APHA/Pt-Co/Hazen vs Gardner colour scales: What’s the difference?

Resin samples typically measured in Gardner Colour

The APHA/Pt-Co/Hazen and Gardner visual colour scales were both originally based on liquid chloroplatinate colour standards but have different history and intended use.

APHA/Pt-Co/Hazen Colour Scale

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.

Gardner Colour Scale

The Gardner Colour is used to differentiate higher chroma yellow samples that also vary in lightness and hue. The Gardner Colour Scale was originally based on liquid chloroplatinate standards before the defintion of the Gardner Colour Scale was re-set in 1958 to a set of 18 colour filters designed to match the colour of the original liquid standards.

The Gardner Colour 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 Colour.

What’s The Difference Between APHA/Pt-Co/Hazen & Gardner?

The point of crossover between APHA/Pt-Co/Hazen and Gardner colour is at the top end of the APHA/Pt-Co/Hazen scale and the corresponding low-end of the Gardner Colour scale. An APHA/Pt-Co/Hazen 500 colour standard is considered to be just under a Gardner 2.

There are some applications where evaluation of process colour quality can be done with a combination of APHA and Gardner Colour. A material may begin as a highly yellow liquid that measures a mid-level Gardner Colour. With refining, the colour 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 colour end point near water. The final measured result may also be used to document a sales quality specification of “APHA Colour less than 30”.