SOLUTION

The color intensity of liquids may be estimated rapidly by using platinum-cobalt standards. This method is particularly applicable to those materials in which the color-producing substances have light-absorption characteristics nearly identical with those in the standards. Colors having hues other than light yellow or reddish yellow cannot be determined with these standards.

The platinum cobalt color standards contain carefully controlled amounts of potassium chloroplatinate and cobaltous chloride. Each platinum cobalt color unit is equivalent to 1 mg of platinum per liter of solution (1 ppm), and the standards are named accordingly. For example, the No. 20 platinum- It standard contains 20 ppm of platinum. These platinum-cobalt standards are also called APHA (American Public Health Association and Hazen standards).