Determination of dissociation constants of weak acids by feedback-based flow ratiometry

Abstract

We applied the principle of feedback-based flow ratiometry, originally developed for continuous titrations, to the determination of acid dissociation constants ( K a). The titrant (NaOH) flow rate F B was varied in response to a control voltage V c from a controller, while the total (titrant+titrand) flow rate F T was held constant. The titrand was aspirated to the flow system at the flow rate of F T− F B, and mixed with the titrant at the confluence point. Downstream, the pH of the mixed solution was measured with a glass electrode. Initially, V c (thus F B) was ramped upward linearly. At the instant the detector sensed the half equivalence point, Eq 1/2, where the buffer capacity is maximum, the algorithm reversed the ramp direction of V c downwards. When Eq 1/2 was sensed again, the algorithm reversed the ramp direction of V c again, upwards. By repeating these processes automatically, time utilization efficiency of the procedure was improved by scanning V c only in the range of interest. When the V c scan range was thus constrained, the pH values corresponding to either a maximum or minimum in V c, were equal to the p K a of the analyte. The p K a values thus obtained for various n-alkyl carboxylic acids and phosphoric acid agreed well with the literature. High throughput (ca. 26 s per determination) was attainable with reasonable precision (R.S.D.<3%).