“APHA” stands for American Public Health Association Color Scale which is the original organization that defined its use in the 1890s. It is also called “Pt-Co” for Platinum Cobalt Scale as preferred by most industrial standards bodies such as ASTM, ISO and JIS. Another name is “Hazen” Scale after the chemist, Allen Hazen, who developed this visual color scale for the APHA.
The APHA Scale was originally developed in the 1890s as a visual indicator of the purity of waste water, where a slightly yellow color is due to the leachates of naturally occurring organic materials such as leaves, bark, roots, humus and peat.
Today, APHA is used as a metric for purity in the chemical, oil, plastics, and pharmaceutical industries. This scale serves to quantify the appearance of yellowness, a visual indicator of product degradation due to exposure to light and/or heat, the presence of impurities, and negative effects of processing.
This visual yellowness color scale is based on a chemical reagent solution, best defined in ASTM D1209 Standard Test Method for Color of Clear Liquids (Platinum Cobalt Scale).
The instrumental APHA/Pt-Co/Hazen method designed to correlate to the ASTM D1209 visual APHA/Pt-Co/Haze color scale is best defined in ASTM D5386 Standard Test Method for Color of Liquids Using Tristimulus Colorimetry.
APHA/Pt-Co/Hazen is a single number, low chroma yellowness metric based on the dilution of a reagent Pt-Co (potassium chloroplatinate) stock solution. Each APHA unit is based on a dilution by volume of 500 parts of Pt-Co stock solution per million parts of distilled water. Distilled water ('water-white') is rated as APHA/Pt-Co 0. The reagent APHA/Pt-Co/Hazen stock solution carries a rating of 500 APHA/Pt-Co/Hazen.
The definition of what is a 500 APHA/Pt-Co/Hazen reagent color standard is found in Section 6 of ASTM D1209.
Section 6 has two parts. A 500 APHA/Pt-Co/Hazen reagent color standard must have a particular chemical composition, and an absorbance criteria. After blanking spectrophotometer on the 10 mm cell and DI water, the measured absorbance of the 500 APHA/Pt-Co/Hazen reagent color standard must be within a defined acceptance window at 4 spectral points in the blue region of the visible spectrum. These absorbance points happen to correspond slopes for the 500 color standard and are quite sensitive to concentration.
If an Pt-Co solution meets both chemical and absorbance criteria, it is an APHA/Pt-Co/Hazen 500 reagent color standard, suitable for visual APHA/Pt-Co/Hazen evaluation of sample solutions per ASTM D1209.
FAQ: “If three (3) sites have their own APHA/Pt-Co/Hazen 500 reagent color standards, would them all measure them as 500 on their instruments?”
Given that there is a range of absorbance acceptance criteria per D1209, there will be some variation in the measured values of multiple APHA/Pt-Co/Hazen 500 reagent color standards when measured on the same instrument, or on multiple instruments.
The saving grace is that while there may be some measurable difference among multiple 500 color standards, this largely disappears at the low end of the APHA/Pt-Co/Hazen color scale where most clients are measuring samples.