Development and evaluation of a spectrophotometric method for the determination of free phthalic acid in potassium hydrogen phthalate

Free phthalic acid is a common impurity in samples of recognised analytical grade potassium hydrogen phthalate. If the pH error of a 0.05 m solution is to be –0.001 or less the free acid content must not exceed 0.025%m/m. A simple ether extraction-ultraviolet spectrophotometric procedure for determining the free phthalic acid content has been developed. The method is capable of detecting less than 0.0003%m/m of free acid and is reproducible to ±8%. A close correlation between the free acid content, the pH and the acidimetric purity has been confirmed.