The APHA / Hazen / Pt-Co scale
APHA, Hazen and Pt-Co are three names for the same clear-liquid colour scale. We explain what it means and where it is used.
One scale, three names
APHA, Hazen and Pt-Co (platinum–cobalt) are three names for the same colour scale used for clear, near-water liquids. The scale describes the degree of yellow tint in units from 0 (distilled water, colourless) to 500. The Pt-Co name comes from the original standard — a solution of potassium chloroplatinate and cobalt chloride whose colour matched successive points on the scale.
The method is standardised in ASTM D1209 and ISO 6271, among others. Although the liquid was historically compared visually against a set of standards, today the measurement is made objectively with a transmission spectrophotometer, which removes subjectivity and lets results be archived.
Where it is used
The APHA/Pt-Co scale is a quality-control standard wherever clarity and the absence of yellowing matter:
- the chemical industry — solvents, plasticisers, resins, glycols;
- the pharmaceutical industry — substances and solutions per pharmacopoeia;
- the food industry — oils, syrups, beverages.
For more strongly coloured liquids, related scales such as Gardner or Saybolt are used. Liquid colour on these scales is measured with transmission spectrophotometers such as the TS4020 or the benchtop YS6060, which report APHA, Gardner, Saybolt and other indices in a single measurement.
Frequently asked questions
How do APHA, Pt-Co and Hazen differ?
They are three names for the same clear-liquid colour scale. APHA comes from the American Public Health Association, Pt-Co from the original platinum–cobalt standard, and Hazen from the method's author. The values are identical.
When should I use APHA vs Gardner?
The APHA/Pt-Co scale is for light, near-water liquids (0–500). Liquids with a more pronounced yellow-brown tint are assessed on the Gardner scale. Many transmission spectrophotometers report both scales at once.
Do I have to compare samples visually against standards?
No. A modern transmission spectrophotometer measures liquid colour objectively and converts it to the APHA/Pt-Co scale, removing the subjectivity of visual assessment and enabling result archiving.