Question

why was lead nitrate positive for egg albumin but not gelatin?

Answer 1

The lead nitrate test is generally used to find out whether Sulphur is presnt or not in the compuond (amino acids). Lead precipitates out as a black lead sulphide (Pbs) precipitate. The lead nitrate reacts with sulphur in egg albumin. The oxidatio state of Pb is +2.

NOTE: Lead nitrate is highly poisonous hence special care has to be taken for its handling.

Test: take NaOH solution in dry test tube.

NaOH has Na⁺ and OH^- ions in the solution.

NaOH---------> Na⁺ + OH^-

After the addition of egg albumin, the reaction becomes,

2Na⁺ + S----------> Na₂S--------> 2Na⁺ + S^2-

The Na⁺ reacts with the sulphur in egg albumin and we get Na₂S (Sodium sulphide) which inturn breaks and forms Na⁺ ans S^2- (sulphide).

Next, S^2- reacts with Pb²⁺ from the lead nitrate and a black precipitate of PbS (lead sulphide) is obtained.

Reaction: S^2- + Pb²⁺ --------> PbS

This confirms the presence of sulphur in egg allbumin. However, sulphur is not present in gelatin. Thus this reaction does not occur in gelatin and it shows a negative test for sulphur.