The Platinum-Cobalt Scale (Pt/Co scale or Apha-Hazen Scale ) is a color scale that was introduced in 1892 by chemist Allen Hazen (1869–1930). The index was developed as a way to evaluate pollution levels in waste water. It has since expanded to a common method of comparison of the intensity of yellow-tinted samples. It is specific to the color yellow and is based on dilutions of a 500 ppm platinum cobalt solution. The colour produced by one milligram of platinum cobalt dissolved in one litre of water is fixed as one unit of colour in platinum-cobalt scale. The ASTM has detailed description and procedures in ASTM Designation D1209, "Standard Test Method for Color of Clear Liquids (Platinum-Cobalt Scale)".
- HunterLab. "FAQ - How to Measure Color of a Sample & Use An Index - HunterLab" (PDF). www.hunterlab.com.
- Designation D1209-00, "Standard Test Method for Color of Clear Liquids (Platinum-Cobalt Scale)," ASTM International, West Conshohocken, PA, 2000
- Hazen, A., "The Measurement of the Colors of Natural Waters," American Chemist Journal (18:264), 1896