Box 7-1 Reagent Chemicals and Primary Standards

The assay value of 100.6% means that a specified analysis for one of the major components produced 100.6% of the theoretical value. For example, if ZnS04 is contaminated with the lower molecular mass Zn(OH)2, the assay for Zn2+ will be higher than the value for pure ZnS04. Less pure chemicals, generally unsuitable for analytical chemistry, carry designations such as chemically pure (CP), practical, purified, or technical.  

For trace analysis (analysis of species at ppm and lower levels), impurities in reagent chemicals must be extremely low. For this purpose, we use very-high-purity, expensive grades of acids such as trace metal grade HN03 or HC1 to dissolve samples. We must pay careful attention to reagents and vessels whose impurity levels could be greater than the quantity of analyte we seek to measure. 

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