ARTICLE BACK TO BASICS pH meters and their electrodes: calibration, maintenance and use lithium-based glass electrodes are now used Following consideration of centrifugation and liquid handling in the exclusively for pH-responsive glass electrodes. February and March issues, respectively, of The Biomedical Scientist, POTENTIOMETRIC MEASUREMENT Almost all conductors of electricity are Peter Riddle now directs attention to pH meters, their electrodes and metal or an electrolyte, with the current being carried by either electrons or ions. the measurement of hydrogen ion concentration. When current passes from metal to electrolyte, or from electrolyte to metal, the type of carrier usually changes suddenly, and As living processes are dependent on the around 1928 the first commercial pH meter whenever there is an interface between the unique ionising solvent we know as water, was produced by the Cambridge Instrument metal and ions of that metal in a solution and as much of classical chemistry was Company. Both electrodes were subject to then an electric potential is produced. concerned with the study of reactions in interference from the components of redox This potential is called the electric potential aqueous solution, the ubiquity of the pH systems that reduced the effectiveness of of that metal. An electrode potential is also meter in present-day biomedical laboratories these electrodes to measure the potential produced when different concentrations of is scarcely surprising. The concept of response due to hydrogen ions. Membrane an ion are separated by a membrane that is electrolytic dissociation was introduced over electrodes, including the glass electrode, semipermeable to that ion. Non-metallic a century ago by Arrhenius, and in 1909 are not subject to these limitations and elements such as hydrogen also have Sorenson demonstrated the importance of electrode potentials. hydrogen ion concentration on enzyme In order to measure an electrode potential, activity and coined the term ‘pH’. Although ‘The hydrogen electrode another voltage source (such as another that same year saw the first systematic study metal/solution interface) is needed to of the glass electrode, it was not until the was the first electrode to be measure it against. Each of the electrodes is development of convenient electronic called a half-cell. The two half-cells arranged techniques capable of accommodating the used for pH measurement together constitute an electrochemical cell, high electrical resistance of the glass in which one of the half-cells maintains a electrode that interest in electrometric pH but it required a supply of constant voltage. The electrode in the half- determination and its importance in cell with the constant voltage is called the biological systems became clear. hydrogen gas and thus its reference electrode, whereas the variable The pH meter is designed to measure the voltage portion is termed the indicator concentration of hydrogen ions in a solution. application was limited’ electrode. It is possible to measure the Basically, three parameters are involved in the measurement: the actual molar concentration of hydrogen ions, the dissociation constant of the acid (pKa), and temperature. pH is Glass Test or standard KCl/Hg Cl /Hg defined as the negative of the logarithm to Ag/AgCl/HCl 2 2 membrane membrane the base 10 of the hydrogen ion concentration (pH = –log10 [H+]). The hydrogen electrode was the first electrode to be used for pH measurement but Indicator glass electrode Reference electrode it required a supply of hydrogen gas and thus its application was limited. The quinhydrone Fig 1. The composition of the two half-cells that permits the electrode appeared in the early 1920s and difference in potential to be detected between them. 202 THE BIOMEDICAL SCIENTIST APRIL 2013 ARTICLE Wire connection point Wire connection point Glass body Glass or plastic body Seal Filled with Silver potassium chloride wire ‘buffer’ solution Silver wire Bulb filled with – + + – Very thin glass potassium – + + – bulb chemically chloride ‘buffer’ – + + – ‘doped’ with solution Silver – + + – lithium ions so as Silver + chloride tip + to react with – – chloride tip + + hydrogen ions – + + + + + – Porous junction – – outside the bulb – – – Voltage produced across thickness of the glass membrane Fig 2. The reference electrode. Fig 3. The indicator (glass) electrode. potential difference between these two concentration through the Nernst equation, is desired, a 0.1 mol/L or 1 mol/L potassium electrodes and calculate the concentration which, in its simple form, is E=KT log chloride solution electrode is preferred of ions in the solution of the indicator (C1/C2) where K = Constant 2.3 (R/zF) because it reaches its equilibrium potential (measuring) electrode. and if either C1 or C2 is known then the more quickly and its potential depends less For example, if a silver wire is immersed in concentration can be calculated from the on temperature than does the saturated type. a solution of silver chloride, ionisation of the measurement of the potential developed. The silver–silver chloride electrode is a very silver metal occurs, with the formation of In making an electrical connection reproducible electrode. The solution silver ions (Ag+) and electrons. An electric between the reference electrode and the surrounding the electrode should be potential now exists between the wire and the sample solution via a salt bridge, a negligible saturated with potassium chloride and silver solution. If two half-cells are used, each with a but reproducible potential is produced. This chloride. silver wire or foil immersed in a different silver potential develops at the interface between solution, and the two solutions are connected two non-identical solutions and is called the INDICATOR ELECTRODES through a meter, a difference in potential liquid–liquid junction potential. A saturated The glass electrode was the first and still most can be detected between them (Fig 1). potassium chloride solution is normally used commonly used electrode for measuring As the potential of each solution depends on as a salt bridge in the reference electrode hydrogen ion activity. A glass electrode consists the concentration of silver ions in it, the because many ions diffuse from the salt of a small bulb of special glass that contains a concentration of ions in one solution can be bridge against which the sample ions must solution of known hydrogen ion concentration predicted if the value for the other solution diffuse. Thus, diffusion of ions from the (eg 0.1N HCL or KCL ‘buffer solution’) and an and the difference in potential between sample to the junction is negligible and the internal reference electrode (usually calomel them are known. A temperature difference junction potential is ≤1 mV and is or silver–silver chloride; Fig 3). The sensor is between the two half-cells would affect the reproducible. normally a platinum wire. reproducibility of measurement. Other minor The pH electrode is manufactured with technical factors, such as coating (eg protein), REFERENCE ELECTRODES different properties, depending on the will affect the measurements. The standard hydrogen electrode is the application. Those used in biomedical Potentiometric methods are based on international standard but is seldom used for laboratories should be linear at about pH 7.0. the quantitative relationship between the routine work because a supply of hydrogen Wide-range pH electrodes (1–14) tend to be potential of a cell as given by the following gas is required and more convenient types non-linear towards the ends of the range. distribution of potential: (together with reliable calibration buffers) are Some are designed to be linear for low pH E = E + E + E available. The saturated calomel electrode values, while others are linear for high pH cell reference indicator junction is a widely used reference electrode that values. These properties are controlled by the Because the reference and junction potentials contains an inert element (eg platinum) in glass formula. are constant, the indicator potential can be contact with mercury, mercurous chloride determined. The potential of the indicator (calomel) and a solution of potassium COMBINATION ELECTRODE electrode can then be related to chloride of known concentration. However, It is inconvenient in many applications to concentration. Electrode systems used in in view of environmental considerations, the have to contend with two measuring biomedical laboratories have precalibrated mercury electrode is rarely used these days electrodes (reference and indicator). read-out devices that give results in and such electrodes are not suitable for Combination electrodes are available into concentration units. varying temperatures or temperatures above which both the reference (usually Ag–AgCl) The cell potential is related to 60˚C (Fig 2). When a high degree of accuracy and indicator electrodes are incorporated. APRIL 2013 THE BIOMEDICAL SCIENTIST 203 ARTICLE The pH membrane is usually recessed in, ‘Both standard moving-pointer INSTRUMENT CALIBRATION and protected by, a plastic housing. Such It is essential that pH electrodes are electrodes will withstand more physical abuse meters and digital display calibrated regularly using the meter and than will conventional pH electrodes. reference electrode with which they are to be The pH of a solution is a function of its units are common, although used in the laboratory. The procedure is as temperature. Voltage output from the follows: