Limit Tests for Lead
Theory : The offcial test is based on the conversion of traces of lead salts present in the pharmaceutical substances to lead sulphide, which is obtained in colloidal form by the addition of sodium sulphide in an alkaline medium achieved by a fairly high concentration of ammonium acetate. The reaction may be expressed as follows :
PbCl2 + Na2S → PbS (Dec) + 2NaCl
The brown colour, caused due to colloidal lead sulphide in the test solution is compared with that produced from a known amount of lead.
Equipment : Nessler Cylinders (or Nessler Glasses) : According to the British Standard Specification No : 612, 966—a pair of cylinders made of the same glass and having the same diameter with a graduation mark at the same height from the base in both cylinders (Figure 1).
The final comparison is made by viewing down through the solution against a light background.
Materials Required :
(i) Lead Nitrate Stock Solution : Dissolve0.1598 g of lead nitrate in 100 ml DW to which has been added 1 ml nitric acid, then dilute with water to 1 Litre.
Note : The solution must be prepared and stored in polyethylene or glass containers free from soluble lead salts.
(ii) Standard Lead Solution : On the day of use, dilute 10.0 ml of lead nitrate stock solution with DW to 100.0 ml. Each ml of standard lead solution contains the equivalent of 10 microgrammes of lead. A control comparison solution prepared with 2.0 ml of standard lead solution contains, when compared to a solution representing 1.0 g of the substance being tested, the equivalent of 20 parts per million of lead.
(iii) Standard Solution : Into a 50 ml Nessler Cylinder, pipette 2 ml of standard lead solution and dilute with DW to 25 ml. Adjust with dilute acetic acid Sp. (IP)* or dilute ammonia solution Sp. (IP) to a pH between 3.0 and 4.0, dilute with DW to about 35 ml and mix.
(iv) Test Solution : Into a 50 ml Nessler Cylinder, place 25 ml of the solution prepared for the test as directed in the individual monograh, dissolve and dilute with DW to 25 ml the specified quantity of the substance being tested. Adjust with dilute acetic acid Sp. (IP) or dilute ammonia solution Sp. to a pH between 3.0 and 4.0, dilute with DW to about 35 ml and mix.
Procedure : To each of the cylinders containing the standard solution and test solution respectively, add l0 ml of freshly prepared hydrogen sulphide solution, mix, dilute with water (DW) to 50 ml, allow to stand for 5 minutes and view downwards over a white surface, the colour produced in the test solution is not darker than that produced in the standard solution.
A few typical examples from the official compendium are given below :