UNIVERSIDAD LmRE FACULTAD DE INGENIEIRA … · cipitated at pH 8.5 as strontium oxalate by slowly adding a saturated solution of ammonium oxalate and letting the precipitate ripen

UNIVERSIDAD LmREFACULTAD DE INGENIEIRA

DEPARTAMENTO DE CIENCIAS BASICAS

CLASSICAL CHEMICAL ANALYSIS

Majors, R., Sample preparation in analyticaL Chemical Analysis Group, Hewlett - Packard. Handbook of instrumentalTechniques for Analytical Chemistry. Frank Settle Editor.

C!assica!analysisis the collective term used to describe an analysis whose measurement step is agravimetric or titrimetric determination. Instrumenta! analysisis the collective term used for ananalysis whose measurement step involves an instrumento Although the use of instrumental analy-sis grew slowly until the 194Os,most analyses before that time were done classically. Since the1940s, instrumental analysis has become more important because of the increasing demands forsensitivity, speed, and economy, so that today instrumental analysis has assumed the major role inthe analytical laboratory.

Gravimetry and titrimetry are based solely on atomic weights (relative atomic masses), experi-mentally measured masses, and atomic-weight ratios obtained from well-characterized chemicalreactions. (When titrations are done on a volume basis instead of a mass basis, the volume must berelated to mass through density.) The dassical methods of gravimetry and titrimetry, along withcoulometry and isotope dilution mass spectrometry (IDMS), are called definitive methods becausethey have exactly defined errors and under the proper conditions need no extemal calibrants. Theresults can be said to include the true value within the statistical uncertainty of the method. Cou-lometry depends upon Faraday's laws relating the moles of a substance reacted to the fundamentalunits of current and time. In IDMS the analytical result is based only on isotope ratio measure-ments. In contrast, a]Jother instrumental methods, including IDMS, reIy on responses that possessno absolute quantitative meaning until they are calibrated with known standards.

In both classical and instrumental analysis, the final determination is almost always precededby one or more preparatory steps. The precision and accuracy of the analytical result are no betterthan the co11ectiveprecision and accuracy of a11the preparatory steps. Most of the preparatorysteps in an instrumental analysis have been adopted directly from classical analysis. Most analyti-cal chemists working today received their education in the instrumental age. Many of the tech-niques used in the preparatory steps in a dassical analysis are not as significantly emphasized as inthe preinstrumental era. Therefore, a brief look at some components of dassical analysis will beuseful.

Gravimetry is the determination of an element or species through the measurement of themass of the reIatively insoluble product of a well-characterized chemical reaction involving thatelement or species. The insoluble product may be a gas evolved from solution, or it may be a non-volatile solid residue from a low-temperature volatilization or a high-temperature ignition. Usu-ally, the insoluble product is a precipita te formed in an aqueous solution.

One or more preparatory steps are almost always necessary before doing a gravimetric de-termination. The element or species being sought (the analyte) must be determined on a homo-geneous specimen that represents the entire sample, be it a solid, liquid, or gas. If the specimen isnot already a solution, it must be put into solution by a dissolution procedure. These indudeopen-beaker acid dissolution, dry ashing, fusion, microwave-oven digestion, and high-tempera-ture, high-pressure digestions (bomb- and Carius-tube techniques). Appropriate precautionsmust be taken to avoid mechanical and volatilization losses. Next, interferences must be elimi-nated through a separation, a series of separations, or masking (complex formation). For gravim-etry, an interference is any element or species that would coprecipitate with, or be ocduded by,the precipitate being sought. These preparatory steps are required before the element or speciesof interest can be determined by precipitating the insoluble element or stoichiometric com-pound.

For high-accuracy work, the systematic errors due to coprecipitation, ocdusion, and thesolubility-product effect must be evaluated. Despite precautions, a1l precipitates contain smalI

Sample Preparation in Analytical Chemistry (Inorganic Analysis) 57

amounts of coprecipitated and occluded substances. These substances can be determined by vari-ous instrumental techniques and appropriate corrections made. Due to the solubility product ef-fect, all filtrates from precipitates contain small amounts of dissolved precipitate, which also canbe determined by instrumental techniques.

An example of a gravimetric determinacion is the assay of a sample for strontium. If all thenecessary preparatory steps have been properly carried out, one has an aqueous solution that con-tains all the strontium in the original sample and is free from all interferences. Strontium is pre-cipitated at pH 8.5 as strontium oxalate by slowly adding a saturated solution of ammoniumoxalate and letting the precipitate ripen (age) overnight. The strontium oxalate is filtered and thefiltrate reserved for instrumental determination of unprecipitated strontium. The precipitate isignited in a quartz crucible at 1100 °C to convert it to strontium oxide. The tared crucible is re-weighed and the mass of strontium oxide found by difference. After the precipitate is checked forimpurities by instrumental means, the amount of strontium in the sample is calculated by sum-ming the strontium in the precipitate and in the filtrate and correcting for any low impurityamounts in the strontium oxide.

The preparation of the sample for the final gravimetric determination has already involvedthe common analytical operations of weighing, transferring, dissolving, and making one or moreseparations. The gravimetric de!ermination itself involves the use of several additional commonanalytical operations. The precipitation technique must minimize coprecipitation and occlusion.In some instances, coprecipitation and occlusion are minimized when the precipitating agent isadded slowly to the analyte. In other cases, adding the analyte slowly to the precipitating agentgives the best results. The next step is filtration, which involves the transfer ofboth the precipitateand the supernatent liquid to the appropriate type of filter paper, followed by the proper washingof the precipita te. Finally, accurate results depend heavily on the use of proper techniques in thedrying and ignition of the precipitate. As with the initial weighing of the sample, the inclusion of abuoyancy correction is essential for the accurate weighing of the precipitate. In the example justgiven, instrumental techniques have been used to provide corrections for errors that were mini-mized but not corrected for in the pastoGravimetric procedures that are fully optimized by instru-mental procedures can have gready improved accuracy and precision and justify the use of theterm definitive method because all known errors have been accounted for.

Titrimetry is the determination of an element or species through the measurement of themass of a chemical necessary to complete a definite chemical reaction in a solution containing thatelement or species. The mass of the chemical is usually obtained indirecdy by measuring the vol-ume of a standard solution (solution of known concentration) of that chemical, but for very accu-rate work a mass titration should be used. Titrimetric methods are based on acid-base reactions,precipitation reactions, complexation reactions, and oxidation-reduction reactions. Endpointsgenerally are determined by observing a color change in an indicator. Endpoints also are com-monly determined by observations of potential differences at electrodes, changes in conductivity,changes in optical absorbance, and many other physical or chemical properties. A titrimetric de-termination is usually preceded by one or more preparatory steps, many of which are the same asor similar to those used in gravimetry. Unlike in gravimetry, coprecipitation and occlusion inter-ferences do not exist except in titrimetric methods that are based on precipitation reactions. How-ever, interferences that cause competing side reactions must be eliminated or accounted for in alltitrimetric methods. In addition, any systematic error resulting from the indicator's deviationfrom marking the true equivalence point must be evaluated and a correction made.

An example of a titrimetric determination is the assay of a sample for vanadium. With instru-mental methods, it is now possible to know exacdy what interfering elements are present in thesample and to design the appropriate procedure to isolate the analyte. As with gravimetry, whenall the necessary preparatory steps have been performed, one should have an aqueous solution

58 Handbook of Instrumental Techniques for Analytical Chemistry

containing a11the vanadium in the sample and no interfering substances. The vanadium is oxi-dized toV'+ with excess potassium permanganate, which is subsequently destroyed by the additionof excess nitrite. The excess nitrite is destroyed with excess urea, which does not interfere with theredox titration for vanadium. The vanadium is titrated to yI-+with standardized ferrous ammoni-um sulfate solution using diphenylamine sulfonate indicator.

Besides the common analytical operations already used in sample preparation, the titrimetricassay also involves the calibration and proper use of volumetric flasks, pipets, and burets, togetherwith any necessary temperature correction. Volumetric flasks, pipets, and burets are calibrated byweighing how much water they contain or deliver and making buoyancy corrections. Gravimetrictitrations eliminate most of these requirements and have the advantage that masses are easier todetermine accurately than volume. The density of a solution changes with temperature, so all vol-umetric procedures must be temperature corrected. The mass of solution does not change withtemperature, so gravimetric titrations do not need to be corrected for temperature.

Finally, one must also understand the reaction mechanism of the indicator used to be able toevaluate the indicator error. In the example given, the diphenylamine sulfonate indicator is itself aredox system that consumes a tiny amount of the V+ analyte, which requires a correction in thefinal calculations. There are other ways to determine the.endpoint of a titration, including poten-tiometric, thermometric, amperometric, spectrophotometric, and conductimetric techniques thatcan eliminate the indicator or improve the ability of the method to determine the endpoint accu-rately.

This brief overview shows that the gravimetric or titrimetric determination is the last in a se-ries of steps that make up a classical analysis. Because most of the preparatory steps are identicalfor instrumental techniques, an accurate and precise instrumental analysis depends on someone inthe analytical chain having training and experience in classical analysis. This type of training is de-emphasized in modern undergraduate analytical chemistry. This summary is not a substitute forthat training, but we try to point the way for self-study through references to well-known texts aswe outline general sample preparation steps.