Second-derivative spectrophotometric assay of mixtures of dicloxacillin sodium and ampicillin sodium in pharmaceuticals.

'Zero-crossing' derivative spectrophotometry has been used for determining binary mixtures of dicloxacillin Na and ampicillin Na, which are penicillins with closely overlapping absorption spectra. The procedure is rapid, simple, nondestructive, and does not require solutions of equations. Calibration graphs are linear (r = 0.9999), with a zero intercept for up to 60 micrograms/mL of each antibiotic. Detection limits at the p = 0.05 level of significance were calculated to be 0.29 and 0.31 microgram/mL of dicloxacillin Na and ampicillin Na, respectively. The method was successfully applied to the assay of commercial injections and capsules for these drugs.