Lead is one of the toxic heavy metals and its presence is undesirable in medicinal substances. The lead contamination can occur due to the storage in glass bottles, lead-lined apparatus and also due to the use of sulfuric acid.
Principle of Limit test for lead
As per B.P, the principle of the limit test is dependent on the formation of brownish coloration when sodium sulfide is added to the dilute solution containing lead salts. The intensity of color varies based on the quantity of lead present.
If the quantity of leads salts is greater than traces of limits, a brownish black colloidal precipitate of lead sulfide.
The color produced is compared with that produced by a standard solution having a known quantity of lead.
Procedure for limit test for lead:
Two separate solutions of the test substance are prepared as a primary solution and auxiliary solution in two 50ml Nessler cylinders.
The primary solution contains a known amount yet higher amount of substance than the auxiliary solution.
Both the solutions are prepared by using hot water and acetic acid. To the auxiliary solution, a fixed amount of dilute lead nitrate solution is added. Then ammonia and potassium cyanide solutions are added to both the solutions. If the solutions are colored, a small amount of burnt sugar is added to both to clear color variations. They are made upto 50ml and if they are turbid, they are filtered and once again made up to 50ml. Both solutions are treated with sodium sulfide solutions to develop the color. The color of the primary should not be greater than the auxiliary to pass the test.
| Steps | Primary Cylinder | Auxiliary Cylinder |
| 1 | Dissolve 4gm of sample in water containing th 10ml of acetic acid. | Dissolve 2gm of the sample in water containing 10ml of acetic acid. |
| 2 | Make the solution alkaline with ammonia solution and then add 1ml of potassium cyanide solution. | Make the solution alkaline with ammonia solution and then add 1ml of potassium cyanide solution. |
| 3 | If the solutions are colored, add small amount of burnt sugar. If turbid then filter the solution. | If the solution is colored, add small amount of burnt sugar. If turbid, then filter the solution. |
| 4 | Make up the solution to 50ml with water and then add 2ml of sodium sulfide and mix. | Dilute to 50 ml with water, add 2ml of sodium sulfide and then mix. |
| 5 | Compare the color with auxiliary | Compare the color with the primary. |
The procedure as per USP:
The principle is based on a reaction between lead and dithizone solutions to form a violet color.
The test and standard solutions are prepared in two separating funnels.
The fixed quantity of sample solution is taken into a separating funnel and to it, 6ml of ammonium citrate, 2ml potassium cyanide, 2ml of hydroxylamine hydrochloride, 2 drops of phenol red are added. The pH of the solution is made alkaline by adding ammonia solution. To this 5ml portions of dithizone solution is added till the entire solution turns green.
To this add 30ml of 1N nitric acid shake for 30 seconds and discard the chloroform layer. To the acidic portion of the solution, add 5ml of dithizone solution, 4ml of ammonium cyanide and shake for 30 seconds.
Prepare a control solution by a known amount of dilute lead solution and repeat as that of the test. The color of the chloroform layer of the test solution should not be deeper than that of the control solution.
Important points:
1. The hot water helps to dissolve the lead faster.
2. Acetic acid is a good solvent for the lead.
3. All the reagents and solvents used in the procedure should be lead-free.
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