Simultaneous determination of chlorzoxazone and acetaminophen in combined dosage forms by an absorbance ratio technique and difference spectrophotometry.

Two spectrophotometric methods have been developed for the simultaneous determination of chlorzoxazone and acetaminophen in their combined dosage forms. No preliminary separation step is required in either method. The first, an absorbance ratio technique using the isoabsorptive point as one of the wavelengths, together with "Q curve" analysis gives accurate and reproducible results for both drugs. The second, a difference spectrophotometric method is based on measurement of absorbance of an alkaline solution relative to that of an acidic solution of identical concentration of the sample at two different wavelengths. The method is suitable for routine analysis of such combinations of the drugs.