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
BS 5339:76 (1993)    Measurement of Color on Hazen Units /Platinum-Cobalt
DIN 53409                  Bestimmung der Hazen-Farbzahl (APHA-Verfahren)
DIN ISO 6271              Einstufung der Farbe nach der Platin Cobalt Skala

Typical application areas of the Pt-Co/Apha-/Hazen color scale include polymer additives, water and waste water, resin
solutions, and solvents such as hexane, glycerol, methanol, mineral oils , etc. The recommendation of this procedure by
the American Public Health Agency (APHA) has established the respective naming.

As in other areas of color classification, there is a trend to replace the complex and subjective visual comparison with
reference solutions by objective procedures. For Apha-/Hazen reference solutions degradation must also be taken into
account since the solutions are not stable with regular use. An objective procedure can be based on the sa
spectrum in the visible spectral range, i.e. 380–780 nm.

Each one-dimensional color scale has its limitations, if the color of the sample differs markedly from the color of the
reference solutions. Color variations must be taken into account and sample solutions should be within a typical range.

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