Colorimetric Determination of Benzoic Acid by Application the Janovsky Reaction

Abstract

Benzoic acid (BA) was converted quantitatively to dinitrobenzoic acid with nitrate in sulfuric acid. This compound was produced characteristic color with alcohol, acetone and alkali. A linear relation was confirmed between the absorbance and the amount of BA.
The procedure was as follows. Up to 3ml of acetone containing up to 3mg of BA was taken in a test tube and evaporated with dried air.
The residue was heated in a boiling water-bath for 40 minutes with 0.2g of potassium nitrate and 0.5ml of sulfuric acid, and the mixture was transferred with 30ml of water into a separatory funnel and shaken with 10ml of iso-butanol for 30 seconds.
After washing with 10ml of 1% sulfuric acid, the iso-butanol layer was separated into a centrifuge tube and was shaken with 0.1g of calcium carbonate for neutralization.
1ml of centrifuged iso-butanol layer was introduced into a colorimetric tube with 2ml of acetone and 1ml of 0.05N sodium hydrooxide.
The absorbance was measured at 555mμ every 5 minutes, after let, for 20 minutes at 20-30°C (four times). The concentration of BA was determined at the maximal absorbance.