FAQ: “Where do HunterLab sphere instruments “top-out” as far as APHA measurements go? I have a request for 0-3000 APHA.”
HunterLab instruments can measure above APHA 500 but there is background information you should be aware of in this high yellow range.
Another name used for this same color scale is “Hazen Color”. When this name is used in a specification, the range is often above the typical 500 units associated with the APHA/Pt-Co, such as “1500 Hazen Color”. However, it is all the same yellow color scale for yellow liquids.
The instrumental correlation to APHA/Pt-Co/Hazen Color in HunterLab sphere instruments can measure above 500, including up to and beyond 3000. We would typically recommend using a 10 mm path length cell for this highly chromatic yellow sample.
It should be noted that when a liquid sample is a deeply saturated yellow color, there is more color information available than just the yellowness quantified by the APHA/Pt-Co/Hazen value. Relative to distilled water, the color will be much darker, much more yellow and tinted red or green. While a high APHA/Pt-Co/Hazen value does measure the intense yellowness above 500, a better and full descriptive metric to quantify all aspects of this high color would be CIE L*, a*, b*. It is recommended that CIE L*, a*, b* values be reported along with the APHA/Pt-Co/Hazen value for APHA measurements > 500.
The type of application where using a single number APHA/Pt-Co/Hazen Color rating > 500 is appropriate is when a process starts out with a liquid sample that is highly yellow, then gradually extracts the impurities that create the yellow color eventually leading to an end point where a low APHA/Pt-Co/Hazen value quantifies that the liquid sample is near “water white”.
While difficult to find, there are APHA/Pt-Co/Hazen liquid color standards that can be used to validate color measurement systems where the sample color APHA/Pt-Co/Hazen value is 500.