Application of Feedback-Based Flow Ratiometry to Simultaneous Determination of Calcium and Magnesium Ions by Chelatometry Using Photometric and Potentiometric Detectors

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We advanced the principle of feedback-based flow ratiometry to stepwise titration using two detectors set in tandem in a flow system. The method was applied to the simultaneous determination of calcium and magnesium ions (Ca2+ and Mg2+, respectively) by a chelatometry. The flow rate (FM) of titrand containing Ca2+ and/or Mg2+ was varied in response to controller output signals (Vc) having triangular wave profile, while the total (titrand + titrant + indicator solution) flow rate (FT) was held constant. Titrant solution (EDTA or EGTA, pH 10) was aspirated to the confluence point of the flow system at the flow rate of FT - FM - FI, where FI was a constant flow rate of indicator solution (EBT), and then mixed with the titrand. Downstream, the analytical signals of the mixed solution were monitored. First, a high-throughput photometric titration of a single analyte (Ca2+ or Mg2+) was examined by a feedback-based flow ratiometry coupled with a subsequent fixed triangular wave controlled flow ratiometry. Maximally 17 titrations could be done per minute with reasonable precision (RSD = ca. 1 %). Next, the algorithm of the former ratiometry was modified so that a photometric detector and a potentiometric detector (Ca2+-selective electrode) could detect the total concentration of both ions and the concentration of Ca2+, respectively, using EGTA as a titrant. The equivalence points of the photometry and the potentiometry were each estimated from the Vc values at the instant of the sensing of the corresponding equivalence signal in the course of upward and downward scans of Vc. Good throughput (ca. 31 s per determination) was attainable with allowable precision (RSD < 1.6 %).

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