Question

Why is it necessary to use acetic acid and not sulphuric acid for acidification of sodium extract for testing sulphur by lead acetate test?

Answer 1

Hint: To solve this question, you need to first understand the objectives and the mechanisms behind the lead acetate test. Then we need to compare the properties of sulphuric acid to the reagent currently being used, to draw a conclusion for not choosing sulphuric acid.

Complete Step-by-Step Answer:
Before we move forward with the solution of the given question, let us first understand some important basic concepts.
Lead acetate test is carried in order to find the presence of sulphur - based compounds like hydrogen sulphide. The entire procedure for lead acetate test can be explained as follows:
Pieces of metallic sodium are gently heated in the fusion tube until they dissolve in the shiny globules. At this point, a small amount of material is added and the tube is heated strongly. The bright red tube is immersed in distilled water in a porcelain dish. Boil the contents for a while, allow to cool, and then filter. The filtrate is called sodium extract or Lassaigne's extract. The extract contains sodium sulphide formed by the reaction of sulphur (present in the compound) with sodium, can be given as:
\[2Na + S \to N{a_2}S\]
Now that we have obtained the extract from the previous reaction, we can use it for our lead acetate test. This Lassaigne's extract is initially acidified with acetic acid and is then made to react with lead acetate to form lead sulphide and sodium ethanoate. This lead sulphide is a black precipitate and is the indicator for the presence of sulphur. The chemical reaction for the same can be given as:
\[N{a_2}S + Pb{(C{H_3}COO)_2} \to 2C{H_3}COONa + PbS \downarrow \]
Now, in the step where we acidify the Lassaigne's extract with acetic acid, if we use something like sulphuric acid for acidification, then the lead acetate used in the next step would first react with sulphuric acid. This reaction would result in the formation of a compound named lead sulphate, which would be formed as a white precipitate. This defies the entire purpose of the test in the first place.
Hence, it is necessary to use acetic acid and not sulphuric acid for acidification of sodium extract for testing sulphur by lead acetate test

Note: The Lassaigne's extract that we obtained in this test can be used for detection of nitrogen as well. We need to first boil the sodium extract with iron sulphate and then acidify it with concentrated sulphuric acid. If a Prussian blue coloured extract is observed, then the test for nitrogen is positive.