Title: Limit Test for Lead
- Objective: The limit test for lead is provided to demonstrate that the content of lead does not exceed the limit given in the individual monograph in terms of ppm, i.e. the parts of Lead, Pb, per million parts (by weight) of the substance under examination. The standard solution against which the comparison of colour is made contains 1ppm of lead.
- Principle: Limit test of Lead is based on the reaction of Lead and Diphenyl thiocarbazone (dithizone) in alkaline solution to form lead dithizone complex which is red in colour.
3. Procedure:
- Preparation of Standard Solutions:
Note: All solutions should be prepared in Distilled/Purified Water.
- Ammonium Citrate Solution Sp.:Dissolve 40 g of Citric acid in 90 ml of water. Add 2 drops of Phenol Red Solution and then add slowly strong Ammonia solution until the solution acquires a reddish colour. Remove any lead that may be present by extracting the solution with successive quantities, each of 30 ml portions of Dithizone Extraction Solution, until the dithizone solution retains its orange-green colour.
- Lead Standard Solution (1% Pb): Dissolve 0.4 g of lead nitrate in water containing 2 ml conc. nitric acid and add sufficient water to produce 250.0 ml.
- Lead Standard Solution (100 ppm Pb): Dilute 1 volume of lead standard solution (1% Pb) to 10 volumes with water immediately before use.
- Lead Standard Solution (10 ppm Pb): Dilute 1 volume of lead standard solution (100 ppm Pb) to 10 volumes with water immediately before use.
- Lead Standard Solution (1 ppm Pb): Dilute 1 volume of lead standard solution (10 ppm Pb) to 10 volumes with water immediately before use.
- Dithizone Extraction Solution: Dissolve 30 mg of Dithizone in 1000 ml of Chloroform and add 5 ml of ethanol (95%). Store the solution in a refrigerator. Before use, shake a suitable volume of the Dithizone extraction solution with about half its volume of a 1% v/v solution of Nitric acid and discard the acid.
- Hydroxylamine Hydrochloride Solution Sp.:Dissolve 20 g of Hydroxylamine Hydrochloride in sufficient water to make approximately 65 ml. Transfer to a separator, add 5 drops of Thymol blue solution and strong Ammonia solution untill the solution becomes yellow. Add 10 ml of 4% w/v solution of Sodium Diethyldithiocarbamate and allow to stand for 5 minutes. Extract this solution with successive quantities, each of 10 ml, of chloroform until a 5-ml portion of the Chloroform extract does not acquire a yellow colour when shaken with Dilute Cupric Sulphate Solution. Add Dilute Hydrochloric acid until the solution is pink and then with sufficient water to produce 100 ml.
- Potassium Cyanide Solution Sp.:Dissolve 50 g of potassium cyanide in sufficient water to make 100 ml. Remove the lead from this solution by extraction with successive quantities, each of 20 ml of Dithizone Extraction Solution until the Dithizone Extraction Solution retains its orange green colour. Extract any dithizone remaining in the cyanide solution by shaking with chloroform. Finally, dilute this cyanide solution with sufficient water so that each 100 ml contains 10 g of potassium cyanide.
- Dithizone Standard Solution: Dissolve 10 mg of dithizone in 1000 ml of chloroform. Store the solution in a glass-stoppered, lead-free bottle, suitably wrapped to protect it from light and store in a refrigerator.
- Dilute Cupric Sulphate Solution (10.0% w/v): Dissolve 10.0 g of Cupric Sulphate Copper (II) Sulphate in water and dilute to 100 ml with water.
- Thymol blue solution: Dissolve 0.1 g of Thymol Blue in 2.15 ml of 0.1 M Sodium Hydroxide and 20 ml of ethanol (95%). Shake well to dissolve and add sufficient water to produce 100 ml.
- Phenol Red Solution: Dissolve 0.1 g of Phenol Red in 2.82 ml of 0.1 M Sodium Hydroxide and 20 ml of ethanol (95%). Shake well to dissolve and add sufficient water to produce 100 ml.
- Dilute Hydrochloric acid (10%w/w): Dilute 26 ml of Hydrochloric acid to 100 ml with water.
- Method:
Transfer the Test Preparation, rinsing with 10 ml of water, or the volume of the prepared sample specified in the monograph to a separator, and, unless otherwise directed in the monograph, add 6 ml of Ammonium Citrate Solution Sp. and 2 ml of Hydroxylamine Hydrochloride Solution Sp (For the determination of Lead in Iron salts use 10 ml of Ammonium Citrate Solution Sp). Add 2 drops of Phenol Red Solution, and make the solution just alkaline (red in colour) by addition of strong Ammonia solution. Cool the solution if necessary, and add 2 ml of Potassium Cyanide Solution Sp. Immediately extract the solution with several quantities each of 5-ml portions
of Dithizone Extraction Solution, draining off each extract into another separating funnel, until the Dithizone Extraction Solution retains its green colour. Shake the combined Dithizone solutions for 30 seconds with 30 ml of a 1% v/v solution of Nitric acid (1 ml in 100 ml), and discard the chloroform layer. Add to the acid solution 5 ml of Dithizone Standard Solution and shake for 30 seconds.
Limit: The colour of the chloroform layer is not more intense than that obtained by treating in the same manner a volume of Lead Standard solution (1 ppm Pb) equivalent to the amount of lead permitted in the substance under examination, in place of the solution under examination.