The test for colour of liquids is carried out by comparing the test solution prepared as specified in the monograph with a standard colour solution indicated in the monograph. The composition of the standard colour solution is selected depending on the hue and intensity of the colour of the test solution corresponding to the limits permitted in the specifications.

Recommended procedure

Unless otherwise specified in the monograph, carry out the comparison in flat-bottomed tubes of transparent glass that are matched as closely as possible in internal diameter and in all other respects (tubes of about 16 mm internal diameter are suitable). Use 10 ml of the test solution and 10 ml of the standard colour solution; the depth of liquid should be about 50 mm. The colour of the test solution is not more intense than the standard colour when viewed down the vertical axis of the tubes in diffused light against a white background.

Stock Colour Standard Solutions

Yellow stock standard TS

To 9.5 ml of cobalt colour TS, add 1.9 ml of copper colour TS, 10.7 ml of dichromate colour TS, 4.0 ml of iron colour TS, dilute to 100.0 ml with sulfuric acid (~10 g/l) TS, and mix.

Red stock standard TS

To 40.5 ml of cobalt colour TS, add 6.1 ml of copper colour TS, 6.3 ml of dichromate colour TS, 12.0 ml of iron colour TS, dilute to 100.0 ml with sulfuric acid (~10 g/l) TS, and mix.

Green stock standard TS

To 3.5 ml of cobalt colour TS, add 20.1 ml of copper colour TS, 10.4 ml of dichromate colour TS, 4.0 ml of iron colour TS, dilute to 100.0 ml with sulfuric acid (~10 g/l) TS, and mix.

Brown stock standard TS

To 35.0 ml of cobalt colour TS, add 17.0 ml of copper colour TS, 8.0 ml of dichromate colour TS, dilute to 100.0 ml with iron colour TS, and mix.

Standard Colour Solutions

The standard colour solution is prepared by suitably diluting the stock standard solutions (yellow, red, green, and brown stock standard TS) with sulfuric acid (~10 g/l) TS. The designation of the standard colour solution is composed of two letters indicating the stock standard solution (Yw for yellow, Rd for red, Gn for green, and Bn for brown) and of a number describing the dilution, as given below:

Standard colour solution numbers 4-7 may be stored in sealed glass containers, protected from sunlight but the more dilute standard colour solutions should be prepared as required.