Continuous on-line feedback based flow titrations. Complexometric titrations of calcium and magnesium

The methodology of continuous feedback-based flow titrations and the principle of compensating errors [Anal. Chem. 72 (2000) 4713; Anal. Chim. Acta 435 (2001) 289] were applied to the determination of calcium and magnesium ions with EDTA. The flow of the titrant, EDTA, varied linearly in response to a controller output voltage while the total flow (FT, the sum of the metal ion sample flow and the titrant flow) was held constant. The sample was pre-doped with a metal ion indicator; the status of the indicator color in the mixed stream was monitored by an optical detector and was used for governing the controller output as well as for interpreting the results of the titrations. The titrant flow initially ramped upward linearly. As a change in the color corresponding to the equivalence point was sensed by the detector, the controller output (instantaneous value VH) reversed its ramp direction, thus decreasing the titrant flow linearly at the same ramp rate. When the predefined absorbance corresponding to the equivalence point was sensed again, the controller voltage (instantaneous value VL) was ramped in reverse once more, going upward. Because of the lag time between a change in the controller output and its effect being sensed by the detector, the controller voltage corresponding to the actual equivalence point was the average of VH and VL. Continuous sensor-governed operation of the controller resulted in a triangular waveform. The mean of this waveform during any cycle gives the equivalence point controller voltage VE. This principle allowed true titrations with good reproducibility (0.2–0.7% R.S.D.) and throughput (33–42 s per titration).