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 pK(a) of the analyte. The pK(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%). (C) 2003 Elsevier Science B.V. All rights reserved.