Стеклянный электрод для измерения pH: история и устройство

Electrode that is pH sensitive

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.

The first studies of glass electrodes (GE) found different sensitivities of different glasses to change the medium's acidity (pH), due to the effects of the alkali metal ions. In 1906, M. Cremer, the father of Erika Cremer, determined that the electric potential that arises between parts of the fluid, located on opposite sides of the glass membrane is proportional to the concentration of acid (hydrogen ion concentration). In 1909, S. P. L. Sørensen introduced the concept of pH, and in the same year F. Haber and Z. Klemensiewicz reported results of their research on the glass electrode in The Society of Chemistry in Karlsruhe. In 1922, W. S. Hughes showed that the alkali silicate glass electrodes are similar to hydrogen electrodes, reversible concerning H+. In 1925, P. M. Tookey Kerridge developed the first glass electrode for analysis of blood samples and highlighted some of the practical problems with the equipment such as the high resistance of glass (50–150 MΩ). During her PhD, Kerridge developed a glass electrode aimed to measure small volume of solution. Her clever and careful design was a pioneering work in the making of glass electrodes.

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 electrode, a bulb made from a specific glass. Internal electrode, usually silver chloride electrode or calomel electrode. 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. Reference electrode, usually the same type as 2. Reference internal solution, usually 3.0 mol/L KCl. Junction with studied solution, usually made from ceramics or capillary with asbestos or quartz fiber. Body of electrode, made from non conductive glass or plastics. The bottom of a pH electrode balloons out into a round thin glass bulb. The pH electrode is best thought of as 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.

The glass electrode has some inherent limitations due to the nature of its construction. Acid and alkaline errors are discussed above. An important limitation results from the existence of asymmetry potentials that are present at glass/liquid interfaces. The existence of these phenomena means that glass electrodes must always be calibrated before use; a common method of calibration involves the use of standard buffer solutions. Also, there is a slow deterioration due to diffusion into and out of the internal solution. These effects are masked when the electrode is calibrated against buffer solutions but deviations from ideal response are easily observed by means of a Gran plot. Typically, the slope of the electrode response decreases over a period of months.

Between measurements any glass and membrane electrodes should be kept in a solution of its own ion. It is necessary to prevent the glass membrane from drying out because the performance is dependent on the existence of a hydrated layer, which forms slowly.