This application note considers the use of the APHA index to measure the color of water white liquids.
APHA is a single number yellowness index where each APHA unit is based on a dilution of the 500 ppm stock solution of PtCo. A detailed description of solution preparation and measurement procedures may be found in ASTM Designation D1209.
While instrumental determination of APHA is allowed by D1209 and D5386, no formula is prescribed by either method. For APHA, HunterLab uses a custom correlation based on known APHA values compared to instrumental measurements. Please note that these correlations apply only to HunterLab instruments able to read in Transmittance (2° Standard Observer/Illuminant C).
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