Limit test for Lead

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.

Principle:

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.

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 drops of phenol red

Add 2 ml of potassium cyanide and 2 ml of hydroxylamine Hydrochloride

 

Add 2 drops of phenol red

Make solution alkaline by adding

ammonia solution.

Extract with 5 ml of dithizone until

it becomes green

Make solution alkaline by adding ammonia solution.

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

Add 4 ml of ammonium cyanide

To the acid solution add 5 ml of standard dithizone solution

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 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.

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 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.

References:

https://www.14impressions.in/2020/12/different-types-of-limit-tests.html
https://www.web-formulas.com/Formulas_of_Chemistry/Limit_Test_of_Lead.aspx
https://www.slideshare.net/eshadesai5/impurities-and-their-limit-test
https://books.google.com/books?id=l6KHDwAAQBAJ

 

Also Read:

Limit Test for Heavy Metals

Limit Test for Iron

Limit Test for Sulphates

Limit Test for Chlorides

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