Determination of acid/base dissociation constants based on a rapid detection of the half equivalence point by feedback-based flow ratiometry

Acid dissociation constants (K-a) were determined through the rapid detection of the half equivalence point (EP1/2) 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 EP1/2, the ramp direction of V-c changed downward. When EP1/2 was sensed again, V-c was increased again. This series of process was repeated automatically. The pH at EP1/2 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 approximate to3%) at a throughput rate of 56 s/determination.