Standard Test Method for Color of Clear Liquids (Platinum-Cobalt Scale)

Abstract

3.1 The property of color of a solvent varies in importance with the application for which it is intended, the amount of color that can be tolerated being dependent on the color characteristics of the material in which it is used. The paint, varnish, and lacquer solvents, or diluents commercially available on today's market normally have little or no color. The presence or absence of color in such material is an indication of the degree of refinement to which the solvent has been subjected or of the cleanliness of the shipping or storage container in which it is handled, or both. (A) This is platinum-cobalt color No. 10 in Guide D365. 3.2 For a number of years the term “water-white” was considered sufficient as a measurement of solvent color. Several expressions for defining “water-white” gradually appeared and it became evident that a more precise color standard was needed. This was accomplished in 1952 with the adoption of Test Method D1209 using the platinum-cobalt scale. This test method is similar to the description given in Standard Methods for the Examination of Water and Waste Water3 and is referred to by many as “APHA Color.” The preparation of these platinum-cobalt color standards was originally described by A. Hazen in the American Chemical Journal4 in which he assigned the number 5 (parts per ten thousand) to his platinum-cobalt stock solution. Subsequently, in their first edition (1905) of Standard Methods for the Examination of Water, the American Public Health Association, using exactly the same concentration of reagents, assigned the color designation 500 (parts per million) which is the same ratio. The parts per million nomenclature is not used since color is not referred directly to a weight relationship. It is therefore recommended that the incorrect term “Hazen Color” should not be used. Also, because it refers primarily to water, the term “APHA Color” is undesirable. The recommended nomenclature for referring to the color of organic liquids is “Platinum-Cobalt Color, Test Method D1209.” 3.3 The petroleum industry uses the Saybolt colorimeter Test Method D156 for measuring and defining the color of hydrocarbon solvents; however, this system of color measurement is not commonly employed outside of the petroleum industry. It has been reported by various sources that a Saybolt color of +25 is equivalent to 25 in the platinum-cobalt system or to colors produced by masses of potassium dichromate ranging between 4.8 mg and 5.6 mg dissolved in 1 L of distilled water. Because of the differences in the spectral characteristics of the several color systems being compared and the subjective manner in which the measurements are made, exact equivalencies are difficult to obtain.