Ferric Chloride Test:

  • Acidify the food product with HCl (1+3) and extract with diethyl ether. Evaporate the solvent on a hot water bath removing last traces of solvent under a current of air. Dissolve the residue in few ml of hot water and add few drops of 0.5% ferric chloride solution. Salmon colour precipitate of ferric benzoate indicates the presence of benzoic acid.

Modified Mohler’s Test:

  • To the aqueous solution of the residue obtained as given under method ‘A’ add one or two drops of 10% NaOH solution and evaporate to dryness. To the residue add 5-10 drops of H2SO4 and a small crystal of potassium nitrate. Heat for 10 minutes in a glycerol bath at 120 – 130 º C. Cool, add 1 ml of water and make distinctly ammonical. Boil the solution to decompose any ammonium nitrite (NH4NO2) formed. Cool and add a drop of fresh colourless ammonium sulphide [(NH4)2S] solution. The sulphide solution can be made by passing H2S in 0.88 ammonia. Do not let the layers mix. Red brown ring indicates benzoic acid. On mixing, colour diffuses throughout the liquid and on heating finally changes to greenish yellow. This change differentiates benzoic acid from salicylic acid cinnamic acid. Salicylic acid and cinnamic acid form coloured compounds which are destroyed on heating.