Pt/Co scale / APHA Color
The Platinum-Cobalt Scale (Pt/Co scale or Apha-Hazen Scale ) is a color scale that was introduced in 1892 by chemist Allen Hazen (1869–1930). The index was developed as a way to evaluate pollution levels in waste water. It has since expanded to a common method of comparison of the intensity of yellow-tinted samples. It is specific to the color yellow and is based on dilutions of a 500 ppm platinum cobalt solution. The colour produced by one milligram of platinum cobalt dissolved in one liter of water is fixed as one unit of colour in platinum-cobalt scale. The ASTM has detailed description and procedures in ASTM Designation D1209, "Standard Test Method for Color of Clear Liquids (Platinum-Cobalt Scale)".
APHA Color, also known as Pt-Co or Hazen Color, measures trace yellowness in liquid samples as an indication of purity.
If an APHA value is negative or much lower than expected, the sample may be off-hue from the APHA/Pt-Co color standards. If an APHA value is very high, the sample may be of the correct yellow hue, but more saturated than the upper limit of the Pt-Co 500 stock solution. In this case, the Gardner Color Index may be more appropriate as it was designed for darker yellow liquids. Alternately, a tristimulus color scale like CIE L*a*b* or Hunter L, a, b could be used. APHA/Pt-Co/Hazen Sample Measurement The instrument must be standardized in Total Transmittance (TTRAN) mode using a transmission cell of the same path length as will be used in the sample measurement. Samples may be measured using a 10, 20, or 50 mm transmission cell or 24 mm ID glass vial. APHA/Pt-Co/Hazen Color measurements are specific to the cell path length. If a transmission cell with a 20 mm path length is used for measurement, the “APHA-20 mm” metric should be selected in the software.
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