Platinum-Cobalt Color Scale for Color Measurement in Water

The Platinum-Cobalt Scale (also known as a Pt/Co scale or an Apha-Hazen Scale) is a color scale that was introduced in 1892 by chemist Allen Hazen. Throughout this article, “APHA,” “Hazen” and “Pt/Co” or “Platinum-Cobalt” will be used synonymously. The Pt/Co color scale measures “yellowness” in liquid and is based on dilutions of a 500 ppm (parts per million) platinum cobalt solution. Its color index is a method for evaluating pollution levels in water or wastewater along with determining product quality and impurities.

Over time, it expanded as a common method of comparing the intensity of yellow-tinted samples. ASTM International, formerly known as American Society for Testing and Materials, has detailed descriptions and procedures in the ATSM Designation D1209, “Standard Test Method for Color of Clear Liquids (Platinum-Cobalt Scale).” The APHA/Hazen/Platinum-Cobalt color test is utilized in various industries from wastewater treatment, chemical, pharmaceutical, petroleum and plastic, and more. Industries using the color test index helps maintain consistency and safety within products.

APHA/Hazen/Platinum-Cobalt Color Scale

The Platinum-Cobalt color scale ranges from 0 to 500 with the lowest value at “0” referring to water as white or “distilled.” A 500 value on the scale means the water is distinctly yellow. The colors of technical liquids are frequently classified according to the APHA/Hazen-/Pt-Co color scale. These color values 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.

National & International Standards

There are several national and international standards, which describe the procedure in more detail:

Typical application areas of the APHA/Hazen/Pt-Co color scale include polymer additives, water and wastewater, 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 unstable with regular use. An objective procedure can be based on the sample 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 distinctly from the color of the reference solutions. One must take into account color variations and sample solutions should be within a typical range.