Continuous on-line true titrations by feedback based flow ratiometry: application to potentiometric acid–base titrations Original Research Article

We applied the methodology of continuous titrations by feedback-based flow ratiometry and the principle of compensating errors [Anal. Chem. 72 (2000), 4713] to potentiometric acid–base titrations. In a typical case, the titrant (a standard base) flow (FB) is varied in response to a controller output voltage, while the total flow (FT, consisting of an acid sample and the titrant flow) is held constant. The titrant flow is initially ramped upward linearly. At the instant, the detector senses the equivalence potential is reached, the controller output (instantaneous value VH) reverses its ramp direction, thus decreasing the titrant flow linearly at the same rate. When the equivalence point is sensed again, the controller voltage (instantaneous value VL) is ramped in reverse once more, going upward. Because of the lag time between a change in the controller output and its outcome being sensed by the detector, the controller voltage corresponding to the actual equivalence point is the average of VH and VL. Continuous sensor-governed operation of the controller thus results in a triangular waveform. The mean dc bias of this waveform during any cycle gives the equivalence point controller voltage VE. This principle of compensating errors allows true titrations in principle, requiring no calibration curves. Even though pH electrode response is slow compared with optical sensors, high throughput (12 s per titration) is attainable with reasonable reproducibility (0.7% R.S.D.). The method was applied to diverse acid–base titrations.