J. Fred Haze! University
of
Pennsylvania Philadelphia
A New Twist to K, Evaluation
A variation of a standard experiment which may serve to enhance student interest and introduce a new technique is to evaluate the concentration of slightly soluble salts by as sing the saturated solution through a cation resin and titrating the resulting acid in the eluate. For example, a known volume of saturated solution of lead chloride or clacium sulfate can be passed through a 80 X 15 mm column (we use drying tubes) of Rexyn 101 (H) and the resulting hydrochloric acid titrated to establish the amount of Pb2+retained by the resin. Proper recognition of the appropriate molar relationship then allows the calculation of the Km. Further novelty can be added by using methyl purple, a mixed indicator, for the titration of the hydrochloric or sulfuric acids thus produced. The solution is purple below pH 4.8, green above pH 5.4, and grey in between. The grey range may be missed because one drop of 0.1M sodium hydroxide may raise the pH of a 5 0 4 sample of the acid by as much as
636
/
Journal o f Chemical Education
three pH units. If an oxalate such as magnesium oxalate is used, phenolphthalein should be chosen as the indicator for the titration of the resulting oxalic acid.
"-'
Resin exchange colvmns ( A represents level of liquid to be maintained above the resin).
