Question

a. Show with the aid of a form of the Nernst equation that when using a pH sensitive glass electrode and reference electrode that the measured potential is proportional to pH.

b. Sketch a diagram of a pH sensitive glass electrode, label all parts and indicate the function of each part.

Answer 1

1. a sensing part of electrode, a bulb made from a specific glass
2. internal electrode, usually silver chloride electrode or calomel electrode
3. internal solution, usually a pH=7 buffered solution of 0.1 mol/L KCl for pH electrodes or 0.1 mol/L MeCl for pMe electrodes
4. when using the silver chloride electrode, a small amount of AgCl can precipitate inside the glass electrode
5. reference electrode, usually the same type as 2
6. reference internal solution, usually 0.1 mol/L KCl
7. junction with studied solution, usually made from ceramics or capillary with asbestos or quartz fiber.
8. body of electrode, made from non-conductive glass or plastics.

Active part of the electrode is the glass bubble. While tube has strong and thick walls, bubble is made to be as thin as possible. Surface of the glass is protonated by both internal and external solution till equilibrium is achieved. Both sides of the glass are charged by the adsorbed protons, this charge is responsible for potential difference. This potential in turn is described by the Nernst equation and is directly proportional to the pH difference between solutions on both sides of the glass.

The majority of pH electrodes available commercially are combination electrodes that have both glass H⁺ ion sensitive electrode and additional reference electrode conveniently placed in one housing. For some specific applications separate pH electrodes and reference electrodes are still used - they allow higher precision needed sometimes for research purposes. In most cases combination electrodes are precise enough and much more convenient to use.

Construction of combination electrode is in large part defined by the processes that must take place when measuring pH. We need to measure difference of potentials between sides of glass in the glass electrode. To do so we need a closed circuit.

Circuit is closed through the solutions - internal and external - and the pH meter. However, for correct and stable results of measurements reference electrode must be isolated from the solution so that they will not crosscontaminate - and it is not an easy task to connect and isolate two solutions at the same time.