pKa determination of verapamil by liquid-liquid partition

The pKa of verapamil was determined by measuring the partition coefficient of verapamil between n-heptane and aqueous buffer solution at various pH values. The magnitude of the effects of ionic strength or temperature on the pKa of verapamil was in agreement with those reported previously. The estimated pKa of verapamil in human plasma was 8.75. The pKa of 2,4,6-trimethylpyridine measured by means of the present partition method was in good agreement with that determined by UV spectrophotometry.