The original references were written by Allen Hazen, a chemist who first defined APHA/Pt-Co/Hazen color scale for the evaluation of water quality on behalf of the American Public Health Association:
Hazen, A. A new color standard for natural waters, American Chemist Journal (14:300), 1892.
Hazen, A. The measurement of the colors of natural waters, American Chemist Journal (18:264), 1896.
Industrial Test Methods referencing the Hazen Color Scale
ISO 2211:1973 Liquid chemical products – Measurement of colour in Hazen units (platinum cobalt scale) is an ISO method that references both Hazen and platinum cobalt color and was subsequently replaced by DIN EN ISO 6271.