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.
why should excess alkali be avoided in the ferric chloride test for acetic acid and benzoic acid?
2 answers
Fe(OH)3 is ferric hydroxide.
Fe(OH)2 is ferrous hydroxide.
Fe(OH)2 is ferrous hydroxide.