Question

Why must Group IV be treated with (NH4)2CO3 in basic solution? What would happen if the solution were acidic when the sodium carbonate was added?

Answer 1

The Group IV cations are Ba²⁺, Ca²⁺, and Sr²⁺ These ions form insoluble carbonates. Hence, a soluble carbonate salt has to be used for the precipitation of these ions in order to identify them. As most of the carbonates salts are insoluble, we are left with very few choices which include: lithium, sodium, potassium and ammonium carbonates. Out of these salts, ammonium carbonate is best preferred in alkaline medium because of its dissociation and further equilibrium as:

NH₄⁺ + CO3 ^2- ⇔ HCO₃^- + NH₃

According to this reaction, an increase in the concentration of NH3 will lead the equilibrium to shift towards left, thereby increasing the concentration of CO_{3 ^2-}  for the easy precipitation of Group IV ions. NH₃ is passed into the solution for this purpose.

This reaction is not carried out in acidic medium, simply because carbonates of Group IV ions are readily soluble in acidic medium. Thus, there will be no formation of the precipitate and the test will be failed.