COLORIMETRIC SOLUTIONS(CS)
(For the Preparation of Matching Fluids,see Color and Achromicity á631ñ.)
These solutions are used in the preparation of the colorimetric standards for certain drugs,and for the carbonization tests with sulfuric acid that are specified in several monographs.Store the solutions in suitably resistant,tight containers.
Comparison of colors as directed in the Pharmacopeial tests preferably is made in matched color-comparison tubes or in a suitable colorimeter under conditions that ensure that the colorimetric reference solution and that of the specimen under test are treated alike in all respects.The comparison of colors is best made in layers of equal depth,and viewed transversely against a white background (see also Visual Comparisonunder Spectrophotometry and Light-Scattering á851ñ).It is particularly important that the solutions be compared at the same temperature,preferably 25
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Cobaltous Chloride CS
—Dissolve about 65g of cobaltous chloride (CoCl2·6H2O)in enough of a mixture of 25mLof hydrochloric acid and 975mLof water to make 1000mL.Pipet 5mLof this solution into a 250-mLiodine flask,add 5mLof hydrogen peroxide TSand 15mLof sodium hydroxide solution (1in 5),boil for 10minutes,cool,and add 2g of potassium iodide and 20mLof dilute sulfuric acid (1in 4).When the precipitate has dissolved,titrate the liberated iodine with 0.1Nsodium thiosulfate VS,adding 3mLof starch TSas the indicator.Perform a blank determination with the same quantities of the same reagents,and make any necessary correction.Each mLof 0.1Nsodium thiosulfate is equivalent to 23.79mg of CoCl2·6H2O.Adjust the final volume of the solution by the addition of enough of the mixture of hydrochloric acid and water so that each mLcontains 59.5mg of CoCl2.6H2O.
Cupric Sulfate CS
—Dissolve about 65g of cupric sulfate (CuSO4·5H2O)in enough of a mixture of 25mLof hydrochloric acid and 975mLof water to make 1000mL.Pipet 10mLof this solution into a 250-mLiodine flask,add 40mLof water,4mLof acetic acid,3g of potassium iodide,and 5mLof hydrochloric acid,and titrate the liberated iodine with 0.1Nsodium thiosulfate VS,adding 3mLof starch TSas the indicator.Perform a blank determination with the same quantities of the same reagents,and make any necessary correction.Each mLof 0.1Nsodium thiosulfate is equivalent to 24.97mg of CuSO4·5H2O.Adjust the final volume of the solution by the addition of enough of the mixture of hydrochloric acid and water so that each mLcontains 62.4mg of CuSO4.5H2O.
Ferric Chloride CS
—Dissolve about 55g of ferric chloride (FeCl3.6H2O)in enough of a mixture of 25mLof hydrochloric acid and 975mLof water to make 1000mL.Pipet 10mLof this solution into a 250-mLiodine flask,add 15mLof water,3g of potassium iodide,and 5mLof hydrochloric acid,and allow the mixture to stand for 15minutes.Dilute with 100mLof water,and titrate the liberated iodine with 0.1Nsodium thiosulfate VS,adding 3mLof starch TSas the indicator.Perform a blank determination with the same quantities of the same reagents,and make any necessary correction.Each mLof 0.1Nsodium thiosulfate is equivalent to 27.03mg of FeCl3.6H2O.Adjust the final volume of the solution by the addition of enough of the mixture of hydrochloric acid and water so that each mLcontains 45.0mg of FeCl3.6H2O.