Analytical Sciences

Determination of Acid/Base Dissociation Constants Based on a Rapid Detection of the Half Equivalence Point by Feedback-Based Flow Ratiometry

Hideji TANAKA and Takahiro TACHIBANA

Faculty of Pharmaceutical Sciences, Tokushima University, Shomachi 1-78-1, Tokushima 770-8505, Japan

Acid dissociation constants (K_a) were determined through the rapid detection of the half equivalence point (EP_1/2) based on a feedback-based flow ratiometry. A titrand, delivered at a constant flow rate, was merged with a titrant, whose flow rate was varied in response to a control voltage (V_c) from a controller. Downstream, the pH of the mixed solution was monitored. Initially, V_c was increased linearly. At the instance that the detector sensed EP_1/2, the ramp direction of V_c changed downward. When EP_1/2 was sensed again, V_c was increased again. This series of process was repeated automatically. The pH at EP_1/2 was regarded as being pK_a of the analyte after an activity correction. Satisfactory results were obtained for different acids in various matrices with good precision (RSD ≈3%) at a throughput rate of 56 s/determination.

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