REPORT FOR ANALYTICAL CHEMISTS
proximately 50 p.p.b. and yet primary standard anhydrous sodium carbonate, a reagent which might normally be selected for fusion of the sample, contains about 10 p.p.m. of sulfur. The iron content of semiconductor materials must be reduced below the parts per million level but analytical grade potassium pyrosulfate (commonly used for fusing refractory materials) normally contains about 20 p.p.m. of iron. The list of examples could be continued almost indefinitely but suffice it to say that most of the time any chemical used to bring the semiconductor material into solu-
tion will introduce more of the pertinent impurity than is present in the sample. In dealing with this problem, the analyst has essentially one of three choices: (1) to purify the chemical reagents used, (2) to concentrate the impurities to a level at which reagent impurities become negligible, or (3) to avoid reagents completely and use physical methods such as activation analysis or emission spectroscopy. Purification of Reagents
An attempt to purify the chemi-
I&atf Cu Added ppb
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MINUTES
FROM
MIXING
Figure 4. Effect of copper on the reduction of ferric iron by thiosulfate
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Fig. 5. Relationship between copper content and rate of reduction of ferric iron
