Air segmented amplitude modulated multiplexed flow analysis with software-based phase recognition: determination of phosphate ion

Integrated continuous flow method with dual feedback-based controls for online analysis and process control

The concept of an integrated automated continuous flow method with dual feedback controls is presented for diluting a stock solution to provide a solution of a given concentration. The one control is used for the online process monitoring by a feedback-based flow ratiometry, where the product (the diluted liquid) is titrated through the rapid bidirectional scan of the product/reagent flow ratio. The feedback control limits the scanning to the necessary range to increase the analytical throughput. The other control is used for the process control to output the product with a preset concentration. The merging ratio of the stock solution and a solvent (diluent) is changed based on the information from the online analysis. The concept was verified by applying it to producing 0.1 mol dm^-3 CH₃COOH. When the stock concentration was changed from 0.1 (reference concentration) to 0.3 and then 0.2 mol dm^-3, the system searched for the suitable merging ratio and converged the output concentration to the reference value within 7.43 min with a relative error below 1.05%. The mean throughput rate of the process analysis was 11.2 titrations min^-1. Successful results were also obtained for the 0.1 mol dm^-3 HCl production. The present concept could be the basis for process control with reduced wasteful output and effluent treatment with eco-friendly treated water discharge, resulting in the contribution to SDGs' goals of 6 (Clean water and sanitation), 9 (Industry, innovation and infrastructure), and 14 (Life below water).

Triangular-wave controlled amplitude-modulated flow analysis for extending dynamic range to saturated signals

We have applied our amplitude-modulated flow analysis concept to extend the dynamic range to saturated analytical signals. Sample solution, the flow rate F_S of which is periodically varied with a triangular control signal V_c, is merged with a reagent solution delivered at a constant flow rate F_R. While the total flow rate F_T is kept constant, a diluent with F_T-(F_S + F_R) flow rate is aspirated from the third channel. The analytical signal V_d obtained downstream is processed by fast Fourier transform to obtain the amplitudes of the wave components in V_d. When the sample concentration C_S is low, V_d shows a triangular profile like V_c; the sum of the amplitudes ΣA is proportional to C_S. When C_S is high, V_d shows a trapezoidal profile because of the V_d saturation. A linear calibration curve can also be obtained for such saturated signals by plotting C_SΣA against C_S. The proof of the concept is validated by applying it to spectrophotometric determinations of a dye (Methylene Blue) and colored complexes of Fe²⁺ - phenanthroline and Fe³⁺ - Tiron.

Introduction of Air-Segmentation Approach to Flow Titration by Feedback-based and Subsequent Fixed Triangular Wave-controlled Flow Ratiometry

An air-segmentation approach has been introduced to a feedback-based and subsequent fixed triangular wave-controlled flow ratiometry to suppress axial dispersion in flow titration. The flow rate of a base solution containing an indicator is linearly varied with a control signal, Vc, supplied by a computer. The solution is merged with an acid solution under a constant total flow rate. Air is introduced to the merged solution in order to segment the solution with air bubbles. Both phases are led to a UV/Vis detector without phase separation. Air signals are removed by signal processing. The effect of the lag time between the merging of solutions upstream and the sensing of the corresponding signal downstream is offset by feedback-based upward and downward Vc scans, and thus the Vc that gives the equivalence composition is determined. Subsequently, fixed triangular wave control is applied to a narrower Vc range with a higher scan rate to enhance the throughput rate (maximally 11.8 titrations/min). Air-segmentation has been found to be effective to reduce axial dispersion and to preserve the titrand/titrant composition upon their just being merged. Consequently, the applicable range is extended especially to lower titrand concentration. The proposed method has been successfully applied to various acid-base titrations, including the nonaqueous titration of the Japanese Pharmacopoeia drug.