Analysis of common ions at low concentrations in water - NPTel

Environmental Chemistry and Analysis Prof. M.S.Subramanian

Indian Institute of Technology Madras

MODULE 2.2

Analysis of common ions at low concentrations in water Ultraviolet And Visible Spectrometry 1 Spectrophotometric instrumentation 2 Analysis by direct absorption 3 Nitrate determination 3

Analysis after formation of derivative 4 Chloride: Automated method using

mercuric thiocyanate and ferric nitrate 4

Fluoride: Zr-Alizarin lake method 5

Nitrite 5

Phosphate 5

Automatic procedures 5

Field Techniques 6

Flame photometry 6

Ion Chromatography 7

Examples Of The Use Of Other Techniques 11 Ammonia 12

Fluoride 13

Sulphate 13

Free chlorine (Residual chlorine) 14

Sulphide 14

Sample Collection and preservation 15

Spectrophotometric determination of H2S 15 Titrimetric method 15 Cyanide 16



MODULE 2.2

Analysis of common ions at low concentrations in water:

In this chapter the application of instrumental techniques to determine the

concentration of ions which are present in mg l -1 concentration range are

discussed . One of the major advantages of the use of instrumental techniques is

that elaborate sample preparation is not necessary. However when it comes to

the analysis of ions at µg l-1 levels, a preconcentration step is needed inorder to

bring the concentration of the analyte ions within the working range of the

instruments. In that case the instrumental part becomes just one part of a more

complex analytical procedure. The analysis of ions present at µg l-1 concentration

level are discussed in the next chapter.

Ultraviolet And Visible Spectrometry:

This technique is based on Beer-Lambert law. That is at sufficiently low

concentrations of the absorbing species the above law is obeyed which can be

expressed mathematically as

A = clε

Where A = absorbance of radiation at a particular wavelength;

o

t

IA logI

⎛ ⎞=⎜ ⎟

⎝ ⎠

= Intensity of incident radiation;

= Intensity of transmitted radiation;

= Molar absorptivity; l mol

oI

tI

ε -1 cm-1

1



C = Concentration of absorbing species (mol l -1);

l = Path length of light beam (cm)

There are many instruments which are used to measure the absorption of

light. These can range from sophisticated laboratory instruments which can

operate over the whole visible-ultraviolet range to portable colorimeters using

natural visible light, which are used as field instruments.

Natural water is colourless since none of the common ions present in

water absorb light in the visible region. The only ions which absorb in the

ultraviolet range above 200nm are nitrate and nitrite.

Since nitrate ions absorb at 220nm, it permits the concentrations of these

ions in water to be determined using spectrophotometer by direct absorption.

The concentration of ions such as chloride, fluoride and phosphate which

do not absorb in the visible region, can however be determined after converting

them into a suitable coloured species by chemical reactions. Nitrate and nitrite

also are converted into suitable coloured products, the absorbances of which can

be measured.

Spectrophotometric instrumentation:

The out line of a simple single-beam spectrophotometer is shown in fig.1.

Source Monochro mator

Sample Detector& read out

fig.1 block diagram of a single beam spectrophotomer

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For absorption in the visible region, a tungsten lamp is used as a source and

that for absorption in uv region hydrogen lamp is used. The light of the desired

wavelength is isolated using diffraction monochromator. Normally for all

absorption measurements 1 cm cells are used and for all uv absorption

measurements cells made of quartz are used. Different types of detectors such

as photocell, photomultiplier or photodiode array detectors are used in the

absorption measurements.

Analysis by direct absorption:

Nitrate determination:

This method is useful only for screening water samples that have low

organic matter contents, i.e., uncontaminated natural waters and potable water

supplies.The calibration curve follows Beer's law upto 11

ppm. Measurement of uv absorption at 220nm enables rapid determination of

nitrate. Because dissolved organic matter also may absorb at 220 nm and

does not absorb at 275 nm, a second measurement made at 275 nm, may

be used to correct the value. The extent of this empirical correction is

related to the nature and concentration of organic matter and may vary from one

water to another. This method is therefore not recommended if a significant

correction for organic matter absorbance is required, although it may be useful in

monitoring levels within a water body with a constant type of organic

matter. Sample filtration is intended to remove possible interference from

suspended particles. Acidification with 1N HCl is designed to prevent interference

from hydroxide or carbonate concentration upto 1000 ppm as CaCO

-3NO

-3NO

-3NO

-3NO

3. Chloride

does not interfere in the determination. However organic dissolved matter,

surfactant, , and Cr(VI) do interfere. -2NO

3



Analysis after formation of derivative:

Chloride: Automated method using mercuric thiocyanate and ferric nitrate:

Thiocyanate ion is liberated from mercuric thiocyanate solution by the

formation of soluble mercuric chloride. In the presence of ferric ion, free

thiocyanate ion, forms a highly coloured ferric thiocyanate complex

(λmax=470 nm) the absorbance of which is proportional to the chloride

concentration.

The interference due to particulate matter can be overcome by filtration or

centrifugation before analysis.

The method is applicable to potable, surface, and saline waters, and

domestic and industrial waste waters. The concentration range of chloride that

can be measured is 1 to 200 ppm.

Fluoride: Zr-Alizarin lake method:

This SPANDS Colorimetric method is based on the reaction

between fluoride and a coloured zirconium-alizarine lake. Fluoride reacts with the

Zirconium alizarin lake, dissociating a portion of it into a colourless complex

anion (ZrF62-). As the amount of fluoride increases, the colour of zirconium dye

lake becomes progressively lighter.

The reaction between fluoride and zirconium ions is influenced

greatly by the acidity of the reaction mixture. If the proportion of acid in the

reagent is increased, the reaction can be made almost instantaneous. The

absorbance measurements can be done at 570nm and concentrations can be

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determined with calibration graph. Both the standards and samples should be

kept for one hour before absorption measurements.

Nitrite:

Nitrite is determined through formation of a purple azo dye produced

at pH 2.0 to 2.5 by coupling diazotised sulfanilamide with

N-(1-naphthyl)-ethylenediaminedihydrochloride. The absorbance of the resulting

purple azo dye can be measured at 543 nm using a spectrophotometer. Beer's

law is obeyed upto 25 µg

-2NO

-1. Higher Concentration of can be determined by

diluting the sample. Free chorine and the following ions which form precipitates

under test conditions such as Sb

-2NO

3+ ,Au3+, Bi3+, Fe3+, Pb2+, Hg2+, Ag+ interfere .

Cupric ion may cause low results by catalysing decomposition of the diazonium

salt.

Phosphate: The procedure for phosphate involves the addition of a mixed

reagent (sulphuric acid, ammonium molybdate,ascorbic acid, antimony

potassium tartrate) to a known volume of sample, diluting to volume, shaking and

leaving for 10 min. A blue phosphomolybdenum complex is produced and the

absorbance is measured at 880 nm.

The concentration is calculated using a predetermined calibration graph

derived from standard solution treated in the same way.

Automatic procedures: have been developed for most of the ions listed in

the previous paragraph.

Instead of mixing reagents for each analysis, streams of each reagent

(segmented by air bubbles to diminish mixing effects) in narrow-bore tubes are

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mixed by combining the flows at a T-junction or within a mixing cell. A sample is

introduced from an automatic sampler as a continuous flow into the reaction

stream. The combined flow is then led into a spectrophotometer and the

absorption measured. The flows of all the reagents and samples are produced

from a multi-channel peristaltic pump.

Field Techniques:

Field techniques are very important and they give immediate

measurement of ion concentration. Using the automated procedures described

above unmanned field stations can be set up. Otherwise portable instruments

can be used to perform the analysis at the site.

Flame photometry:

This technique is used in water analysis for determining the concentration

of alkali and alkali metal ions such as sodium, potassium and calcium. The

following diagram (fig 2) shows the basic components of a flame photometer.

Monochromator Detector& read out

Nebuliser & burner

samplesolutionin

flame

lens

Fig2. Outline of a flame photometer

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The liquid sample to be analysed is sprayed under controlled conditions

into the flame where the water evaporates, leaving behind the inorganic salts as

minute particles. These salts decompose into constituent atoms or radicals and

may become vapourised. The vapours containing the metal atoms are excited by

thermal energy of the flame and this causes electrons of the metallic atoms to be

raised to higher energy levels. When these excited electrons fall back to their

original positions, they give off discrete amounts of radiant energy. The emitted

radiation is passed through the monochromater where the desired region

isolated. A photocell and an amplifier are then used to measure the intensity of

isolated radiation. Normally for alkali metals a propane-compressed air mixture is

used as a fuel.

A linear concentration range (for sodium and potassium 1 to 10 mg l-1 and

for calcium 10-50 mg l-1) is within the range expected for environmental water

samples.The method is simple and sample preparation is not needed. However

care has to be taken that the calibration of the instrument and analytical

measurements are performed quickly after each other.

Ion Chromatography:

The major application of this instrument is for inorganic anions in

environmental analysis. This is a kind of ion-exchange chromatography used for

the separation of inorganic and some organic cations and anions using a

conductivity detector after suppressor column. The schematic diagram of an ion

chromatograph is shown in fig 3a.

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Na2SO4

H2SO4

H2SO4

bath solution2Na+

CO32-

Eluate containingNa+,CO32-,HCO3-,+analyte anions

SO42-

2H+

H2CO3

Sulfonated polyethylene hollow fiber

Eluate containingCO2,H2O,H+ +analyte anions

Fig 3b Micro membrane suppression

Eluentreservoir

Pump

Sample injection

Separator column

micro membrane suppressor column

conductivitydetector

Fig 3a Schematic diagram of

ion chromatograph

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The stationary phase is a pellicular material, the particles consisting of an

impervious central core surrounded by a thin outer layer (about 2µm thick)

incorporating cation-or anion-exchange sites. The thin layer contributes to faster

rates of exchange that results in higher efficiencies. For the separation of anions,

mobile phases containing solutions of electrolytes such as Na2CO3 or NaHCO3

are used. For the separation of cations, HCl solution is used as mobile

phase.The detection of low concentration of ionic solutes in the presence of high

concentrations of eluting electrolyte is not possible.

Unless the latter is removed. This is achieved by using a micro membrane

suppressor column (fig 3b) immediately after the separator column, which

converts the electrolyte into unionised water, leaving the solute ions as the only

ionic species in the mobile phase.

The reactions for the separation of inorganic anions on an-ion-exchange

column in the form using sodium hydrogen carbonate as mobile phase

are summarised in the following equation.

-3HCO

Separator Column:

n n3 3 3 n

n 2 33 4 4

n (Res N R HCO ) x (Res N R ) x nHCO

where x F ,Cl ,NO ,SO ,PO etc.

+ − − + −

− − − − − −

+ → +

=

i i 3−

.............................(1)

Suppressor reactions:

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The most wide spread method of achieving low back ground conductivity is

by the use of "micromembrane suppressor" as illustrated in fig.3b. The effluent

flows between two semi-permeable membranes which separate it from the

counterflow of sulphuric acid. The migration of each ion is determined by the

relative concentrations of the ions in the two liquids, the ion moving into the

solution of lower concentration.

For the analysis of anions in environmental water samples which are found at mg

l-1 concentrations by this technique, the sample has to diluted before injection.

This along with the filtration of the water sample is often the only sample

preparation necessary for carrying out the analysis. It takes only a few minutes

for the analysis of common anions that are present in water as shown in fig.4.

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0 2

NO2- 1.2 ppm

NO3- 1.4 ppmSO42- 1.1 ppm

Cl-

Br- 1.4 ppm

4 6 8 12 1410

1.0 ppm

Time (Min)

Fig 4 Separation of ions in environmental water

Although ion chromatography is quite commonly used for the analysis of

anions in water, some common cations (Na+, K+, NH4+, Ca2+, Mg2+) also are

being analysed using both the ion suppression system and also conventional

chromatographs with conductivity detection.

Examples Of The Use Of Other Techniques:

The most widely used methods covered so far are only for the analysis

common ions. There are however, a few frequently used methods which have

not been covered. For example the analysis of species such as ammonia,

fluoride and sulphate, free chlorine, sulphide and cyanide which have not been

discussed previously are described here.

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Ammonia:

In environmental water ammonia is the only alkaline gas commonly

found. The ammonia could be determined by a simple acid-base titration

provided it is isolated from solution. This may be done by addition of magnesium

oxide to ensure that the sample is slightly alkaline. Ammonia is then distilled off

into known excess acid. By tiltrating the excess acid with standard alkali, the

ammonia concentration can be determined.

Ion selective electrodes (i.e. an electrode whose potential measured with

respect to a reference) respond in a linear manner to the logarithm of the activity

of analyte over a four to six order of magnitude range of activity. Electrodes do

not consume unknown samples, and they introduce negligible contamination.

Response time is seconds or minutes. Since the electrodes respond only to the

activity of the uncomplexed ions, the ligands must be absent or masked.

In order to know the concentrations, an inert salt is often used to bring all

the standards and samples to a high constant ionic strength. If the activity

coefficients remain constant, the electrode potential gives concentrations directly.

Commercial electrodes are available for the detection of anions (e.g. for halides,

NO3-, CN- , SCN-, S2-); cations (e.g. for H+,Na+,K+,Ca2+,Cd2+,Cu2+,Pb2+) and

gases (e.g. NH3,O2,CO2,NO2).

In the ammonia selective electrode which is a gas sensing type, the

ammonia diffuses through a permeable membrane and causes a pH change in a

small volume of internal solution which is sensed by a glass electrode. Before

taking the measurement, concentrated sodium hydroxide solution is added to

samples and standards which serves to increase the pH to above 11 so that all

ammonia is in the unprotonated form and provides a constant ionic strength.

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Fluoride:

Fluoride ion can be estimated, based on potentiometric measurements

with a membrane electrode consisting of a single crystal of europium doped

lanthanum fluoride, LaF3. The purpose of Eu doping is to improve electrical

conductivity. The membrane is cut as a 1-mm thick disc, a few mm in diameter,

the disc is sealed into the end of a rigid plastic tube, filled with an equimolar

solution of KCl and NaF, into which dips an AgCl electrode. A reference

electrode (saturated calomel electrode) is inserted into the test solution along

with the fluoride electrode. The potential difference is measured. Fluoride

concentration down to 10-6 M can be measured. This electrode shows extremely

high specificity to the analyte ion, the only pretreatment necessary being the

addition of buffer solution to maintain constant pH and ionic strength.

Alternatively fluoride can be determined by spectrophotometry or by ion-

chromatography both of which have been discussed already. Fluoride reacts with

zirconium-alizarin lake to form colourless ZrF62- and the dye. The colur of the dye

lake becomes progressively weak with increase in amount of F-.

To the sample solution is added drop of sodium arsenite solution to

remove residual chlorine if any. Then enough of zirconyl-alizarin reagent is

added.The solution is thoroughly mixed and absorbance measurement were

done at 570 nm after keeping the samples and standards for 1 hr.

Sulphate:

Ion chromatography is the only instrumental method for the direct

determination of sulphate. Sulphate may be precipitated either with Ba2+ or

2-aminoperimidinium salts. The precipitate may be weighed for a direct

determination of the sulphate as a gravimetric method. Other methods using

13



insoluble salt precipitation are indirect, estimating the excess of the cation after

precipitation of the sulphate. Excess barium may be determined by titration with

EDTA or by atomic absorption.Excess 2-aminoperimidinium ions may be

estimated by visible spectrophotometry.

Free chlorine (Residual chlorine):

The effectiveness of chlorination of raw water for public water supply

can be checked by estimating free or residual Cl2 in water samples. Free

available Cl2 consists of Cl2 ,HOCl, and HClO2. Free Cl2 reacts instantly with N,

N-diethyl phenylene diamine (DPD) indicator to form a red colour. This colour is

discharged by the addition of ammonium iron(II) sulphate solution.

The water sample is treated with DPD reagent solution and titrated

with standard ferrous ammonium sulphate solution until it becomes colourless.

The amount of free chlorine in the water sample can be determined from the

volume of ammonium iron (II) sulphate consumed.

Sulphide:

The major effluents containing sulfide in significant quantities are

tannery wastes, sulphide dye liquors, oil refinery wastes, viscose rayon wastes,

septic sewage and ammonical gas liquor.

Two methods namely colorimetric and titrimetric methods are used.

The colorimetric procedure is simple to carryout, rapid, sensitive and applicable

to sulphide concentrations of 0.2 to 20 ppm. Titrimetric procedure, though is

applicable to samples containing more than 1 ppm sulphide, it is generally used

for samples of higher concentration and containing more interferences.

14



Sample Collection and preservation:

Sample should be collected with least aeration as sulfide is

volatilised and oxygen destroys sulphide. Samples have to be preserved by the

addition of zinc acetate solution.the pH of the solution should be adjusted to > 9

by the addition of sodium hydroxide. The sample should contain representative

porportion of suspended solids.

Spectrophotometric determination of H2S :

In this method the absorbing solution used contains CdSO4 and

NaOH. The precipitated cadmium sulphide on acidification releases S2- ions

which interacts with FeCl3 and -diethylphenylene diamine to give a dye ethylene

blue which can be measured at 670 nm. Beer's law is obeyed from 0.1 to 10 ppm

S2-. Gases like SO2, O3, NH3 do not interfere.

Titrimetric method:

This procedure measures total sulphides excepting acid insoluble

metallic sulphides.

To the Sulphide in acetate solution is added a known excess of iodine

solution which is later acidified. The excess iodine is back titrated with standard

thiosulphate solution using starch as indicator. Knowing the amount of the

standardised iodine consumed, and the volume of sample taken, the amount of

sulphide present in the water sample can be calculated.

………………………..(4)

2 2H S I 2H 2I S++ → + +

22 2 3 4 6I S O 2I S O2− −+ → + − …………………….(5)

1 ml 0.05N I2=0.85mg H2S

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Cyanide:

This method is applicable to all types of industrial effluents, domestic

waste waters, polluted waters, raw and treated waters.

The cyanide in the sample is distilled and absorbed in alkali. CN- in the

alkaline distillate on treatment with bromine water is converted to cyanogen

bromide (CNBr) which reacts with p-phenylenediamine reagent to form a red dye.

The absorbance of the dye is measured at 530nm and the concentration of the

cyanide can be read from a calibration graph.

Most of the simple cyanides M(CN)x are readily converted to HCN by

acid distillation. The complex cyanides such as alkali ferri and ferrocyanides are

not converted to HCN during distillation. Acid cuprous chloride and /or

magnesium chloride are added to the sample which converts the complex

cyanide to simple cyanides which are then converted to HCN by distillation.

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Analysis of common ions at low concentrations in water - NPTel

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