A nonaqueous differentiating titrimetric procedure is presented for determining dosage forms containing acetylsalicylic acid and barbiturate combinations. Preliminary extraction of the active components is avoided. The titrant is sodium methoxide in benzene-methanol and the titration solvent is methyl is0 butyl ketone. Titrations are effected potentiometrically. An additive procedure is suggested when the ratio of acetylsalicylic acid to barbiturate is greater than 10 to 1. T h e effect of additives and other active constituents is considered. HE A.O.A.C. method for the analysis of acetyl-Tsalicylic acid and phenobarbital tablets is a spectrophotometric procedure involving preliminary separation of the components on a column of diatomaceous earth1 which has been treated with dipotassium phosphate. It is based on a procedure developed by Banes (1) for determining phenobarbital in the presence of salicylates, and modified by Byers (2). Although satisfactory results are obtainable b y this procedure, i t is tedious and time-consuming.
In a previous paper (3) synthetic mixtures of acetylsalicylic acid and a variety of barbiturates were analyzed b y a differentiating nonaqueous titration procedure. Titration was effected with a Fisher titrimeter equipped with a glass-calomel or antimony-calomel electrode system. The titration solvent was methyl isobutyl ketone, and the titrant was sodium methoxide in benzenemethanol ( 1 O : l ) . In the present study the method is applied t o numerous dosage forms which contain acetylsalicylic acid and barbiturates in addition t o additives, excipients, and other active constituents. The dosage form is titrated differentially without preliminary extraction of the active constituents. T h e effect of interfering substances on the sensitivity of the procedure is considered.
EXPERIMENTAL
Apparatus.-All titrations were performed po- tentiometrically with a Fisher titrimeter, model 35,