Clarity of Solution
Standard Suspension
Dissolve 1.0 g of hydrazine sulfate in sufficient water to produce 100.0 ml and set aside for about 6 hours. To 25.0 ml of this solution add 25.0 ml of a 10.0 percent w/v solution of hexamine, mix well and allow to stand for 24 hours. Keep in a glass container with a smooth internal surface in which the suspension does not adhere to the glass. Store in this manner, the suspension is stable for about 2 months.Prepare the standard suspension by diluting 15 ml of the well-mixed suspension to 1000 ml with water. The standard suspension should be used within 24 hours of preparation.
Opalescence Standards
Prepare opalescence standards by mixing aliquots of the standard suspension with water as indicated in Table 1. Each opalescence standard should be shaken well before use.
Method
Transfer to a flat-bottom test-tube of neutral glass, 15 to 25 mm in diameter, a suitable volume of the solution under examination such that the test-tubes are filled to a depth of 40 mm. Into another matched test-tube add the same volume of the freshly prepared opalescence standard. After 5 minutes, compare the contents of the test-tubes against a black background by viewing under a diffused light down the vertical axis of the tubes.Clarity or opalescence
Express the degree of opalescence in terms of the opalescence standard. A liquid is considered clear if its clarity is the same as that of water or of the solvent used for preparing the solution under examination or if its opalescence is not more than that of opalescence standard OS 1.Colour of Solution
Special Reagents
Ferric Chloride Colorimetric Solution (FCS)
Dissolve about 55 g of ferric chloride hexahydrate in enough of a mixture of 25 ml of hydrochloric acid and 975 ml of water to produce 1000 ml. Pipette 10 ml of this solution into a 250-ml iodine flask, add 15 ml of water, 3 g of potassium iodide and 5 ml of hydrochloric acid and allow the mixture to stand for 15 minutes. Dilute with 100 ml of water and titrate the liberated iodine with 0.1 M sodium thiosulphate using 0.5 ml of starch solution as an indicator, added towards the end of the titration, as an indicator. Carry out a blank titration. 1 ml of 0.1 M sodium thiosulphate is equivalent to 0.02703 g of FeCI3,6H2O. Adjust the final volume of the solution by the addition of enough of the mixture of hydrochloric acid and water so that each ml contains 0.045 g of FeCI3.6H2O.The solutions should be stored protected from light standardized before use.
Cobaltous Chloride Colorimetric Solution (CCS)
Dissolve about 65 g of cobaltous chloride in enough of a mixture of 25 ml of hydrochloric acid and 975 ml of water to produce 1000 ml. Pipette 5 ml of this solution into a 250 ml iodine flask, add 5ml of hydrogen peroxide solution (10 volume) and 15 ml of sodium hydroxide solution, boil for 10 minutes, cool and add 2 g of potassium iodide and 60 ml of dilute sulphuric acid. Dissolve the precipitate by gentle shaking, if necessary, and titrate the liberated iodine with 0.1 M sodium thiosulphate using 0.5 ml of starch solution, added towards the pink endpoint, as an indicator. Carry out a blank titration. l ml of 0.1 M sodium thiosulphate is equivalent to 0.02379 g of CoCl2,6H2O. Adjust the final volume of the solution by the addition of enough of the above mixture of hydrochloric acid and water so that each ml contains 0.0595 g of CoCl2.6H2O.Cupric Sulphate Colorimetric Solution (CSS)
Dissolve about 65 g of cupric sulphate in enough of a mixture of 25 ml of hydrochloric acid and 975 ml of water to produce 1000 ml. Pipette 10 ml of this solution into a 250 ml iodine flask, add 40 ml of water, 4 ml of acetic acid, 3 g of potassium iodide, and 5 ml of hydrochloric acid and titrate the liberated iodine with 0.1 M sodium thiosulphate using 0.5 ml of starch solution, added towards the pale brown end-point, as indicator. Carry out a blank titration. 1ml of 0.1 M sodium thiosulphate is equivalent to 0.02497g of CuS04,5H2O. Adjust the final volume of the solution by the addition of enough of the above mixture hydrochloric acid and water so that each ml contains 0.0624 g of CuS04.5H2O.Reference Solution
Prepare by mixing the volumes of colorimetric solutions and hydrochloric acid (1 per cent w/v HCl) as indicated in Table 2.NOTE - Reference solutions must be prepared immediately before use from the Colorimetric solutions which may be stored in refrigerator.
Method
Transfer to a flat bottom test tube of neutral glass 15 to 25 mm in diameter, a suitable volume of a liquid been examined such that the test tube is filled to a depth of 40 mm. Into another matched test tube add the same volume of water or of the solvent used for preparing the solution being examined or of the reference solution stated in the individual monograph.Compare the colors in diffused light viewing vertically against a white background.
Please advise why indicator is added so late, at the end, not before the titration ?
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