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 ASTM has detailed description and procedures in
ASTM Designation D1209, "Standard Test Method for Color of Clear Liquids (Platinum-Cobalt Scale).
The colors of technical liquids are frequently classified according to the Apha-/Hazen-/Pt-Co color scale. These three designations are commonly used in different application areas, but they are based on identical procedures. The Apha- /Hazen-/Pt-Co color values complement the Iodine color scale for weak yellow/brown hues.
Technical liquids often have a slight yellow color due to contamination or decomposition products. Traditionally, color classification for this type of sample is performed by visual comparison of the sample with yellow reference solutions in defined vessels. According to a proposal by A. Hazen in 1892, the Pt-Co/Apha-/Hazen color scale uses an acidic solution of potassium hexachloro- platinate(IV) and cobalt(II) chloride. The reference solutions are designated according to their platinum content in mg/L in the range 0–500. The solutions can be obtained from commercial suppliers.
There are several national and international standards which describe the procedure in more detail:
|ASTM D 1209||Standard Method for Color of Clear Liquids (Platinum-Cobalt Scale)|
|BS 5339:76 (1993)||Measurement of Color on Hazen Units /Platinum-Cobalt Scale|
|DIN 53409||Bestimmung der Hazen-Farbzahl (APHA-Verfahren)|
|DIN ISO 6271||Einstufung der Farbe nach der Platin Cobalt Skala|