Question

why should excess alkali be avoided in the ferric chloride test for acetic acid and benzoic acid?

Answer 1

The reason the test works is because the Fe(3+) ion formed reacts with the benzoate ion formed to give a coloured solution.

However, in an excess of alkali, the Fe(3+) ion instead reacts with the OH(-) ions in order to form ferrous hydroxide, which interferes with the test and coloured compound formation.

Answer 2

Fe(OH)3 is ferric hydroxide.
Fe(OH)2 is ferrous hydroxide.