Application of digital-movie-based flow colorimetry to hue-based end point detection of acid-base titration by...

A digital-movie-based flow colorimetry for pH measurement using a universal indicator has been applied to the end point detection of acid-base titrations. A two-channel flow system of feedback-based flow ratiometry, primarily consisting of two peristaltic pumps, a digital microscope-based detector, and a laptop computer, was constructed; a Visual Basic.NET program written in-house was used for automating the analytical processes. While maintaining the total flow rate (F_T) constant, a titrand solution was merged with a titrant solution, both containing the same concentration of Van Urk's universal indicator, under varying flow ratios. Downstream, the video image was captured with a digital microscope and its color was expressed as RGB values, hue, and luminance. The end point was determined from the rapid change of hue reflecting the color transition of the universal indicator around the equivalence point. A stepwise titration of multivalent acid (i.e., H₃PO₄) was also possible by setting hue values corresponding to the first (1st) and second (2nd) equivalence points as criteria to determine the respective end points. The hue-based approach was validated by the titrations of CH₃COOH and H₃PO₄ (1st and 2nd equivalence points) with 0.1 mol dm^-3 NaOH and to those of NaOH and NH₃ with 0.1 mol dm^-3 HCl. The method was applied to determine Japanese Pharmacopoeia (JP) borax and JP citric acid. The respective assay results were 100.0 ± 0.0% and 100.0 ± 0.0%, both meeting the JP specifications. The developed method is simple, high throughput (1 titration/min), versatile, and does not require indicator replace depending on the equivalence point pH.