What Is Lead Sulfide Test?
Lead sulfide test also referred to as lead acetate test is a biochemical test for the detection of amino acids like cysteine and cystine. The test is a specific test for the detection of amino acids containing sulfur, S-S group in cysteine and S-H group in cysteine. It is important to note that even though the test is specific for the detection of amino acids, methionine does not give a positive result in this test.
Objective Of Lead Sulpfide Test
- To detect the presence of sulfur-containing amino acids in a sample.
- To detect protein -containing cysteine and cystine in a given sample.
- To distinguish between sulfur-containing and non-sulfur containing amino acids.
Principle Of Lead Acetate Test
The sulfur-containing amino acid such as cysteine, cysteine, and methionine (sulfhydryl/thiol group) reacts with lead acetate under alkaline conditions to form a brown precipitate. These sulfur-containing amino acids are degraded in strongly alkaline media to release sulfide ion (S2-) in the form of H2S (hydrogen sulfide). The sulfide ions can react with lead (II) acetate to form a brownish-black precipitate.
Reagent And Material Required
- 2% lead acetate solution in water
- 40% NaOH
- Test Sample/solution
- Test tubes
- Test tube stand
Procedure For Lead Acetate Test
- Add 1ml test solution in dry test tube.
- Similarly, take 1 ml distilled water in another test tube as control.
- Add 2ml of 40 % NaOH and mix well.
- Now add 1ml Foli’s reagen (lead acetate) to all test tubes.
- Heat over the flame of Business burner.
- Observe any color change and note it down.
Lead Acetate Test Results Interpretation
- Positive Test: A positive test of the Lead sulfide test is indicated by the formation of black precipitate at the bottom of the test tube. This confirms the presence of cysteine or cystine in the solution.
- Negative Test: A negative result of the Lead sulfide test is indicated by the absence or lack of black residue in the test tube. This confirms the absence of cysteine or cystine.
Uses of Lead Sulfide Test
- The test is used to detect sulfur-containing amino acids like cysteine and cystine.
- It helps to distinguish between different groups of amino acids.
- The detection of cystine in urine is a pathological symptom of diseases like cystine stones in the kidneys and bladder.