Edwin H. Klehr
schoolof
Civil Engineering and Environmental Science University of Oklahoma Norman, 73069
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A Radiometric Method for S d f a b Analysis
Every chemist in charge of teaching analytical chemistry a t the undergraduate level is faced with the obvious problem of selecting experiments which serve several educational purposes simultaneously. The experiment should afford an opportunity to consider important basic concepts of analytical chemistry; it should he indicative of newer developments; it should be somewhat open-ended; and it should require careful attention to good technique. Rurther, an experiment should allow the student to consider basic problems in experimental design. This last feature is usually absent from most experiments in standard analytical textbooks. The use of radiometric methods, as distinct from the use of radiation sources such as activation analysis, scems to offer a convenient opportunity to present to the student experiments which meet the above criteria. An appealing phase of these methods involves the incorporation of radioactive tracers into classical methods in order to achieve greater sensitivity, reduce analysis time, minimize interferences, etc. As an example, it should be possible to modify a classical gravimetric procedure by incorporating a radiotraccr so that the final precipitate would be counted instead of weighed. Furthermore, if a controlled excess of labelcd reagent were used, it should be possible to carry out the desired determination by making a count rate measurement on thc supernate rather than on the precipitate after filtration. To explore these concepts, a simple radiometric method for sulfate analysis was developed and tested with good results. Principles
Thc classical gravimetric procedure involves the addition of a barium chloride solution to the acidified sample, digestion of the barium sulfate precipitate, filtration and weighing of the precipitate. I n the radiometric modification, '33Bais used to label the barium chloride, and the precipitate is counted with a simple scintillation or Geiger-Muller System. The amount of barium sulfate is thus determined by the measurement of an amount of radioactivity rather than a weighing. The counting rate of the precipitate is proportional to the sulfate concentration, provided that thc procedure can be done reproducibly. Since counting v i t h a scintillation or G-11 system yields only a relative measurement (i.e., the counting rate is only proportional to the amount of 133Ba,and hence to the amount of barium in the counting sample) the radiometric method needs to be standardized. It is interesting to consider an alternative procedure. If a controlled excess of labeled barium chloride solution 644
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Journal of Chemical Education
is added to the sample and the barium sulfate precipitate filtered out, the counting rate of the supernate should be related to the original sulfate concentration of the sample. This follows since the counting rate of the supernate would he proportional to the amount of barium remaining in solution, which in turn is inversely related to the sulfate concentration. This method would have at least one advantage in that liquid samples of gamma emitters can generally be counted in well scintillation systems with greater reproducibility than can solid samples with an end-window G-M or scintillation system. Obviously, the method needs to be standardized, and the amount of barium chloride solution added to the sample is now critical. The use of radioactive material, in the amounts and concentrations required for this experiment, pose no more hazardous conditionsin the laboratory than the use of sulfuric acid, sodium hydroxide, etc. However, the experiment does provide an opportunity to discuss radiation safety, and to introduce students to some basic procedures and precautions in the handling of radioactive materials. Since only generally licensed quantities of radioisotopes are needed, waste disposal is not a serious problem, but here again use should be made of the opportunity to discuss the very serious topic of radioactive waste disposal. No special apparatus is needed other than a nuclear detection system. The procedures developed can be performed u~itha simple well scintillation assembly set up for gross gamma counting and a flat scintillation assembly or an end-window G-M system. Such basic equipment is apparently available on most campuses. I t should be pointed out that the concepts of this experiment are simple enough so that with a minimum of basic instruction or background reading, the students could actually consider the design of the experiment for themselves and decide what questions they need to answer, what points of the procedure are likely to he critical, etc. This assumes, of course, that they already have a fair understanding of the principlesof gravimetric analysis. Procedure 1) Place 25 ml of sample (unknown or standard) into a 50-ml beaker. 2 ) Add 2 drops of 1: 1 HCI. 3) Heat ta just below bailing. 4) Add 10 ml labeled BaCI. solution (Ba*CIa )with stirring. 5) Digest precipitate at 70-75% for 30 min. 6 ) Cool to room temperature. 7) Filter with 0.45 p membrane filter paper, using suction. Catch filtrate in a 50-ml centrifuge tube. Do not wash precipitate at this stage. 8 ) Remove centrifuge tube from filtration assembly and adjust final volume of supernate to 35 ml.
9 ) Count 5 ml aliquot in well scintillation system. 10) Wash Ba*SO, precipitate on filter paper with 3 4 small volumes of beneene and air dry precipitate. 11) Mount and count the precipitate with either a flat NaI scintillation system or an end-window G-M system. 12) From the appropriate calibration curves, determine the sulfate concentration from the counting rate. Note The unevenness of precipitate deposition will effect the geometry of counting. This effect can be minimized by counting each sample 4 times, with 90' rotation of the sample. An average of the 4 counting rates should be calculated and used as the counting rate of the silmple. This is especially important for lorn sulfate samples where the amount of precipitate will be small. Discussion o f Results
Some of the interferences in the gravimetric method which would cause a high counting rate of the supernate (high results) u-ould cause a low counting rate of the precipitate (low results). Hence, perhaps an average of the two determinations would give the best value for the sulfate concentration of a sample. Since the procedure stipulates that a constant amount of Ba*CI2solution is added to all samples (or standards) the sum (and average) of the counting rates of the supernate and precipitate should be a constant for a given Ba*C12 solution. However, since self-absorption, selfscattering, etc., of a radioactive sample is always a factor, the sum will not be a precise constant but show
Stmdordirotion curves: 0 ,rtondordirotion curve for supernate counting; $tmdwdizatim cune for precipitate counting.
A,
solution can be used to give a set of standardized samples in the range of 0-500 mg/l. Typical Results
Reagents a n d Solutions
