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Limit Test of Lead

Lead is a most undesirable impurity in medical compounds and comes through use of sulphuric acid, lead lined apparatus and glass bottles use for storage of chemicals.

Principle:

Limit test of lead is based on the reaction of lead and diphenyl thiocabazone (dithizone) in alkaline solution to form lead dithizone complex which is read in color.

Dithizone is green in color in chloroform and lead-dithizone complex is violet in color, so the resulting color at the end of process is red.


Procedure:

Test sample Standard compound
A known quantity of sample solution is transferred in a separating funnel A standard lead solution is prepared equivalent to the amount of lead permitted in the sample under examination
Add 6ml of ammonium citrate Add 6ml of ammonium citrate
Add 2 ml of potassium cyanide and 2 ml of hydroxylamine hydrochloride Add 2 ml of potassium cyanide and 2 ml of hydroxylamine hydrochloride
 Add 2 drops of phenol red Add 2 drops of phenol red
Make solution alkaline by adding ammonia solution. Make solution alkaline by adding ammonia solution.
Extract with 5 ml of dithizone until it becomes green Extract with 5 ml of dithizone until it becomes green
Combine dithizone extracts are shaken for 30 mins with 30 ml of nitric acid and the chloroform layer is discarded Combine dithizone extracts are shaken for 30 mins with 30 ml of nitric acid and the chloroform layer is discarded
To the acid solution add 5 ml of standard dithizone solution To the acid solution add 5 ml of standard dithizone solution
Add 4 ml of ammonium cyanide Add 4 ml of ammonium cyanide
Shake for 30 mins Shake for 30 mins
Observe the color Observe the color

Observation:

The intensity of the color of complex, is depends on the amount of lead in the solution. The color produce in sample solution should not be greater than standard solution. If color produces in sample solution is less than the standard solution, the sample will pass the limit test of lead and vice versa.

Reasons:

Ammonium citrate, potassium cyanide, hydroxylamine hydrochloride is used to make pH optimum so interference and influence of other impurities have been eliminated.
Phenol red is used as indicator to develop the color at the end of process
Lead present as an impurities in the substance, gets separated bye extracting an alkaline solution with a dithizone extraction solution.

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