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A new analytical technique for the assay of pharmaceutical substances at low concentration levels has been proposed and applied to the determination of vitamin C as an example of its application. It is based on the chemical reaction of vitamin C with iodine in a sodium citrate-hydrochloric acid medium, followed by measurement of the frequency change of a silver-plated piezoelectric quartz crystal. The frequency change is proportional to the vitamin C concentration from 2 x 10(-7) to 4 x 10(-6) M. The method was applied to the assay of vitamin C in pharmaceutical preparations.

According to the response properties of piezoelectric crystal quartz sensors to the solution conductivity the frequency shift response was derived to show a linear dependence on the concentrations of vitamin B1 (VB1) and vitamin C (VC). This was experimentally verified and a calibration model for simultaneous determinations of the two vitamins was established. The usefulness of the technique was evaluated by quantitation of mixtures of unknown composition using common multiple linear regression (MLR) and partial least squares (PLS). The average relative standard deviations for six samples were 4.46% for VB1 and 6.63% for VC with MLR; and 1.97% for VB1 and 2.74% for VC with PLS, respectively.