APHA color

APHA color, also referred to as the Hazen scale, and more appropriately as the Platinum Cobalt(Pt/Co) scale, is a color standard named for the American Public Health Association and defined by ASTM D1209. It was originally intended to describe the color of waste water, but its usage has expanded to include other industrial applications. APHA color is a color scale sometimes referred to as a "yellowness index" that is used to assess the quality of liquids that are clear to yellowish in color.
It is similar to the Hazen color test, to which it is sometimes incorrectly referred. The Hazen color test uses a Pt/Co solution and was developed for water treatment facilities where the Color of water could be used as a measure of concentration of dissolved and particulate material. Slight discoloration is measured in Hazen units (HU). Impurities can be deeply colored as well, for instance dissolved organic compounds called tannins can result in dark brown colors.
History.
Developed in the 1890s by chemist Allen Hazen (1869–1930), this color scale's original purpose was to assist in the determination of the quality of public water supplies. Since then other uses of APHA color have been demonstrated in chemical, pharmaceutical, beverage, plastic and petroleum industries