What is APHA and How is it Calculated?


APHA-10, -20, and -50 mm

The American Public Health Association (APHA) Index was developed in the 1890s as a visual indicator of the purity of wastewater, where color is due to the presence 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 light and/or heat, the presence of impurities, and the effects of processing.

APHA-10, -20, and -50 mm are designed to yield APHA/PtCo values that closely correlate to APHA/PtCo standard solution values as defined by ASTM D1209. They are calculated from the YI E313 yellowness index in accordance with ASTM D5386 using a proprietary formula.

A transmission cell with 10-, 20-, or 50-mm path length is required for this metric. The sensor should be standardized in TTRAN mode using a clear liquid as a blank.

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