Development of the Glass Electrode and the pH Response

The glass electrode is the most commonly used device to access the pH of an aqueous solution. It attains highly accurate measurements via simple and well-established procedures. However, the reasons why the glass electrode potential scales with hydrogen ion concentration according to almost Nernstian potential values have been long-standing challenges to explain. Only in the past 50 years has an understanding of the glass electrode response to pH been achieved, as elucidated by Nikolsky and Baucke and other researchers. In essence, the potential of a glass electrode derives from a solid−liquid electrolyte ionic process that entails both hydrated glass surface groups and diverse ions in solution. Thus, a process that is deceptively complicatedand usually overlookedunderpins a boundary potential difference. This notwithstanding, other interpretations of the glass electrode response have been contributed. Most notably, Cheng has viewed the device as an electrical circuit condenser responding to adsorbed hydrogen or hydroxyl ions. In addition, Morrison has attributed the glass electrode response to a double layer surface− liquid interface potential. The purpose of this article is to portray an unusual mix of complexity and viewpoint disparity over the years. Knowledge and appreciation of this mix offer a more complete picture for students and educators of aqueous solution chemistry. This information is most appropriate to lecture and laboratory courses on quantitative analysis.