Determination of Total Cation Conient by Ion Exchange: Unknowns for Use in QuantitativeAnalysis The determination of total cation content by ion exchange is a common experiment in the quantitative analysis laboratory. A sample of a mixture of ionic compounds is passed through a cation exchange column in the H+ form. Assuming that exchange is complete, one equivalent of hydronium ions is liberated for each equivalent of cations added. The acid is readily determined by titration with standard base. Details of the experimental procedure are described in several
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tort. 1 5
One dit'firulty with this rxprrimrnt has brrn the lark oisuitable solid unknwmu of accurately determined anlt mntent. We haw fwnd thnt thr rhlorid~unknowns qold by Thorn Smith Chemists, lnc" make excellent u n k n w n s fur t h i i r x p 4 m m t . Prior tu the middle 1970's. these u n k n w m werr nwnnrrd bv mixinr ourr KC1 and nure NaCl In auth acssc. the ttml cation content (expressed as wt% NaC) may be obtained &om the-analyzed wt% CI- by simple gravimetrie conversion:
a
Since the mid-l97O1s,a small fraction of impurities (most notably sodium ferrocyanide) has been added to these unknowns
to prevent caking. A more exact total cation content based on a listing of the approximate impurity concentrations obtained from Thorn Smith gives a wt% Na+ less than 1ppt higher than the value obtained using the above equation. The chloride unknowns have been in use in the cation exchange experiment a t the University of Florida for over a year. A 250-mm X 12-mm column of Amberlite IR-120H C.P. (or equivalent resin) is used. The sample must he passed through the column slowly (2.5-4 mllmin) to ensure completeness of exchange; the acid may be rinsed from the column a t a faster flow rate (20-30 mllmin). Analyses by the author indicate that good accuracy and precision can be obtained by using the chloride unknowns. Relative errors are of the order of 3 ppt or less on five different unknowns, with RSD's of about the same magnitude. The median relative errors and RSD's of quantitative analysis students are comparable. One unknown gives systematically high results and is not assigned to students. What causes this particular problem is not known. Specific infomation about sample numbers and values may be obtained from Thorn Smith Chemists, Inc. Presented at the Florida ACS Meeting-in-Miniature.Tallahassee, FL. May 9, 1961. 'Harris, W.E. and Kratoehvii. B., "Chemical Separations and Measurements: Background and Procedures for Madern Analysis." W. B. Saunders Co., Philadelphia. 1974, pp. 230-233. Pietrzyk, D. J. and Frank, C. W.. ''Analytical Chemistry," 2nd ed., Academic Press, New York, 1979, pp. 644-645. Benston. M. L., and Jaaneenmakl. M. K., Wuantitative Chemistry," Van Nostrand Reinhold Co.. New Yark, p. 122. Bassen. J.. Denny. R. C., Jeffery. G. H.. andMendham. J.. "Vagel'sTextbook ot Quantitative Inorganic Analysis." 4thed.. Longman, Inc.. New York. 1978, p. 183. Williams. Kathryn R., "Experiments in Quantitative Analysis: A Laboratory Manual,'' Burgess Publishing Co., Minneapolis, 1981, pp. 65-67. BThornSmilh Chemists. inc.. 7755 Narrow Gauge Road. Beulah, MI 49617.
Kathryn R. Williams University of Florida Gainesville, FL 3261 1
338
Journal
of Chemical Education
