Determination of Ampicillin in the Presence of Cloxacillin

A spectrophotometric method is described for the assay of ampicillin in the presence of cloxacillin in pharmaceutical preparations. It involves the reaction of hydrolysed ampicillin with formaldehyde in an acidic phosphate buffer (pH 2.5) and measurement of the absorbance of the product at 373 nm after dilution with 2 M sulphuric acid. The method is selective for ampicillin in the presence of cloxacillin. The absorbance had a linear relationship with concentration for the range studied (5-35 micrograms ml-1). By this method the ampicillin content of combined preparations of ampicillin and cloxacillin has been successfully determined in capsules, injections and a syrup.