Digital-movie-based flow colorimetry for pH measurement with universal indicators

A continuous, simple, and versatile pH monitoring method based on digital-movie-based colorimetry is proposed. A constructed flow system was of a two-channel configuration mainly composed of two peristaltic pumps, a digital microscope-based detector, a pH meter with a flow-through combination pH-reference electrode, and a laptop computer. While the total flow rate (FT) was held constant, the flow rate (FB) of a base solution for Britton-Robinson buffer containing a universal indicator (Yamada's indicator or Van Urik's indicator) was changed in proportion to the control signal (Vc) from the computer. An acid solution for the buffer containing the indicator was aspirated to the confluence point at the flow rate of FT – FB and mixed with the base solution. Thus, buffer solutions with arbitrary pH could be easily prepared. The image of the mixed solution was captured with the microscope downstream; the pH of the solution was measured with the pH meter at the most downstream. An in-house program written in Visual Basic .NET was developed to control the system, acquire and analyze the signals (image data and pH), and display the results automatically; the color of the image was expressed as tristimulus values (i.e., R, G, B), hue, and luminance. The relationships between these color-specific values and pH were analyzed after optimizing the Vc scan rate. Van Uik's indicator was superior to Yamada's regarding the applicable pH range (ca. 2.5 – 10). A sigmoid-like calibration curve was established between hue and pH, which was used to determine sample pH. The proposed method was validated by measuring the pH of different drugs and vinegar samples.