Determinations of lead, zinc, copper, and tin are important in certain
mining regions and in places where the water has a solvent action on
pipes and other containers. The use of certain “germicides” also makes
it necessary to test for some of these metals.
Lead, zinc, and copper may be determined colorimetrically or
electrolytically. The colorimetric methods are not so accurate as a
combination of both, and are chiefly of value as qualitative tests.
It is possible to make a rough estimation of the amount of lead in clear
waters by acidifying with acetic acid, saturating with hydrogen sulfide,
and comparing the color produced with that produced by standard lead
solutions in Nessler tubes, treated in similar manner. This method,
however, is not applicable if the water is colored or contains iron.
_Reagents._—1. Standard lead solution. Dissolve 1.60 grams of lead
nitrate (Pb(NO_{3})_{2}) in 1 liter of distilled water. One cc. of this
solution contains 1 mg. of lead (Pb). As a check it is desirable to
determine lead as sulfate in a measured portion of this solution.
2. Standard copper solution. Dissolve about 0.8 gram of copper sulfate
crystals (CuSO_{4}.5H_{2}O) in water and, after the addition of 1 cc. of
concentrated sulfuric acid, dilute the solution to 1 liter. Determine
the copper in 100 cc. of this solution in the usual way by electrolytic
deposition. Dilute the solution so that 1 cc. contains 0.2 milligram
copper (Cu). This solution is permanent.