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Limit test for Lead

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Limit test for Lead

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


The limit test of lead is based on the reaction of lead and diphenylthiocabazone (dithizone) in alkaline solution to form lead dithizone complex which is read in color. Dithizone is green in color in chloroform and leaddithizone complex is violet in color, so the resulting color at the end of the process is red.


Test sampleStandard compoundA known quantity of sample solution is transferred in a separating funnelA standard lead solution is prepared equivalent to the amount of lead permitted in the sample under examinationAdd 6ml of ammonium citrateAdd 6ml of ammonium citrateAdd 2 ml of potassium cyanide and 2 ml of hydroxylamine hydrochloride

Add 2 drops of phenol redAdd 2 ml of potassium cyanide and 2 ml of hydroxylamine Hydrochloride

Add 2 drops of phenol redMake solution alkaline by adding

ammonia solution.

Extract with 5 ml of dithizone until

it becomes greenMake solution alkaline by adding ammonia solution.

Extract with 5 ml of dithizone until

it becomes greenCombine dithizone extracts are shaken for 30 mins with 30 ml of nitric acid and the chloroform layer is discardedCombine dithizone extracts are shaken for 30 mins with 30 ml of nitric acid and the chloroform layer is discardedTo the acid solution add 5 ml of standard dithizone solution

Add 4 ml of ammonium cyanideTo the acid solution add 5 ml of standard dithizone solution

Add 4 ml of ammonium cyanideShake for 30 minsShake for 30 minsObserve the colorObserve the color


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


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 an indicator to develop the color at the
end of the process. Lead present as impurities in the substance gets separated by extracting an alkaline solution with a dithizone extraction solution.


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