Reusable pH Test Strip for Extremely Acidic pH Detection

Abstract

Commercial pH test strips suffer from low sensitivity in the extremely acidic range (pH 0.5–1.5) and are typically single-use due to indicator leaching, causing environmental concern. To address these issues, we developed a reusable pH test strip based on the strong adsorption between Congo red and a nylon film. It was prepared by a simple soaking-ultrasonic washing process (optimal washing times is 15) and can be reused at least 100 times. After a single calibration, it maintains a deviation within ±0.1 from the pH meter following 11 days of storage in water and within ±0.15 after 30 days, demonstrating excellent stability. The response shows low temperature sensitivity in the range of 10.2–39.1 °C and no response to common ions (Na+, K+, Ca+, Mg2+, Cu2+, Fe3+, Zn2+, Pb2+, and Hg2+). Using a smartphone, the Total Huev parameter, and IQA software, the strip enables accurate pH determination of HCl solutions and simulated gastric fluids. Thus, it serves as a low-cost alternative to pH meters for preparing acidic solutions (pH 0.5–1.5) and for preliminary screening of the Zollinger–Ellison syndrome, reducing damage to pH electrodes from highly acidic media. Furthermore, the strip identifies four concentrated acid reagents (H2SO4, CH3COOH, HNO3, and HCl) via kinetic response, naked-eye color comparison, or principal component analysis. Future work includes extending this reusable platform to broader pH ranges to mitigate environmental pollution from disposable test strips. The exceptional leaching resistance of the Congo red-nylon film makes it promising for pH imaging.