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.
Measure True Color 