APHA

Background
Prior to the 1890s, descriptions of slightly colored clear liquids were based on the concept of "water white." There were many interpretations of "water white" and it became necessary to have a more specific method for describing the color of waste water. In 1892, chemist A. Hazen described the American Public Health Association (APHA) color index. The index was initially used to evaluate the color of waste water by comparison with dilutions of a platinum-cobalt (PtCo) stock solution. The index was used as an indication of purity, as the color of waste water is produced by undesirable impurities and organic materials.

As the chemical, petroleum, plastic, and pharmaceutical industries grew, they also developed the need to measure the color of their products, some of which were similar in hue to the PtCo solutions in A. Hazen's method. In many cases, the color of their almost "water white" products was an indication of purity, the level of refinement, or the cleanliness of the product container. In 1952, ASTM Test Method D1209 was adopted using a PtCo scale similar to that described by A. Hazen.

APHA is a single number yellowness index where each APHA unit is based on a dilution of the 500 ppm stock solution of PtCo. Distilled water has an APHA value of zero. The stock solution has an APHA value of 500. The PtCo scale and Hazen scale are also based on this sample reagent dilution and have units equivalent to APHA units. APHA is the name used in HunterLab systems, although it could also be called the PtCo scale. A detailed description of solution preparation and measurement procedures may be found in ASTM Designation D1209, "Standard Test Method for Color of Clear Liquids (Platinum-Cobalt Scale)."