Limit Test for Colour of solution

Title: Limit Test for Colour of solution

 

1. Objective: Using identical test tubes of colourless, transparent, neutral glass with a flat base and an internal diameter of 15-25 mm, compare the liquid to be examined with a reference suspension.

 

2. Principle: The color of chemicals is a physical property of chemicals that in most cases comes from the excitation of electrons due to absorption of energy performed by the chemical. What is seen by the eye is not the color absorbed, but the complementary color from the removal of the absorbed wavelengths.

 

3. Procedure:

• Preparation of Standard Solutions and Reagents:

Note: All solutions should be prepared in Distilled/Purified Water.

• 1M Sodium Thiosulphate Solution: Dissolve 25 g of Sodium Thiosulphate  and 0.2 g of  Sodium Carbonate in purified water and dilute to 1000 ml with the same solution.
• Starch Solution: Triturate (dissolve) 1 g of soluble Starch with 5 ml of water and add stirring continuously, to 100 ml of boiling water containing 10 mg of Mercuric Iodide.
• Sodium Hydroxide Solution (20% w/v): Dissolve 20.0 g of Sodium Hydroxide in water and dilute to 100.0 ml with Purified water.
• Dilute Sulphuric acid: Dilute 57 ml of concentrated Sulphuric acid to 1000 ml with water.
• Hydrogen Peroxide solution (10 volume): Dilute Hydrogen Peroxide solution (20 volume) with an equal volume of water.
• Ferric Chloride Colorimetric Solution (FCS):  Dissolve about 46 g of Ferric Chloride Hexahydrate in enough of a mixture of 25 ml of concentrated 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 concentrated Hydrochloric acid and allow the mixture to stand for 15 minutes. Dilute with 100 ml of water and titrate the liberated iodine with 0.1M Sodium Thiosulphate using 0.5 ml of Starch solution, added towards the end of the titration, as indicator. Perform a blank determination and make any necessary correction.

1 ml of 0.1M Sodium Thiosulphate is equivalent to 0.02703 g of FeCI₃,6H₂O (Ferric Chloride Hexahydrate).  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.

Note: The solutions should be stored protected from light and standardised before use.

 

• Cobaltous Chloride Colorimetric Solution (CCS):  Dissolve about 60 g of Cobaltous Chloride in enough of a mixture of 25 ml of concentrated Hydrochloric acid and 975 ml of water to produce 1000 ml. Pipette 5 ml of this solution into a 250 ml iodine flask, add 5 ml 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 1M Sodium Thiosulphate using 0.5 ml of Starch solution, added towards the pink end-point, as indicator. Carry out a blank titration and make any necessary correction.

Each ml of 0.1M Sodium Thiosulphate is equivalent to 0.02379 g of CoCl₂,6H₂O (Cobaltous Chloride).  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 0.0595g of CoCl₂,6H₂O.

• Cupric Sulphate Colorimetric Solution (CSS): Dissolve about 63 g of Cupric Sulphate in enough of a mixture of 25 ml of concentrated 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 1M Sodium Thiosulphate  using 0.5 ml of Starch solution, added towards the pale brown end-point, as indicator. Perform a blank determination and make any necessary correction.

Each ml of 0.1M Sodium Thiosulphate is equivalent to 0.02497g of CuSO₄,5H₂O (Cupric Sulphate).  Adjust the final volume of the solution by the addition of enough of the mixture Hydrochloric acid and water so that each ml contains 0.0624 g of CuSO₄,5H₂O.

 

• Reference Solution:Prepare by mixing the volumes of colorimetric solutions and Hydrochloric acid (1% w/v HCl) as indicated in Table No. 01.

Note: Reference solutions must be prepared immediately before use from the Colorimetric solutions which may be stored in Refrigerator.

 

• Method:

Transfer to a flat bottomed 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 match 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. Examine the colours of liquid in diffused light by viewing vertically against a white background.

Colourless Solution: A solution is considered colourless if it has the same appearance as water or the solvent used for preparing the solution or is not more intensely coloured than Reference Solution BS8.

Table No. 01

Colour of  Reference Solution	Reference solution	FCS (ml)	CCS (ml)	CSS (ml)	Hydrochloric Acid (1% w/v HCI) (ml)
Yellow             Greenish   Yellow       Brownish  Yellow           Brown               Red	YS1 YS2 YS3 YS4 YS5 YS6 YS7 GYS1 GYS2 GYS3 GYS4 GYS5 GYS6 GYS7 BYS1 BYS2 BYS3 BYS4 BYS5 BYS6 BYS7 BS1 BS2 BS3 BS4 BS5 BS6 BS7 BS8 RS1 RS2 RS3 RS4 RS5 RS6 RS7	24.0 18.0 12.0 6.0 3.2 1.6 0.8 24.0 14.5 8.5 5.0 5.8 2.9 2.9 24.0 18.0 12.0 6.0 3.0 1.5 1.0 22.5 15.0 11.2 7.5 3.7 1.5 0.8 0.4 10.0 7.5 5.0 3.8 2.5 1.3 0.5	6.0 4.5 3.0 1.5 0.8 0.4 0.2 0.5 0.1 0.05 0.05 0.05 0.05 0.05 10.0 7.5 5.0 2.5 1.5 0.8 0.4 22.5 15.0 11.2 7.5 3.7 1.5 0.8 0.4 20.0 15.0 10.0 7.6 5.0 2.6 1.0	0 0 0 0 0 0 0 0.5 0.1 0.05 0.05 0.05 0.05 0.05 4.0 3.0 2.0 1.0 0.5 0.2 0.1 18.0 12.0 9.0 6.0 3.0 1.2 0.6 0.2 0 0 0 0 0 0 0	70.0 77.5 85.0 92.5 96.0 98.0 99.0 75.0 85.8 91.4 94.9 194.1 197.0 397.0 62.0 71.5 81.0 90.5 95.0 97.5 98.5 37.0 58.0 68.6 79.0 89.6 95.8 97.8 99.0 70.0 77.5 85.0 88.5 92.5 96.1 98.5

 

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