The APHA/Pt-Co/Hazen and Gardner visual color scales were both originally based on liquid chloroplatinate color standards but have different history and intended use.
In the chemical, pharmaceutical and plastics industries, APHA/Pt-Co/Hazen is used to differentiate trace amounts of yellowness in near colorless or low chroma liquid samples. APHA/Pt-Co/Hazen has a range from 0 (distilled water) to 500 (pale yellow) and is used to differentiate small amounts of yellowness in near “water-white” liquids.
The Gardner Color is used to differentiate higher chroma yellow samples that also vary in lightness and hue. The Gardner Color Scale was originally based on liquid chloroplatinate standards before the defintion of the Gardner Color Scale was re-set in 1958 to a set of 18 color filters designed to match the color of the original liquid standards.
The Gardner Color scale ranges from 0 (distilled water) to 1 (light yellow), then continues to a dark, murky 18. Over the 0 to 18 range, there is an increase in yellowness, decrease in lightness and change in hue from a red yellow to green yellow. Most applications range from 0 to 8 in Gardner Color.
The point of crossover between APHA/Pt-Co/Hazen and Gardner color is at the top end of the APHA/Pt-Co/Hazen scale and the corresponding low-end of the Gardner Color scale. An APHA/Pt-Co/Hazen 500 color standard is considered to be just under a Gardner 2.
There are some applications where evaluation of process color quality can be done with a combination of APHA and Gardner Color. A material may begin as a highly yellow liquid that measures a mid-level Gardner Color. With refining, the color quality improves and results in a less yellow Gardner 2 or 1. As the refining process continues, the products are more water white. APHA can be used to measure the product as it is further refined to desired color end point near water. The final measured result may also be used to document a sales quality specification of “APHA Color less than 30”.