Glass electrode Unit-3 | Engineering Chemistry | Btech Tutorials | KlassPM

A glass electrode is a type of ion-selective electrode made of a doped glass membrane that is sensitive to a specific ion. The most common application of ion-selective glass electrodes is for the measurement of pH. The pH electrode is an example of a glass electrode that is sensitive to hydrogen ions. Glass electrodes play an important part in the instrumentation for chemical analysis and physicochemical studies. The voltage of the glass electrode, relative to some reference value, is sensitive to changes in the activity of a certain type of ions. Construction A typical modern pH probe is a combination electrode, which combines both the glass and reference electrodes into one body. The combination electrode consists of the following parts (see the drawing): a sensing part of the electrode, a bulb made from a specific glass internal electrode, usually a silver chloride electrode or a calomel electrode the internal solution, usually a pH=7 buffered solution of 0.1 mol/L KCl for pH electrodes or 0.1 mol/L MCL for pM electrodes when using the silver chloride electrode, a small amount of AgCl can precipitate inside the glass electrode the reference electrode, usually the same type as 2 reference internal solution, usually 0.1 mol/L KCl junction with studied solution, usually made from ceramics or capillary with asbestos or quartz fibre. body of the electrode, made from non-conductive glass or plastics. The bottom of pH electrode balloons out into a round thin glass bulb. The pH electrode is the best thought of like a tube within a tube. The inner tube contains an unchanging 1×10−7 mol/L HCl solution. Also inside the inner tube is the cathode terminus of the reference probe. The anodic terminus wraps itself around the outside of the inner tube and ends with the same sort of reference probe as was on the inside of the inner tube. It is filled with a reference solution of KCl and has contact with the solution on the outside of the pH probe by way of a porous plug that serves as a salt bridge. Galvanic cell schematic representation This section describes the functioning of two distinct types of electrodes as one unit which combines both the glass electrode and the reference electrode into one body. It deserves some explanation. This device is essentially a galvanic cell that can be schematically represented as: Glass electrode || Reference Solution || Test Solution || Glass electrode Ag(s) | AgCl(s) | KCl(aq) || 1×10−7M H+ solution || glass membrane || Test Solution || junction || KCl(aq) | AgCl(s) | Ag(s) In this schematic representation of the galvanic cell, one will note the symmetry between the left and the right members as seen from the centre of the row occupied by the "Test Solution" (the solution whose pH must be measured). In other words, the glass membrane and the ceramic junction occupies both the same relative place in each respective electrode (indicative (sensing) electrode or reference electrode). The double "pipe symbol" (||) indicates a diffusive barrier that prevents (glass membrane) or slowing down (ceramic junction), the mixing of the different solutions. By using the same electrodes on the left and right, any potentials generated at the interfaces cancel each other (in principle), resulting in the system voltage being dependent only on the interaction of the glass membrane and the test solution. The measuring part of the electrode, the glass bulb on the bottom, is coated both inside and out with a ~10 nm layer of a hydrated gel. These two layers are separated by a layer of dry glass. The silica glass structure (that is, the confirmation of its atomic structure) is shaped in such a way that it allows Na+ ions some mobility. The metal cations (Na+) in the hydrated gel diffuse out of the glass and into solution while H+ from solution can diffuse into the hydrated gel. It is the hydrated gel, which makes the pH electrode an ion-selective electrode. H+ does not cross through the glass membrane of the pH electrode, it is the Na+ which crosses and allows for a change in free energy. When an ion diffuses from a region of activity to another region of activity, there is a free energy change and this is what the pH meter actually measures. The hydrated gel membrane is connected by Na+ transport and thus the concentration of H+ on the outside of the membrane is 'relayed' to the inside of the membrane by Na+. All glass pH electrodes have extremely high electric resistance from 50 to 500 MΩ. Therefore, the glass electrode can be used only with a high input-impedance measuring device like a pH meter, or, more generically, a high input-impedance voltmeter which is called an electrometer. Unit-3 Electrochemical, Engineering Chemistry by B.tech KlassPM. https://www.klasspm.com/engineering/osmania-university_2/be_155/bio-medical-engineering_59/1-year-i-semester_28/engineering-chemistry-i_16