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4.2 COLOUR OF SOLUTION

4.2 COLOUR OF SOLUTION

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

Method

      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. The colour of the test solution is not more intense than that of the matching fluid when viewed down the vertical axis of the tubes in diffused light against a white background.

Colorimetric Solutions (CS)

      COBALT(II) CHLORIDE CS Dissolve about 65 g of cobalt (II) chloride (CoCl2.6H2O) in enough of a mixture of 25 ml of hydrochloric acid and 975 ml of water to make 1000 ml. Pipette 5 ml of this solution into a 250-ml iodine flask, and add 5 ml of hydrogen peroxide TS (10 volumes) and 10 ml of a 30 per cent w/v solution of sodium hydroxide. Boil gently for 10 minutes, allow to cool and add 60 ml of 1 M sulfuric acid and 2 g of potassium iodide. Close the flask and dissolve the precipitate by shaking gently. Titrate the liberated iodine with 0.1 M sodium thiosulfate VS, using starch TS, added towards the end of the titration, as indicator. The endpoint is reached when the solution turns pink. Perform a blank determination with the same quantities of the same reagents, and make any necessary correction. Each ml of 0.1 M sodium thiosulfate VS is equivalent to 23.79 mg of CoCl2.6H2O. Adjust the final volume of the solution by adding enough of the hydrochloric acid and water mixture so that each ml contains 59.5 mg of CoCl2.6H2O.

      COPPER(II) SULFATE CS Dissolve about 65 g of copper (II) sulfate (CuSO4.5H2O) in enough of a mixture of 25 ml of hydrochloric acid and 975 ml of water to make 1000 ml. Pipette 10 ml of this solution into a 250-ml iodine flask, add 40 ml of water, 4 ml of acetic acid, 5 ml of hydrochloric acid, and 3 g of potassium iodide. Titrate the liberated iodine with 0.1 M sodium thiosulfate VS, using starch TS, added towards the end of the titration, as indicator. The end-point is reached when the solution shows a slight pale brown colour. Perform a blank determination with the same quantities of the same reagents, and make any necessary correction. Each ml of 0.1 M sodium thiosulfate VS is equivalent to 24.97 mg of CuSO4.5H2O. Adjust the final volume of the solution by adding enough of the hydrochloric acid and water mixture so that each ml contains 62.4 mg of CuSO4.5H2O.

      IRON(III) CHLORIDE CS Dissolve about 55 g of iron(III) chloride (FeCl3.6H2O) in enough of a mixture of 25 ml of hydrochloric acid and 975 ml of water to make 1000 ml. Pipette 10 ml of this solution into a 250-ml iodine flask, add 15 ml of water, 5 ml of hydrochloric acid and 3 g of potassium iodide. Close the flask, allow to stand protected from light for 15 minutes and add 100 ml of water. Titrate the liberated iodine with 0.1 M sodium thiosulfate VS, using starch TS, added towards the end of the titration, as indicator. The end-point is reached when the solution turns colourless. Perform a blank determination with the same quantities of the same reagents, and make any necessary correction. Each ml of 0.1 M sodium thiosulfate VS is equivalent to 27.03 mg of FeCl3.6H2O. Adjust the final volume of the solution by adding enough of the hydrochloric acid and water mixture so that each ml contains 45.0 mg of FeCl3.6H2O.

Matching Fluids

      To serve as standards of comparison, twenty matching fluids, each designated by a letter of the alphabet, are provided, the composition of each being as indicated in the accompanying table. To prepare the matching fluid required, pipette the prescribed volumes of the colorimetric solutions and water into one of the matching tubes, and mix the solution in the tube. Make the comparison as directed in the individual monograph, under the viewing conditions previously described. The matching fluids, or other combinations of the colorimetric solutions, may be used in very low concentrations to measure deviation from colourlessness.

Matching Fluids

 

APPENDICES • 4.2 COLOUR OF SOLUTION
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หมายเหตุ / Note : TP II 2011 PAGE 420-421