cal reagents gets one caught in a vicious circle. The analysis of the reagent which one has purified re quires other reagents which must also be purified and these in turn must be analyzed by other purified reagents and so on ad infinitum. This approach is obviously illogical and in general has been abandoned in these laboratories. However, for specific elements as opposed to the general spectrum of impurities, a reagent may be freed sufficiently for high sensitivity analyses. For ex ample, copper is an impurity which even in the very low parts per bil lion region has a deleterious effect on some semiconductor devices, par ticularly those based on germanium. One of the more sensitive tests for copper employs its catalysis of the reduction of ferric iron by thiosulfate. The disappearance of a col ored ferric complex will indicate the reduction. Using the salicylate complex, the reaction is 2Fe+»(Sal);+ 2S203-S -^ 2Fe+! + SiO,"' + 2x Sal"
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Since the ferric salicylate complex is colored but the products colorless, the progress of the reaction may be followed by the color intensity. The only reagent present in large amount is water which may be puri fied sufficiently by distillation in quartz. The bleaching takes place at a measurably slow rate in the presence of omy minute traces of copper. Figure 4 shows a plot of absorbance at a fixed wave length against time for a series of solutions with added copper. After a short induction period, there is a period of about 10 minutes when the rate of fading is substantially linear. A
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Circle No. 26 on Readers' Service Card
VOL. 33, NO. 6, MAY 1961 ·
25 A
