Limit Test for Lead—Principle, Procedure & Observation | P.I.C.| B.Pharma. 1st sem | Pharma Hub +

Limit test -
This is a quantitative or semi quantitative test designed for identification and control of impurities present in the compound.

In limit test, we compare test sample and standard sample & observe them.

In this video we are going to study about Limit test for Lead.

Principle—
Limit test of Lead is based on the reaction of lead and dithizone in the presence of alkali to form Lead Dithizone complex which appears as red color in the solution.

Actually, the color of dithizone is green & the color of lead-dithizone complex is violet so they both appear as red in the solution.

Requirements -
Reagents
standard lead solution
ammonia sol
ammonium citrate
hydroxylamine hydrochloride
potassium cyanide
standard dithizone solution
1% v/v solution of Nitric Acid
dithizone extraction solution.

Apparatus
2 nessler cylinder
glass rod
beakers
separating flasks

Procedure
1. For test sample
Transfer the given quantity of sample in 'T' marked nessler cylinder.
Add 6 ml of ammonium citrate.
  - Add 2 ml of potassium cyanide and 2 ml of hydroxylamine hydrochloride.
  - Make the solution alkaline by adding ammonia solution.
Extract with 5 ml of dithizone extraction solution, until it becomes green.
Dithizone extracts are shaken for 30 minutes 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.
Shake for 30 minutes.
Observe the colour.  

2. For standard sample
A standard lead solution (1 ppm Pb) is prepared equivalent to the amount of lead permitted in the sample under examination.
Add 6 ml of ammonium citrate.
  - Add 2 ml of potassium cyanide and 2 ml of hydroxylamine hydrochloride.
  - Make the solution alkaline by adding ammonia solution.
Extract with 5 ml of dithizone extraction solution, until it becomes green.
Dithizone extracts are shaken for 30 minutes 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.
Shake for 30 minutes.
Observe the colour.  

Compare the intensity of the color of both cylinders.

If the intensity of red color developed in test sample is less than that of the standard , the sample will pass the test or vice versa.

Thank you for watching
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