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An assay of mixtures of phenobarbital and pentobarbital is described which exploits the difference in rate of degradation of the drugs in alkali. The procedure entails measurement of peak absorbance of dilutions (pH 10) of phenobarbital and a mixture of the drugs before and after heating them in 0.1M trisodium phosphate for 15 minutes. In four trials of the procedure on replicated samples, the average recovery of the phenobarbital was 97.3% (range 89.5–105.2%) and of the pentobarbital 98.1% (range 94.0–101.0%). Synthetic tablet mixtures containing phenobarbital-pentobarbital combinations were assayed with satisfactory results after chromatographic separation of the barbiturates from the mixture.

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