Lead is one of the most undesirable impurities and enters through storage containers like bottle caps, as well as some apparatus.
It has two different variants of limit tests. The first one is more specific for lead.
- Test.1: Reaction with Dithizone: This test is official in IP and USP.
Reaction for the Limit Test for Lead
Principle of Limit Test for Lead
Limit Test for Lead is based upon the chemical reaction between lead and diphenyl thiocarbazone (dithizone) in an alkaline solution to form lead dithizone, which is red. (Dithizone itself is green in colour and the lead dithizone formed is violet in colour. Thus, the net resultant colour of the solution becomes red). The lead present in the impurity is first extracted using the dithizone extraction solution. To avoid interference by other metals and make the pH optimum, reagents like ammonium citrate, KCN, and NH₂OH.HCI is employed. Phenol red is used as an indicator to develop colour at the end of the process.
Procedure of Limit Test for Lead
Take two 50 ml Nessler cylinders. Label one as “Test” and the other as “Standard”
| Standard | Test |
| 1. A standard lead solution (1 ppm Pb) is prepared equivalent to the amount of lead permitted in the sample under examination. | 1. A known quantity of the sample solution is transferred in a separating funnel. |
| 2. Add 6 ml of ammonium citrate. | 2. Add 6 ml of ammonium citrate. |
| 3. Add 2 ml of potassium cyanide and 2 ml of hydroxylamine hydrochloride. | 3. Add 2 ml of potassium cyanide and 2 ml of hydroxylamine hydrochloride. |
| 4. Make the solution alkaline by adding ammonia solution. | 4. Make the solution alkaline by adding ammonia solution. |
| 5. Extract with 5 ml of dithizone in 5. chloroform solution, until it becomes green. | 5. Extract with 5 ml of dithizone in 5. chloroform solution, until it becomes green. |
| 6. Dithizone extracts are shaken for 30 minutes with 30 ml of nitric acid and the chloroform layer is discarded. | 6. Dithizone extracts are shaken for 30 minutes with 30 ml of nitric acid and the chloroform layer is discarded. |
| 7. To the acid solution add 5 ml of standard dithizone solution. | 7. To the acid solution add 5 ml of standard dithizone solution. |
| 8. Add 4 ml of ammonium cyanide. | 8. Add 4 ml of ammonium cyanide. |
| 9. Shake for 30 minutes. | 9. Shake for 30 minutes. |
| 10. Observe the color. | 10. Observe the colour. |
Table: Procedure for Limit Test for Lead
Observation
The colour comparison of Standard and Test solutions is done to decide whether the sample passes or fails.
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