Differential Colorimetric pH Measurement with a Microfluidic Chip Using a Novel Sample Injection Method

This paper presents a novel approach to pH measurement of colored liquids using differential optical absorption measurements within a microfluidic chip and a novel sample injection method. Sample is automatically introduced and mixed with a pH indicator solution by coupling the mechanical motion of a syringe pump to two syringes with different cross-sectional areas. Predefined and consistent mixing ratios are confirmed using a Y-shaped microfluidic chip. The dynamic performance of the injection method is also optimized to eliminate flow fluctuations using an electrical circuit analogy and conducting electrical simulations. A polymethyl methacrylate microfluidic chip with a passive micromixer and two observation windows is also fabricated. The mixing capability of the chip is tested by introducing samples with different color dye concentrations ranging from 150 to 450 µL/L at a flow rate of 15 µL/min. Absorbances are measured from observation windows before and after mixing the colored sample with DI water, confirming uniform and consisting mixing ratio of 2:1 of colored sample to DI water. Finally, pH measurements of colored samples are made. Phenol red pH indicator is introduced at a flow rate of 15 µL/min and mixed with colored samples of varying pH values between 5.5 and 8.5 and sample to color dye volume ratio of 600:1, 600:4, and 600:8. The effect of the color of the sample is eliminated by differential absorbance measurements, before and after the colored sample is mixed with the pH indicator, revealing a linear response with the experimented pH values.