Appendix IV B. Colour of Solution

The examination of the degree of coloration of liquids in the range brown-yellow-red is carried out by one of the 2  methods below, as prescribed in the monograph.

A solution is colourless if it has the appearance of water  R or the solvent or is not more intensely coloured than reference solution  B₉.

Using identical tubes of colourless, transparent, neutral glass of 12  mm external diameter, compare 2.0  ml of the liquid to be examined with 2.0  ml of water  R or of the solvent or of the reference solution (see Tables of reference solutions) prescribed in the monograph. Compare the colours in diffused daylight, viewing horizontally against a white background.

Using identical tubes of colourless, transparent, neutral glass with a flat base and an internal diameter of 15  mm to 25  mm, compare the liquid to be examined with water  R or the solvent or the reference solution (see Tables of reference solutions) prescribed in the monograph, the depth of the layer being 40  mm. Compare the colours in diffused daylight, viewing vertically against a white background.

Yellow Solution Dissolve 46  g of ferric chloride  R in about 900  ml of a mixture of 25  ml of hydrochloric acid  R and 975  ml of water  R and dilute to 1000.0  ml with the same mixture. Titrate and adjust the solution to contain 45.0  mg of FeCl₃,6H₂O per millilitre by adding the same acidic mixture. Protect the solution from light.

Titration. Place in a 250  ml conical flask fitted with a ground-glass stopper, 10.0  ml of the solution, 15  ml of water  R, 5  ml of hydrochloric acid  R and 4  g of potassium iodide  R, close the flask, allow to stand in the dark for 15  min and add 100  ml of water  R. Titrate the liberated iodine with 0.1  M sodium thiosulphate, using 0.5  ml of starch solution  R, added towards the end of the titration, as indicator.

1  ml of 0.1  M sodium thiosulphate is equivalent to 27.03  mg of FeCl₃,6H₂O.

Red Solution Dissolve 60  g of cobalt chloride  R in about 900  ml of a mixture of 25  ml of hydrochloric acid  R and 975  ml of water  R and dilute to 1000.0  ml with the same mixture. Titrate and adjust the solution to contain 59.5  mg of CoCl₂,6H₂O per millilitre by adding the same acidic mixture.

Titration. Place in a 250  ml conical flask fitted with a ground-glass stopper, 5.0  ml of the solution, 5  ml of dilute hydrogen peroxide solution  R and 10  ml of a 300  g/l solution of sodium hydroxide  R. Boil gently for 10  min, allow to cool and add 60  ml of dilute sulphuric acid  R and 2  g of potassium iodide  R. Close the flask and dissolve the precipitate by shaking gently. Titrate the liberated iodine with 0.1  M sodium thiosulphate, using 0.5  ml of starch solution  R, added towards the end of the titration, as indicator. The end-point is reached when the solution turns pink.

1  ml of 0.1  M sodium thiosulphate is equivalent to 23.79  mg of CoCl₂,6H₂O.

Blue Primary Solution Dissolve 63  g of copper sulphate  R in about 900  ml of a mixture of 25  ml of hydrochloric acid  R and 975  ml of water  R and dilute to 1000.0  ml with the same mixture. Titrate and adjust the solution to contain 62.4  mg of CuSO₄,5H₂O per millilitre by adding the same acidic mixture.

Titration. Place in a 250  ml conical flask fitted with a ground-glass stopper, 10.0  ml of the solution, 50  ml of water  R, 12  ml of dilute acetic acid  R and 3  g of potassium iodide  R. Titrate the liberated iodine with 0.1  M sodium thiosulphate, using 0.5  ml of starch solution  R, added towards the end of the titration, as indicator. The end-point is reached when the solution shows a slight pale brown colour.

1  ml of 0.1  M sodium thiosulphate is equivalent to 24.97  mg of CuSO₄,5H₂O.

Using the 3 primary solutions, prepare the 5 standard solutions as follows:

Using the 5 standard solutions, prepare the following reference solutions.

For Method  I, the reference solutions may be stored in sealed tubes of colourless, transparent, neutral glass of 12  mm external diameter, protected from light.

For Method  II, prepare the reference solutions immediately before use from the standard solutions.