Quantifying the Cross-Sensitivity of Glass pH Electrodes in Alkaline Solutions

Abstract

Standard glass pH electrodes are ubiquitous instruments used in research and in classrooms to measure the hydrogen ions present in a solution. While many chemists and educators have communicated ways to support teaching conceptual understanding of solution pH and the function of pH probes and dyes, the community lacks a methodology that enables students to move beyond a qualitative discussion of pH probe cross-sensitivity to other cations such as Li+ and Na+. Here, we present an affordable method whereby students learn to quantify such instrumental limitations and cross-sensitivities. The reversible hydrogen electrode (RHE) detects changes in hydrogen ion activity via electrochemical potential and serves as the baseline in this study to standardize the cation sensitivity of several glass membrane pH probes. Increasing additions of a salt, e.g., NaCl, in their corresponding alkali solutions, e.g., 0.10 M NaOH, were used to observe the shift in measured pH between the RHE (nominally only H+) and glass pH probes (H+ plus other cations). The generated working curves were found to be specific to each glass pH electrode, the cation identity, and the solution pH. Taken together, this report provides a methodology that equips instructors and research mentors to teach undergraduates about systematic error associated with instrumentation and specifically how to improve the accuracy of pH probe measurements in saline or alkaline environments.