Lead Sulphide Test
Objective
 This test is for sulphur containing amino acids cysteine
and cystine. Methionine does not respond to this
test.
Principle
 Sulphur containing amino acids, such as cysteine and
cystine upon boiling with sodium hydroxide (hot
alkali) yield sodium sulphide. This reaction is due to
partial conversion of the organic sulphur to inorganic
sulphide, which can detected by precipitating it to
lead sulphide, using lead acetate solution.
 Cysteine or cystine respond to this test but the
sulphur present in the methionine doesn’t respond to
this test.
 Casein and Gelatin which contain metheonine give
negative test
Reagents
 NaOH 40% solution in H2O
 Lead Acetate solution
Procedure
 Take 2 ml of give solution in test tube.
 Add 2ml of 40% NaOH solution.
 Boil for 2 minutes
 Add ml of lead acetate solution.
Interpretation
 Brown or black ppts formed if cysteine or cysteine
containing protein is present in the solution