmentalscience leading to considerable research in the developmentof reliable copper ISE methods for the analysis of naturalsamples (i.e., seawater, lakes, rivers, soils, etc.) [1 – 4, 11, 12,19 – 22, 25 – 68].

2.1. Free and Total Copper

It is significant to note that early and seminal papers usingthe crystalline membrane copper ISE demonstrated theutility of this ISE for the analysis of trace copper in theenvironment [1 – 4]. Notably, Smith and Manahan [2]demonstrated that standard addition potentiometry withthe Cu ISE can be used to measure accurately Cu2þ levelsdown to 10�8 M in tap water and natural water samples, as

Table 1. Examples of applications of ISEs in environmental analysis.

ISE Environmental application References

Cu ISE Analysis of Cu2þ in natural waters [3, 19, 27, 33, 43]Cu ISE Copper complexation capacity in natural waters and soil peats [69 – 74]Fe ISE Analysis of Fe3þ in natural waters [75, 76]Cd ISE Analysis of Cd2þ in natural waters [77 – 80]Hg ISE Analysis of Hg2þ in natural waters [81 – 83]F ISE Analysis of F� in natural waters [84 – 87]Pb ISE Analysis of Pb2þ in natural waters and culture media [6, 7]Cr ISE Analysis of HCrO4

� in natural waters [88, 89]CN ISE Analysis of CN� in natural waters [90 – 92]UO2

2þ ISE Analysis of UO22þ in tap water and seawater [93]

Anion, cation & gas ISEs Analysis of CN�, NO2�, F�, NH4

þ and NH3 in natural waters [94 – 96]H2PO4

� & Sm3þ ISE Analysis of PO43� directly or indirectly by titration in natural waters [97, 98]

Mg ISE Analysis of Mg2þ in seawater [99]Cu ISE Determination of water hardness [100]TFP ISE Back titration of excess 2-aminoperimindium ion for SO4

2� in seawater [101]Anion ISEs Analysis of corrosive anions (e.g., Cl�, NO3

� and SO42�) on electronic

equipment or in reinforced concrete structures[102, 103]

1989Ion-Selective Electrode Potentiometry in Environmental Analysis



long as the sample containers are cleaned rigorously inconcentrated acids and a dilute solution ofEDTA, ultrapurereagents are used, and the electrode release of copper isminimized through the use of a complexing antioxidantbuffer. Similarly, Jasinski et al. [3] showed that the copperISE can be used to analyze residual amounts of copper aslow as 3� 10�9 M in artificial and real seawater, ifprecautions are taken to minimize the contamination ofsamples using clean techniques and reagents, as well asemploying highly stirred solutions to facilitate the dispersalof electrode released copper out of the electrode?s diffusionlayer into the bulk sample. Most significantly, Jasinski et al.[3] showed that a standard addition analysis of seawater atpH 8 yielded a characteristic titration shaped curve and aconcomitant super-Nernstian response for the Cu ISE,noting that this effect is indicative of copper complexationwith ubiquitous seawater ligands, but theoretical Nernstianresponse was evident in the same seawater sample at pH 3due to a negation of the complexation capacity of seawaterligands due to protonation of their metal coordinatingfunctional groups. Notably, the research of Barica [25] inCanadian Prairies at pH 7.8 to 8.6 also demonstrated adouble Nernstian response to added copper(II), but thisanomalous response was found to be highly reproducibleand useable when the ISE was calibrated under theseanalytical conditions. ThepioneeringworkbyZirino and co-workers [1, 4] demonstrated that the potential of the Cu ISEin seawater, indicative of free copper, correlated beautifullywith anodic stripping voltammetry (ASV) determinationsof total copper, and that the copper ISE is susceptible tooxidation by oxygen, but its effect is minimal, and theauthors subsequently adopted this combined ISE and ASVapproach in the shipboard monitoring of free and totalcopper in samples taken along the coast from San DiegoUSA to Pisco Peru. Essentially, the outstanding potential ofthe copper(II) ISE for the analysis of Cu2þ in natural waterswas demonstrated in this very early research, and providedanalytical chemists with an excellent foundation for thedevelopment of reliable ISE methods for environmentalanalysis.Camusso et al. [26] reported an interesting study on

variations in the activity of Cu2þ in Lake Orta Italy as afunction of depth, as monitored using a conductivity-temperature-depth (CTD) device fitted with a jalpaitecopper(II) ISE, along with pH, oxygen and conductivitysensors. In this important paper, it was shown that respira-tion below the surface layer is responsible for a diminutionin dissolved oxygen levels and a concomitant decrease insolution pH due to nitrate formation via the oxidation ofammonium ions, and theCu2þ levels in the lakemirrored theexpected changes in speciation as acidification led todissociation of natural copper(II) complexes into freecopper(II). In essence, this study highlighted the tremen-dous potential of the copper(II) ISE for in-situ field studiesof copper in the environment using submersible instrumen-tation.In a landmark paper, Belli and Zirino [19] studied the

response of the crystallinemembrane copper ISE in real and

artificial seawater. It was found that a standard additionanalysis of real and artificial seawater at pH 8 yielded asuper-Nernstian response to added copper, but the theoret-ical Nernstian response was obtained in bothmedia at pH 2.Conversely, the response of the copper ISE in the spikedartificial seawater at pH 8 yielded the theoretical Nernstianresponse when plotted against the MICROQL calculatedfree copper levels obtained using the well-known copper-inorganic speciation of artificial seawater. The authors usedsimulations for copper speciation in seawater to prove thatthe apparent super-Nernstian response of the copper ISE instandard addition analyses of seawater is due to forcing theISE data to fit the incorrect parameter in total copper sincethe copper partitioning factor that dictates the level of freecopper in seawater is variable at different levels of addedcopper. Last but not least, Belli and Zirino [19] noted thatthe response of the copper ISE in chloride-based copperbuffers was independent of the salinity at the concentrationsof seawater, clearly demonstrating that the electrode is freeof the usual chloride interference that is problematic duringthe analysis of saline solutions containing high concentra-tions of total copper. Clearly, this research demonstratedthat the copper ISE can be used in determinations of totalcopper in acidified seawater.DeMarco [27] undertook an evaluation of the response of

crystalline membrane copper ISEs in seawater media, andfound that all commercially available ISEs yielded aNernstian response in saline copper buffers in the range10�15 to 10�9 M Cu2þ. Notwithstanding, it was found that theelectrode employing a crystallinemembrane of jalpaite (i.e.,Ag1.5Cu0.5S) was the only electrode capable of providingNernstian response that was collinear with the responseobtained in chloride-free unbuffered standards in the range10�6 to 10�1 M Cu2þ. Clearly, this outcome demonstratedthat the response of the jalpaite copper ISE in salinecalibration buffers is free of the usual chloride interferenceeffect that is evident at 10�6MCu2þ or higher, andDeMarco[27] argued that this is due to kinetic limitations of thechloride-induced membrane surface reactions that arecontingent on the activity of copper, which is at ultra-tracelevels below 10�9 M. Most significantly, the response of thecopper ISE in artificial seawater comprising low levels oftotal copper (i.e., 3� 10�9 and 3.6� 10�8 M) and 10�6 Mglycine yielded ISE determined free copper levels within�0.2 log(aCu2þ) units of those calculated using the well-known complex formation and ionization constants forcopper(II)-glycine, copper(II)-hydroxy, copper(II)-carbon-ate, etc. complexes and their associated ligands. Withoutdoubt, the work of De Marco [27] demonstrated that thecopper ISE is capable of detecting free copper in seawatercomprising nanomolar concentrations of total copper.De Marco and co-workers [12, 28, 29] conducted impor-

tantmechanistic studies on the electrode kinetic and surfacechemical behavior of the jalpaite copper(II) ISE in sea-water. In contaminated and uncontaminated SanDiegoBaysamples, Zirino et al. [12] noted a surface excess and lowerlimit of detectionof 2� 10�8M total copperwith a static ISE,with the influence of hydrodynamic flow at either a rotating




disc electrode (RDE) copper(II) ISE or static copper(II)ISE residing in a flowing stream in the wall-jet cell of a CFAor FIA analyzer allowing dispersal of electrode releasedcopper into the bulk solution and lowering the ISEdetectionlimit to approximately 10�10 M. In a separate study, DeMarco [28] usedX-ray photoelectron spectroscopy (XPS) todemonstrate that continuous aging of the membrane inartificial seawater leads to a modification of the sensorsurface to a copper deficient sulfide phase together withelectrode fouling due to silver chloride formation, andadsorption of the natural organic ligands in seawateralleviates this chloride interference by promoting thepeptization of deposited silver chloride, therebyminimizingthe seawater-induced fouling of the membrane. A subse-quent electrode kinetic study of the jalpaite copper(II) ISEin seawater using electrochemical impedance spectroscopy(EIS) [29] demonstrated that the behavior of the sensor inseawater is compliant fully with the generally acceptedresponse mechanism involving the reduction of copper(II)and concomitant copper(I) ion-exchange at copper sulfidesites. Significantly, the EIS charge transfer kinetics of theISE in the presence of varying levels of commercial humicacid [29] showed that natural organic ligands exert a mildinterference effect on the response of the copper(II) ISE, asevidenced by the ligand-induced facilitation of the copper(-II) reduction process, but potentiometric data revealed thatthe typical level of humic acid in seawater (i.e., 1 mg L�1) istolerable leading to an error of about�0.1 log(aCu2þ) units.Also noteworthy was the observation that a RDE jalpaitecopper(II) ISE yielded a more substantial change in theinfluence of rotation speed in natural seawater, as predictedusing the Levich equation, noting that this observation issymbolic of a hydrodynamic-induced desorption of naturalorganic ligands from the surface of the jalpaite copper(II)ISE. In any event, an interesting FIA studywith a solid-statechalcogenide copper(II) ISE in unbuffered saline copperstandards at concentrations above 10�6 M Cu2þ [30]demonstrated that the usual chloride interference associ-ated with complexation of copper(I) generated through thereductive ion-exchange of copper(II) is eliminated by thephenomenon of kinetic discrimination in which the cop-per(II) and chloride response processes are time resolved inthe FIA transient due to significant differences in thekinetics or non-steady state signals of the ISE towards thesespecies. Accordingly, contrary to popular belief that organicligands interfere stronglywith the responseof the copper(II)ISE in natural waters [17, 18], this mechanistic researchdemonstrated that ubiquitous seawater organic ligandsexert a mild interference, and can in fact protect the sensoragainst the chloride-induced fouling of the sensor duringcontinuous exposure to seawater together with the possi-bility of compensating for these interference effects underthe influence of hydrodynamic flow at an RDE or in CFAand/or FIA.In a brief review on the use and misuse of crystalline

copper(II) ISEs in seawater, Mackey and De Marco [31]showed that a jalpaite copper(II) ISE pre-equilibrated inreal seawater, if handled using clean techniques and a

cleaning protocol entailing exposure of the ISE to asacrificial copper(II) buffer comprising low activities ofCu2þ, then it is possible to conduct reliable determinations ofCu2þ in seawater media.The aforesaid mechanistic research on the response

mechanism of the copper(II) ISE in seawater inspired anumber of subsequent studies by Zirino and co-workers [11,32, 33] into the analysis of copper(II) in seawater using thejalpaite copper(II) ISE. First, De Marco et al. [11] demon-strated that overnight conditioning of the electrode inseawater is needed to stabilize its response in seawaterenabling fast response times and short electrode contacttimeswith the sample, therebyminimizing contamination ofthe sample through electrode dissolution. Furthermore,these authors showed [11] that ISE determined freecopper(II) levels in organic-free or UV photooxidizedseawater are commensurate with those calculated usingthe well-known inorganic copper(II) speciation in seawater,and the analysis of seawater at pH 2 using a standardaddition method gave ISE total copper levels consistentwith those measured using graphite furnace atomic absorp-tion spectrometry (GFAAS). Next, Zirino et al. [32] re-spectively measured free, total and labile copper(II) levelsat various sites in San Diego Bay using ISE, GFAAS andASV, and found that the data is reconcilable with anequilibrium model for the binding of copper to naturalorganic ligands, thereby providing stability constants forseawater copper(II)-organo complexes that are comparablewith previously published findings. Last, Zirino et al. [33]used a RDE jalpaite copper(II) ISE in the analysis of freecopper(II) activities in San Diego Bay demonstrating thatthe electrode release of copper is diminished substantially atrotation speeds in excess of 4,800 revolutions per minute,thereby allowing a reliable determination of the truethermodynamic activity of Cu2þ. Most significantly, theauthors [33] showed that, betweenmeasurements, storageofthe ISE in a sacrificial copper(II) buffer in completedarkness minimized electrode carry-over of Cu2þ andundesirable photochemical effects with this sensor, therebyproviding a substantially improved reproducibility of ap-proximately �0.06 log(aCu2þ) units. Furthermore, Zirinoet al. [33] also showed that continuous bathing of acopper(II) ISE in natural seawater progressively titratedthe ubiquitous organic ligands with electrode released Cu2þ

yielding a breakthrough point in the potentiometric re-sponse of the ISE, as evidenced by a sharp rise in theelectrode potential that is commensurate with Nernstianresponse to the GFAAS determined values of releasedcopper(II), signifying that it is possible to utilize the ISEcorrosion-based release of copper in autotitrations of thecopper complexing capacity. Further credence for thiscopper ISE autotitration method was provided by the EISelectrode kinetic data in seawater [29] showing that sea-water autotitrated by the electrode release of copper(II)correlates with a relative constancy in ISE potential up tothe breakthrough point in the autotitration, at which pointthe copper(II) ISE kinetics are consistent with a response toelevated levels of Cu2þ. Clearly, these papers provided




unequivocal evidence of the utility of the jalpaite copper(II)ISE in precise and accurate determinations of free and totalcopper in seawater.There are other interesting papers on the use of a jalpaite

copper(II) ISE in the analysis of Cu2þ in natural waters [34 –42]. For example, Stella and Ganzeril-Valenti [34] used acopper(II) ISE to monitor variations in free copper(II) as afunction of pH in the Ticino and Po Rivers, and the authorsused this data in the elucidation of the inorganic complex-ation of these natural waters. Significantly, Cabaniss andShuman [35] as well as Holm and co-workers [36, 37]adopted a combined copper(II) ISE potentiometry andfluorescence quenching (FQ) method in studies of thecopper(II) binding of natural organic matter in seawater,rivers and lakes, and these workers demonstrated that bothmethods gave comparable values for free copper(II) insamples containing low levels of total copper(II) (i.e.,<10�7

M). Similarly, Rozan et al. [38] used a combination of ASVand ISE to determine free copper(II), natural organicligands and copper complexing capacities inmodel copper(-II) buffers incorporating ethylenediamine and EDTA, aswell as copper binding with Suwanne River and DismalSwamp fulvic acids, and the results showed that bothtechniques are able to measure comparable levels of labilecopper(II) when the detection limits of the techniquesoverlapped. Ghode and Sarin [39] illustrated that a cop-per(II) ISE may be used in copper speciation in naturalwaters by showing that copper(II) speciation in the presenceof polyamino carboxylic acids such as EDTA are internallyconsistent with the theoretically expected binding ofcopper(II) by these ligands, while Zolotov et al. [40] showedthat a Chelex 100 preconcentration column used in combi-nation with a copper(II) ISE FIA potentiometric techniqueis capable of providing accurate levels of total copper(II), ascompared to GFAAS, in tap water, waste water andseawater. Most recently, Eriksen et al. [41] developed aCFA method for the analysis of free and total copper inPacificOcean seawater, and the total copper levels obtainedby ISE potentiometry in acidified seawater comparedfavorably with those determined using GFAAS, whileRivera-Duarte and Zirino [42] used copper(II) titrationsof San Diego Bay seawater to deduce the copper(II)complexing capacities and concomitant levels of freecopper(II) yielding results that compared favorably withtypical values in estuarine and coastal samples.To surmise, the aforementioned research has illustrated

the validity of using a copper(II) ISE in determinations offree and total copper(II) in naturalwaters, and this avails thecopper(II) ISE to studies of the bioavailability and toxico-logical response of copper in environmental science.

2.2. Free Copper and Toxicology Studies

Therehavebeennumerous studies on the useof a copper(II)ISE in toxicological studies of aquatic organisms in naturalwaters [20 – 22, 43 – 49]. First, Sunda and co-workers [20 –22] investigated the toxicological response of a marine

bacterium [20], an estuarine diatom [21] and unicellular alga[22] in seawater and river water, and found that ISEdetermined free copper(II) levels correlated very stronglywith toxicological data for these aquatic organisms. In aseries of similar papers [43 – 49], the influence of copper(II)complexation on the toxicity of various fresh and marinewater species wasmonitored in parallel with copper toxicitystudies. In all cases, the biological measure of toxicity (i.e.,LC50, survival rates, embryo hatching rates, copper accu-mulation by the organism) yielded excellent correlationsbetween toxicity and free copper(II) levels as measuredusing the copper(II) ISE.Notwithstanding, the ability of the copper(II) ISE for

reliable analyses of free copper(II) in samples containingtrace levels of copper(II) has been a controversial topic dueto the outdated thesis that this ISE possesses a sensitivity ofapproximately 10�6 M total copper [74], and this view issometimes perpetuated by environmental researchers [50]despite the overwhelming evidence that the copper(II) ISEis functional in seawater and other aquatic media.Ultimately, proof that the copper(II) ISE is indeed a

reliable tool for the environmental analysis of free copper(-II) in natural waters was provided in recent papers byEriksen et al. [43] and Rivera-Duarte et al. [44] whoconducted copper speciation and toxicity studies of amarine diatom and three larval species using the jalpaitecopper(II) ISE. In each of these studies, seawater containingnanomolar concentrations of total copper (MacquarieHarbour seawater in Eriksen et al?s study [43], and SanDiego Bay seawater in the study of Rivera-Duarte et al.[44]) was spiked with varying levels of copper, and total andfree copper levels were assayed by GFAAS and copper(II)ISE potentiometry respectively, while the toxicologicalresponses towards marine species (viz., growth inhibitionof the marine diatom [43] and LC50 values for larvaldevelopment of three marine invertebrates [44]) wererecorded simultaneously. In both studies, there was a clearcorrelation between the toxicological response of marinespecies to copper in seawater, and the free copper(II) levelsdetermined using the ISE, not total copper levels assayedusing GFAAS. Furthermore, these studies also found that atoxicological response occurs at levels above 10�11 M Cu2þ,and this data is consistent with literature data for toxico-logical studies of marine species.In conclusion, the body of research on the behavior of the

copper(II) ISE in natural waters demonstrates unequivo-cally that this sensor is suitable for determinations of freecopper(II) in natural waters; a master variable that isresponsible for the bioavailability and toxicity of copper(II)in the environment. Despite the many alleged artifacts thatare inherent in assays of ultratrace levels of free copper(II)in challenging and complex samples such as natural waters,the correlation of copper toxicity data for a variety ofaquatic organisms with ISE determined free copper(II)levels, togetherwith the expected threshold for toxicologicalresponse above 10�11 M Cu2þ, as accepted widely in thescientific literature, shows that all of the arguments aboutthe aberrant response of the copper ISE are largely




irrelevant, and that this (and other) ISE(s) can providehighly useful data in environmental surveys of the fate andimpact of copper (and other metals) in the environment.

2.3. Copper Titrations in Determinations of CopperComplexing Capacities

The role of ubiquitous organic ligands in natural waters onthe complexation of metals is very significant, and it iswidely accepted that copper(II) in seawater is almostcompletely complexed by these chelating ligands [51]. InMackey?s and Zirino?s [51] interesting and insightfulcommentary about the role of copper(II) complexation bythe organic ligands in seawater, it was proposed that there isa lack of equilibrium between spiked metals and theubiquitous organic ligands in seawater, and they cited 10key experimental observations to support this claim.Accordingly, these authors questioned the merit of ligandcompetition/metal titration methods (e.g., ASV, CSV, ISE,etc.) that assume inherently the establishment of a thermo-dynamic equilibrium in the computation of the associatedcopper(II)-organic complex formation constants and cop-per(II) complexing capacities. Nevertheless, the followingsectionwill demonstrate that this approach is representativeof the steady-state or quasi-equilibrium establishedwith thespeciation of trace metals in seawater, and this may indeedbe indicative of the exact conditions that are controlling thebinding of copper(II) and its concomitant bioavailability inseawater.An alternative approach to the characterization of metal

speciation in environmental samples is viametal titrations ofthe ubiquitous organic ligands in natural samples. Of themany analytical techniques trialed, the copper(II) ISE hasgained widespread use in studies of the stability constantsand copper complexing capacities of natural organic ligandsin swamps, lagoons, rivers, seawater, lakes, ponds, soilextracts, peats, synthetic seawater media, etc. [52 – 74].Clearly, the copper(II) ISE has demonstrated its robustnessand broad utility in copper speciation studies of naturalorganic ligands in a wide range of environmental samples,and this method offers an attractive alternative to compet-itive ligand / adsorptive stripping voltammetric techniques[23, 24, 104, 105].Notably, there have been an array of studies into the

copper(II) complexing capacities of ubiquitous organicligands in natural waters using ISE potentiometric titrationmethods. For example, various authors [55, 58, 63, 66, 68]demonstrated that it is possible to determine the copper(II)speciation of natural waters using the data obtained inpotentiometric titrations, and the stability constants formodel ligands (e.g., carbohydrates, EDTA, glycine, alanine,tiron, etc.) together with natural organic ligands arecommensurate with the literature data highlighting thevalidity of this approach.Most impressive are reports into comparable copper(II)

ISE titration and spectrometric studies of the copper(II)complexing capacity of natural waters [52, 54, 57, 64, 71]. In

all cases, the data obtained using the ISE and alternatespectrometric methods (e.g., differential spectrometry, ion-exchange/GFAAS, dialysis titration, fluorescence quench-ing, and UV-visible spectrometry) were comparable.A unique and interesting approach has employed a

combination of potentiometric stripping analysis (PSA)and copper ISE potentiometry to elucidate the copper(II)complexing capacity, conditional stability constants ofcopper(II)-organic complexes and free copper(II) in sea-water, thereby giving an integrated view of the copper(II)speciation in the presence of natural organic ligands [56, 72,73].Xue and Sunda [74] published a seminal paper on

comparative measurements of free copper(II) levels, organ-ic ligand concentrations, and the stability constants ofcopper(II)-organic complexes in lake waters using ligandcompetition/ASVat three detection windows as well as ISEpotentiometry. It is evident that the copper(II) ISE iscapable of providing reliable and comparable free copper(-II) values in the lake samples investigated at total copperlevels in excess of 10�8 M; however, the ISE yielded higheraCu2þ values by aboutþ0.75 log(aCu2þ) unitswhen the totalcopper level in the sample dropped to beneath the thresholdISE detection window of 10�8 M. Significantly, this impor-tant research demonstrated unequivocally that, althoughthe metal speciation in natural waters is kinetically con-trolled and the system is in a steady-state or quasi-equilibrium, ISE and ASV are capable of providingquantitative analytical data about the steady-state copper-(II) speciation of natural waters, and this information isindeed representative of the true solution chemistry ofnatural waters.In summary, the aforementioned copper complexation

studies in natural waters have highlighted the problemsassociated with copper ISE in determinations of freecopper(II) in environmental samples containing <10�8 Mtotal copper, and illustrate that a ligand competition/ASVmethodology is ideally suited to these samples. Neverthe-less, it is important to note that Zirino et al. [12] demon-strated that the high detection limit of 10�8 M total copperevident with a static copper(II) ISE through contaminationof the electrode?s Nernst diffusion layer at this level may bealleviatedbyusing ahydrodynamic flow regime (e.g.,RDEs,FIAorCFA) to lower the copper(II) ISE?s detection limit toapproximately 10�10 M. Without doubt, the preferredanalytical methodology for the analysis of free copper(II)in environmental samples would utilize a copper(II) ISE ineither the RDE, FIA or CFA analysis modes.

3. Other Trace Metals in the Environment

Section 2 demonstrated clearly that the vast majority ofenvironmental applications with ISEs has concentrated onthe analysis of free and total copper, along with coppercomplexing capacities in natural samples. Notwithstanding,these same analytical approaches are also applicable toother metal ISE systems, and there has been a limited




amount of research on metal speciation in natural watersusing crystalline membrane and glassy membrane iron(III)[75, 76, 106 – 111], cadmium(II) [77 – 80, 112 – 114] andmercury(II) [81 – 83, 115, 116] ISEs, as well as polymericISEs for chromium(VI) [88, 89] and lead(II) [6, 7].

3.1. Iron(III) in Seawater

Iron is an important limiting trace metal nutrient in theenvironment [104, 105, 117], as it limits the growth ofphytoplankton and biomass production in certain oceaniczones. As iron finds its way predominantly into oceans byatmospheric dust deposition and by upwelling of water, it ispresent at very low levels in isolated areas such as TheSouthern Ocean near Antarctica, and this was exemplifiedby an iron fertilization event that led to a phytoplanktonbloom in The Southern Ocean, as observed using satelliteimaging of the fertilization zone [118, 119]. Clearly, a robustiron(III) ISE that permits a monitoring of free iron(III) inseawater would be of immense interest to environmentalscientists and analytical chemists.Using an extension of the approach for lowering the

detection limit of the copper(II) ISE based on the positiveinfluence of hydrodynamic flow on the ISE [12], De Marcoet al. [75] developed a chalcogenide glass iron(III) ISECFAmethod for the determination of free iron(III) in acidifiedand UV photooxidized Southern Ocean or open oceanseawater at pH 3.86 comprising a total iron(III) concentra-tion of 5� 10�10 M, as determined by GFAAS. Notably, theCFAISEmethod yielded a log(aFe3þ)¼ 11.1� 0.15 units for10 repetitive injections of seawater against calibration dataobtained using saline iron(III)-citrate buffer solutions, andthis outcome correlated brilliantly with the iron(III) speci-ation calculated result of log(aFe3þ)¼ 10.8. The results ofthis study demonstrated that the CFAFe(III) ISEmethod isextremely useful in measurements of free iron(III) in UVphotooxidized seawater.In two important papers byDeMarco and co-workers [76,

106], the response of the iron(III) chalcogenide glass ISEwas investigated in raw and UV photooxidized open oceanseawater together with a collection of saline calibrationbuffers comprising a variety of iron(III) binding ligands, andthe concomitant responsemechanism of the iron(III) ISE inseawater media was explored using EIS and XPS of theelectrode kinetics and surface chemistry of the sensor inseawater media. This research demonstrated that the iron(-III) ISE is capable of providing a near collinear response forthe electrode in salicylate and citrate buffers over aconcentration range of 10�23 to 10�1 M aFe3þ; however,EDTA revealed an offset in ISE response, although thebehavior was still Nernstian [76]. Furthermore, this ligandinterference is also manifested by natural seawater ligands,and it was shown that the ISE can be used to providemeaningful response data in organic free, UV photooxi-dized seawater yielding a free iron(III) level commensuratewith the well-known inorganic speciation of iron(III) inseawater [76]. Most significantly, the EIS and XPS mech-

anistic study [106] showed that the observed electrode slopeof 30 mV/decade for the iron(III) ISE in saline media isconsistent with a reversible dual response process involvingboth electron transfer and ion-exchange that is compliantwith its concomitant Nernst equation, and that the iron(III)RDE is also compliant with the Levich equation for thediffusion controlled response in seawater showing that it isalso possible to facilitate the dispersal of electrode releasediron out of the Nernstian diffusion layer by using hydro-dynamic flow in either the RDE, FIA or CFA modes ofanalysis. By analogy to the response of the copper(II) ISE inseawater, which also illustrated a compliance to the Levichequation for an RDE [12, 33], it is clearly possible to lowerthe electrode release of iron from the iron chalcogenide ISEto 10�10 M or less permitting the analysis of open oceansamples comprising sub-nanomolar levels of analyte, as hasbeen demonstrated in the CFA analysis of iron(III) inSouthern Ocean seawater [75].A further mechanistic study of the iron(III) chalcogenide

glass ISE using secondary ion mass spectrometry (SIMS),EIS and XPS in various media [108, 109] suggested thatchloride, hydroxide, nitrate, etc. in seawater together withubiquitous organic ligands do not pose a serious problem forthe electroanalysis of Fe3þ in seawater, but it is necessary tocondition the electrode in seawater prior to analysis, whileregular recalibration, reconditioning (preferably overnight)and frequent polishing is required, if the ISE?s response inseawater is to remain internally consistent with its responsein iron(III) calibration standards.In recent research, De Marco and co-workers [110, 111]

have developed a novel synchrotron radiation-grazingincidence X-ray diffraction (SR-GIXRD)/potentiometry/EIS capability to monitor in-situ the so-called modifiedsurface layer (MSL) of the iron(III) chalcogenide glass ISEin artificial and real seawater, and it was shown that thesurface crystalline phases of this sensor (i.e., metal sele-nides) are attacked by chloride and hydroxide in artificialseawater, but are protected by the natural organic ligands inseawater. The EIS/SR-GIXRD data [110] illustrated that adestruction of the MSL in artificial seawater is due to acomplete removal of all surface crystalline phases, and thatraw seawater comprising natural organic ligands togetherwith a mimetic seawater ligand system containing ethyl-enediamine, salicylic acid and EDTA is capable of protect-ing the MSL of the iron(III) ISE against this oxidativedissolution process. Unpublished data has shown that thismimetic seawater ligand calibration buffer provides mean-ingful aFe3þ data in seawater, and this paper will bepublished soon.In conclusion, the previous research on the response of

the iron(III) ISE in seawater has shown that the adoption ofeither an RDE, FIA or CFA approach in tandem with amimetic seawater ligand calibration system can be used inthe reliable electroanalysis of free iron(III) in seawater, andthis may provide a valuable research tool for analyticalchemists interested in studying this important trace metalnutrient in the environment.




3.2. Cadmium(II) in Natural Waters

Cadmium toxicity is of great concern to environmentalscientists especially in rapidly industrialized regions likeEast Asia, noting that monsoonal rains can wash cadmiumonto flooded farming lands. Despite the environmentalimportance of cadmium, there have been very few studies ofcadmium in the environment using a cadmium(II) ISE.Several authors used the cadmium(II) ISE in studies of

cadmium(II) speciation in environmental samples. Forexample, Simoes et al. [112] studied the speciation ofcadmium in artificial and real seawater using a crystallinemembrane cadmium(II) ISE andASV, and it was found thatboth methods gave comparable stability constants for thebinding of cadmiumbyubiquitous organic ligands. Similarly,Lee et al. [113] studied the cadmium binding affinity to soiland commercial humic acids using ultrafiltration (UF)coupled with GFAAS in combination with the cadmium(II)ISE, and identified two ligands, with the ISE and UF/GFAAS techniques yielding comparable formation con-stants for cadmium(II)-organic complexes. Last but notleast, Fish and Brassard [114] demonstrated that naturalorganicmatter (NOM) in sediment samples induced foulingof the cadmium(II) ISE and a concomitant drift in the ISEpotential as a function of soaking in samples. These authorsdemonstrated that a dialysis membrane is capable ofeliminating the diffusion of NOM to the electrode surface,therebyobviating the usual long-termageing of the sensor inreal samples.Trojanowicz et al. [77] developed an FIA method for the

analysis of free cadmium in natural waters, and used thismethod in the analysis of free cadmium in lake and riversamples. Excellent agreement between GFAAS and ISElevels for free cadmium(II) was observed in these environ-mental samples, and the use of masking agents for lead(II),copper(II) and iron(III) can eliminate these chemicalinterferences in FIA assays of free cadmium(II). AlthoughTrojanowicz et al. [77] only demonstrated the applicabilityof their ISE-FIA technique in contaminated estuarinesamples, it is highly likely that the influence of hydro-dynamic flow in FIAwill permit a diminution in the level ofelectrode released cadmium to sub-nanomolar levels, as isthe case with the copper(II) ISE [12], thereby enabling a useof the cadmium(II) ISE in the electroanalysis of open oceanseawater samples comprising subnanomolar levels of totalcadmium.In an important cadmium(II) speciation study of sea-

water, Sunda [78] determined free cadmium(II) in rawseawater using a Chelex 100 ion-exchange method inconjunction with the cadmium(II) ISE technique, andboth methods yielded a ratio of free-to-total cadmium of10�1.5, while Sherman et al. [79] used the cadmium(II) ISE inmeasurements of free cadmium(II) in pond water andsediments, noting that this master variable correlated withthe toxicity data (i.e., LC50) for Fathead Minnows.Last, Mortensen et al. [80] developed an array electrode

employing a variety of chalcogenide glass ISEs, andused thissensor in conjunction with pattern recognition techniques

such as artificial neural networks (ANN), principal compo-nent analysis (PCA) and partial least squares regression(PLS) to analyze waste incinerator exhausts in chemicalplants.In summary, the crystalline membrane cadmium(II) ISE

possesses a comparable utility to the copper(II) ISE, andmay be used in long-term surveying of water quality in theaquatic environment.

3.3. Mercury(II) in Seawater

Mercury is a highly toxic mineralogical pollutant that posesa serious risk to the environment, as evidenced by theMinamataBay incident in Japanwhere an entire communitywas poisoned as a result of contaminated seafood. Notwith-standing, there has been a limited amount of research intothe development of an ISE sensor technique for themonitoring of mercury(II) in the environment.Shatkin et al. [81] developed a HgS/Ag2S composite

membrane that gave a near-perfect Nernstian response toHg2þ in saline mercury(II) buffers in the range 10�15 to 10�2

M aHg2þ. Similarly, Yin et al. [82] studied the interaction ofHg2þwith soil derived humic acid, and found that calculatedequilibrium and ISE determined free mercury(II) levelswere within the experimental uncertainty, and De Marcoand Shackleton [83] along with De Marco et al. [115]demonstrated that the mercury(II) chalcogenide glass ISEwas able to provide a Nernstian response over a dynamicrange of 20 orders of magnitude and that a seawaterpassivation interference effect may be circumvented byusing a standard addition analysis technique and/or CFAanalysis.Last but not least, De Marco et al. [116] used a variety of

cutting-edge materials and surface characterization tech-niques to elucidate the mechanistic chemistry of themercury(II) chalcogenide glass ISE in seawater media,and showed that the ubiquitous organic ligands of naturalwaters are able to prevent detrimental silver chloridefouling of the sensor through the peptization of precipitatedsilver chloride, noting that this desirable effect was alsoobserved with the copper(II) electrode [28].Regrettably, as open ocean seawater and other uncon-

taminated natural samples comprise picomolar amounts ofdissolved mercury [10], it is unlikely that a mercury(II) ISEwill possess sufficient analytical sensitivity to permit thereliable electroanalysis of mercury in uncontaminatedenvironmental samples, but the mercury(II) ISE shouldsuffice for contaminated coastal, lake, river, estuarine,seawater, etc. samples. Accordingly, the electroanalysis ofmercury in uncontaminated samples using a mercury(II)ISE necessitates the adoption of a preconcentration tech-nique such as the Chelex 100 method developed by Zolotovet al. [40] in the analysis of trace amounts of copper(II) invarious uncontaminated environmental samples.




3.4. Lead(II) and Chromium(VI)

An optimized polymeric lead(II) ISE was employed in acomparative inductively coupled plasma/mass spectrometry(ICP/MS) and ISE analysis of total lead in natural watersamples [6], and demonstrated that the ISE method is atleast as accurate as ICP/MS down to lead(II) levels ofapproximately 0.5 mg L�1. This excellent analytical sensitiv-ity makes the sensor ideal for the analysis of lead(II) inenvironmental samples, and Slaveykova et al. [7] have usedthe ISE to monitor free lead(II) levels as function of thetoxicological response of an aquatic algae in the presence ofSuwannee River fulvic acid, noting a clear correlationbetween lead(II) uptake by aquatic algae and ISE deter-mined levels of free lead(II).Last but not least, Choi andMoon [88, 89] have developed

a liquid supported HCrO4� ISE for the analysis of chro-

mium(VI) in waste waters. The authors also used this newchromium(VI) sensor in continuous flowmonitoring, whichnot only provided scope for automation of the analyses, butalso helped to control the electrode release of analyte intothe ISE?s Nernst diffusion layer, thereby enabling traceanalyses in environmental samples.It is clearly evident that all of the metal ISEs surveyed in

this section have very similar response attributes to thesolid-state copper(II) ISE that has been used extensivelyand successfully in the analysis of natural waters, and thesesensors are therefore expected to fulfill an important role inenvironmental studies of these trace metals in the environ-ment.

4. Fluoride in the Environment

Fluoride is an environmentally important analyte, as it isutilized extensively in the minerals processing industry, aswell as in drinking water, and finds its way into theenvironment. Its toxicological effects such as dental fluo-rosis, chronic toxicosis in animals, and its influence onnutrient cycling due to reaction with environmental nu-trients such as organic carbon, aluminum and iron, impactdramatically on the ecosystem [86]. Accordingly, this is ananalyte that has attracted attention in the analysis ofenvironmental samples using ISEs.In an important study byLowandBloom [86], the fluoride

ISEwas used tomeasure theAeolian deposition of fluoride-based compounds in the Tamar Valley of Tasmania Aus-tralia surrounding its nearby aluminum smelter. Mostsignificantly, it was found that, within a 3 km radius of thesmelter where the observed fluoride fluxes are very high(viz., sometimes as high as 12.568 mg m�2 day�1), plantdamage is clearly observable, and these high levels are atleast two orders of magnitude higher than the normalbackground level. This study identified a need to lower thefluoride emission levels of operational aluminum smelters,and it was highlighted that contemporary practices inaluminum smelting would make a substantially loweremission possible.

In an important environmental surveyof fluoride inPolishnatural waters, drinking waters and the urine of kinder-garten children living in the vicinity of the natural waterssampled [87], it was shown that the fluoride ISE is able toprovide accurate data on the levels of fluoride in the urine ofinfants, thereby providing an indicator for the impact offluoride on young children who are most sensitive tofluorosis.Similarly, Bhagavan and Raghu [120] used a fluoride ISE

to study the fluoride content of bore waters, along with theblood and urine samples of villagers in the AnantapurDistrict of India. Significantly, itwas found that high fluoridebearing bore waters in upstream check dams led to expect-edly high blood and urine fluoride levels of the villagers,especially in the 5 – 11 year-old males. Although thesurveyed young males did not show any detectable signs ofdental or skeletal fluorosis, the authors urged the need forfurther medical studies with these subjects. Furthermore,the authors recommended that the inhabitants of this areaengage in a higher dietary intake of calcium and tamarind tocombat this potentially difficult environmental problem.Pierre et al. [121] conducted a study into the urinary

excretion of fluoride from aluminum industry workersexposed to common pot-room pollutants (i.e., HF, NaF,AlF3, Al2O3, etc.) using a fluoride ISE, and the data enabledthe establishment of a kinetic model to account for theexcretion of fluoride and aluminum compounds in humans.Furthermore, it was noted that a subject expressed theirmaximum in excretion rate several hours after a shift with aminimum rate achieved at the end of a shift. Consequently,an appropriate occupational hygiene sampling regime musttake account of this important characteristic, if reliableexposure data is to be obtained.Rix and co-workers [84, 85, 122] developed a standard

addition method for the analysis of fluoride in naturalwaters, and adapted this approach to a microprocessorcontrolled instrument [85, 122] capable of automatedanalyses. These researchers applied their technique to avariety of seawater samples (viz., Port Lonsdale Victoria,Corio Bay Geelong, Murray River Swan Hill, Albert ParkLake and Hepburn Springs Victoria), and found that thesensitivity of the ISE was sufficient to detect the ambientlevels of fluoride and that the adjoining super-phosphatemanufacturing industry in Geelong had contaminated theshallow regions of Corio Bay with high concentrations offluoride.Clearly, an ISE approach to environmental analysis is also

adaptable to anion analyses, if a robust and reliable sensor isavailable for the analytical task.Obviously, the single crystallanthanum fluoride ISE possesses the requisite physical,chemical as well as analytical robustness and ruggedness tomake it useful in environmental analysis.

5. Miscellaneous Analytes

Despite the suitability of ISEs to field analyses, and awidespread appeal for using these devices in early detection/




monitoring devices, there are very few environmentalstudies outside of the metal and fluoride assays that havebeen described in depth in the previous sections. In thissection, the authors will present a brief overview ofmiscellaneous ISE applications in environmental science.Given the extreme toxicity of cyanide and recent environ-

mental disasters such as poisoning of fish in the DanubeRiver, there have been several attempts to develop acrystalline membrane cyanide ISE for the electroanalysis ofnatural waters. Hefter and Longmore [90] used a cyanideISE in continuous monitoring of cyanide in seawater, anddemonstrated that the well documented chloride electrodeinterference may be corrected when the chloride selectivitycoefficient and concentration are known. Alternatively,Neshkova and co-workers [91, 92] explored a new gener-ation of electrodeposited thin film silver chalcogenidecyanide ISEs as FIA detectors for the analysis of naturalwaters, noting that these electrodes possess sufficientsensitivity for the analysis of environmental samples andthat the electrodepositionmethodused in the preparationofthe sensor lends itself to in-situ electrode regeneration; animportant attribute when these electrodes are used in long-term monitoring in field instrumentation providing apossible source of electrode fouling during continuouselectrode exposure to complex media such as seawater. Inessence, the novel approach of Neshkova and co-workers[91, 92] is a promising method that is expected to gainwidespread use in future environmental research.A very interesting recent paper by Metilda et al. [93] on

the use of an ion imprinted polymer ISE documents a noveluranyl ISE incorporating a uranyl-organic complex trappedwithin a polymer framework that is utilized in a standardPVCmembrane electrode. In this work, the authors showedthat this new ISEhas a detection limit of 2� 10�8M,which issuitable for the analysis of uranyl ions in seawater, and acomparative analysis of seawater gave uranyl ion concen-trations that are as accurate as those determined usingneutron activation analysis (NAA). Moreover, this is anexcellent example of timely and important ISE research, asthe world is striving for energy solutions to air pollution andgreenhouse gas emission problems through an intensifiedutilization of nuclear energy, and the globalmining activitiesfor uranium-based compounds are poised to increase.There is a large list of papers describing the use of ISE

devices in the analysis of cations and anions in a variety ofenvironmental samples. Herewith is a list of ISE applica-tions on miscellaneous analytes:

– ISEs have been used in themonitoring of cyanide, nitrite,fluoride and ammonia in the Houston Ship channel [94]and the Wadi El Raiyan Lakes in the Egyptian desert[95];

– an ISFETdevicewas developed for on-linemonitoring ofpotassium and ammonium pollution in environmentalsamples [96, 123];

– a cobalt-wire ISE has been used in the analysis ofphosphate in waste waters [97];

– a combined nitrate/pH titrimetric system was developedfor the analysis of anoxic activated sludge [124];

– a commercial ammonia ISE method was developed forthe analysis of seawater [125];

– a solid-state polyprrole ammonium ISEwas employed inFIA for the analysis of a variety of natural waters [126] ;

– ISEs have been adapted to continuous on-line monitor-ing of nitrate and ammonium in waste water, sewerageeffluent and Danube River water [127];

– ISEs have been utilized in the monitoring of chloride innatural waters [128 – 130];

– calcium, magnesium and water hardness in naturalwaters have been assayed using ISEs [99, 100, 131, 132];

– a tetraphenylborate ISE has been used in back titrationsof sulfate in seawater in the presence of excess 2-aminoperimindium ion [101];

– ISEs have been developed to monitor chloride, nitrateand sulfate arising from aerial deposition onto alumi-nium and zinc metallic substrates as a means of studyingthe corrosion impact of polluted urban air exposure onelectronic equipment [102];

– a robust solid-state chloride ISE was employed in themonitoring of chloride in concrete as a tool for monitor-ing the corrosion tendency of steel in reinforced concretestructures [103];

– nonionic surfactants that are emerging environmentalpollutants have been studied in rivers using ISE FIA[133];

– copper, ammonium and nitrate ISEs were used toanalyze reservoir water [134];

– a potassium ISE was used in the monitoring of irrigationwater [135] and soils [136].

The aforementioned bulleted list of applications in environ-mental science leaves little to the imagination about the vastarray of opportunities that exist for ISEs in environmentalmonitoring of pollutants, nutrients, harmful and corrosivereagents, emerging pollutants, etc.Notwithstanding, there is a series of recent papers that

highlight the many innovative approaches that are possiblewith ISEs in environmental science. First, Kounaves [137]described the development of an ISE electrode array thatmay be used with a robotic chemical laboratory for theanalysis of calcium, potassium, sodium and chloride as ameans of detecting bacterial growth on Mars. Next, Dzya-devych et al. [138] described an enzyme sensitized ISFETpH sensor system for the detection of acetylcholinesteraseas an early warning system for detecting organophosphoruspesticides in the environment, and the results showed thatthis new sensor is capable of providing accurate analyticaldata that follows the biotoxicity response of a luminescentbacterium. Last, Lei et al.?s [139] excellent review onmicrobial biosensors described an ISE approach in whichpH, NH4

þ, Cl�, etc. or gas sensors (pCO2 and pNH3) havebeen sensitizedby amicrobial biofilm so theymaybeused totarget environmentally important analytes such as organo-phosphate fertilizers, penicillin, tryptophan, urea, trichloro-ethylene, etc.




6. Future Directions and Recommendations

Like all electrochemical sensors, ISEs are ideally suited touse in “electronic tongues” in which an array of carefullyselected ISEs with appropriate cross-sensitivities may beused to establish a sensor response surface that can beinterrogated using standard pattern recognition techniquessuch as artificial neural networks, principal componentanalysis, partial least squares regression etc.Krantz-RQlckeret al. [140] presented an excellent review of the workundertakenusing chalcogenide ISEarrays for the analysis ofdrinking waters, and this approach is clearly transferable tothe analysis of natural waters in the environment.Portable battery-operated microprocessor controlled

electrochemical instrumentation has been available formany years [141], and this approach is clearly amenable toenvironmental analysis in the field, either in in-situ usingsubmersible CTD instrumentation (viz., Camusso et al.?sISE study of copper in Lake Orta [26]) or in on-sitemonitoring programs.Without doubt, a critical issuewith the adoption of ISEs in

environmental analysis is a matching of their analyticalsensitivity with the immense challenges of trace analytedetection and the potential of electrode fouling in complexnatural samples containing a plethora of potential interfer-ing species. Previous research has illustrated the desirableeffects of hydrodynamic flow in RDE, FIA and CFA ISEanalytical devices onboth the detection limit and adsorptionphenomena in complex environmental samples [12, 29, 33,142]. Accordingly, it is strongly recommended that environ-mental analyses with ISEs are undertaken in a flow analysismode using techniques such as FIA or CFA.Nowadays, it is well-known that polymeric membrane

ISEs have excellent analytical attributes, as compared tomany analytical techniques, and their superior selectivityand low limits of detection make them ideally suited toenvironmental analysis [143]. Unfortunately, conventionalpolymeric ISEs do not possess the physical and chemicalrobustness of their crystalline membrane ISE counterparts,and this becomes problematic during environmental anal-ysis, particularly in field studies. Nevertheless, it is presentlypossible to make physically and chemically robust solid-contact ISEs on metallic substrates using a toughenedmethyl methacrylate/decyl methacrylate copolymer mem-brane, and this approach has been usedwith a range of solid-contact polymeric ISEs (e.g., silver, lead, calcium, potassiumand iodide) providing nanomolar response with goodresponse times and excellent reproducibility [144]. Theauthors? strongly recommend this class of polymeric ISE foruse in the analysis of trace analytes in the environment.An exciting analytical approach with polymeric ISEs in

environmental analysis is the concept of switchtrodes [145].With this method, two polymer membrane ISEs areprogrammed to give kinetically controlled detection limitsso as to produce a peak-shaped differential signal betweenthe electrode pairwhen the activity of the sample is resonantwith the intermediate activity needed for switchtroderesponse.As the ISEs are in a characteristic super-Nernstian

response domain under these conditions, a switchtrodeexpresses a chemical amplification, which is expected to beuseful in the environmental analysis of trace analytes.Belli and Zirino established that the standard addition

analysis of metals in natural waters comprising ubiquitousorganic ligands is problematic, as a variation in the complex-ing ability of the sample changes at different levels of addedanalyte, and this gives rise to an anomalous super-Nernstianresponse in environmentalmonitoring [19].As themajority ofISEs are prone to matrix interference effects (e.g., chlorideadsorption, ligand adsorption, aging phenomena, etc.), aninability to conduct standard addition calibrations makes itextremely difficult to undertake reliable analyses in environ-mental samples. Notwithstanding,Malon et al. [146] reportedanexciting approachwithpolymericmembrane ISEs inwhichthe ISE response may be interrogated through a variation inthe composition of the filling solution, and it is possible to usethis approach in a so-called backside calibration of the ISEwhile it bathes in an unperturbed sample. Notably, theseauthors utilized the new backside calibration methodology inthe analysis of lead(II) in Budapest, Warsaw and ZQrich tapwaters, as well as ZQrich Sihl River water, and the analyticaldata demonstrated that the newmethod is at least as accurateas standard addition potentiometry. Clearly, this method isexpected to provide the environmental chemistry communitywitha robustand rugged ISEmethod for theanalysis ofmetalsin natural waters.The holy grail for environmental analysis would entail the

long-term deployment of ISEs and their associated instru-mentation at field sites, and real-time and remote monitor-ing of their response characteristics using telemetry andcomputer technology (see Figure 2 and reference [147]).Unfortunately, this approach is fraught with danger as theISE devices are likely to become fouled by biological films,and the sample matrix itself is likely to foul the sensorsurface rendering its response unrepresentative of thecalibration data set unless steps are taken to regularly cleanand recalibrate the electrodes. Nevertheless, it is possiblethat ISEs for toxic trace metals such as copper, cadmium,mercury, lead, etc. will be resistant to biofouling events sincethe membrane dissolution-based release of the analyte willdisplay an antifouling effect, and the desirable influence ofhydrodynamic flow in RDE, FIA and CFA devices mayeliminate this chemical-based fouling of the membrane. Inany event, this approach has its merits, and it deserves theattention of environmental scientists interested in real-timeremote monitoring of the water quality of natural samples.Most importantly, the long-term deployment of an ISE in

a complex environmental sample such as seawater isexpected to alter its response characteristics through anadulteration of themembrane surface, thereby changing theISE response characteristics against calibration standards,as well as degrading its stability and reproducibility in thesample. As a worst-case scenario, the ISE surface maybecome poisoned and/or passivated during long-term ex-posure to the sample matrix (i.e., chloride, hydroxide,organic ligands, etc.), thereby yielding a totally non-func-tional ISE. Accordingly, these factors will conspire during




long-term deployment of ISEs in the field to provide highlyunreliable ISE analytical data. Under these conditions, it isessential to regularly recalibrate and rejuvenate the electro-des, as required, and this high level of maintenanceprecludes the use of ISEs in remote sensing applications,especially involving the long-term deployment of ISEsensors. Notwithstanding, short-term in-situ field studiesusing properly conditioned and recently calibrated ISEs arerealizable, as evidenced by the investigations of Camussoet al. [26] on the stratification of copper(II) in Lake OrtaItaly using a copper(II) ISE fitted to a CTD device.It is the authors? honest opinion that fieldmonitoringwith

ISEs will be achievable if environmental scientists under-take on-site or shipboard determinations of analytes, asillustrated in the previous and seminal papers by Zirino andco-workers [4, 33], using an FIA or CFA method that isknown to ameliorate the problems of electrode drift,electrode fouling, electrode passivation, electrode dissolu-tion and electrode carry-over arising from the cleansingeffect of hydrodynamic flow on the surface of the ISE [11].Accordingly, it is strongly recommended that field researchin environmental science is undertaken using this conserva-tive and realizable strategy.

7. Acknowledgements

The Australian Research Council (ARC) and AustralianInstitute of Nuclear Science and Engineering (AINSE) areacknowledged for financial support.

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