This article throws light upon the four types of electrode used in electrochemical techniques.
The four types of electrode are: (1) The pH Electrode (2) Ion Selective and Gas Sensing Electrodes (3) The Clark Oxygen Electrode and (4) The Leaf Disc Electrode.
Contents
Type # 1. The pH Electrode:
Principles:
Perhaps the most convenient and accurate way of determining pH is by using a glass electrode. The pH electrode depends on ion exchange in the hydrated layers formed on the glass electrode surface.
Glass consists of a silicate network amongst which are metal ions coordinated to oxygen atom, and it is the metal ions that exchange with H+. The glass electrode acts like a battery whose voltage depends on the H+ activity of the solution in which it is immersed.
The size of the potential (E) due to H+ is given by the equation:
E = 2.303 RT/F log ([H+]i/ [ H+]o
where [H+]; and [H+]o are the molar concentrations of H+ inside and outside the glass electrode respectively. In practice, [H+]; is generally 10-1, because the electrode contains 0.1 M HCL. Since pH= – log [H+], it follows that the developed potential is directly proportional to the pH of the solution outside the electrode. Glass electrodes are particularly useful because of lack of interference from the components of solution.
On the whole these molecules are not readily contaminated by molecules in solution, and if other ions are present they do not cause any significant interference. However, at high pH they do respond to sodium. Inaccuracies also occur under very acid conditions.
A glass electrode consists of a thin, soft glass membrane that is situated at the end of a hard glass tube, or sometimes an epoxy body. Also present in the glass electrode is an internal reference electrode of the silver/silver chloride (Ag/AgCL) surrounded by electrolyte of 0.1 M HCL. This internal reference electrode gives rise to a steady potential.
Thus the varying potential of the glass electrode can be compared with a- steady potential produced by an external reference electrode such as the standard calomel electrode by joining internal and external reference electrode.
The external reference electrode can either be a separate probe or built around glass electrode giving a combination electrode. If a combination electrode is used, the level of the test solution must be high enough to cover the porous plug (liquid junction) but not as high as the level of salt bridge solution (KCL) in the external electrode because it is essential for KCl to diffuse out slowly into the test solution.
Whatever reference electrode is used the measured voltage is the result of the difference between that of the reference and the glass electrodes. In practice, however, there are other potentials present in the system. These include so-called asymmetric potential, which is poorly understood but which is present across the glass membrane even when the H+ concentration is the same on both sides.
Also included are the potentials due to Ag/AgCl and to the liquid junction to the reference electrode, which gives the potential because the K+ and CI– do not diffuse at exactly the same rate and, therefore, generate a small potential at the boundary between the sample and the KCl in the reference electrode. The measured potential for glass electrode should thus also include constants to account for the additional potential within the device.
Therefore, the equation becomes:
E = E* + 2.303 RT/F log ([H+]i/[H+]o),
where E* includes the standard electrode potential for glass electrode, and the constant junction potential present in the system.
At 25°C this equation becomes:
E = E* + 0.059 pH,
where E* now also includes a term to account for the internal H+ concentration. As already known there is a 59 mV change for a 10-fold change in the activity of a monovalent ion; this means that a change of one pH unit produces a 59 mV change.
A pH electrode is used in conjunction with a pH meter. This records the potential due to H+ concentration but is designed to take a little current from the circuit. A large current flow will cause changes in the ion concentration and hence changes in pH; this is prevented by having a high resistance present. The pH meter, glass electrode and reference calomel electrode are designed so that pH gives a zero potential.
Operation Of pH Electrode/Meter:
pH electrodes are available in variety of different shapes and sizes for many different applications. Intracellular pH can also be measured by using a miniature probes (micro electrodes). However, majority of them are based on same principle and operated in a similar fashion.
It is important that the outer layer of glass on glass electrode remains hydrated, and so it is normally immersed in a solution. Thus thin glass layer is fragile and thus care must be taken not to break it or scratch it, or to cause a buildup of static electric charge by rubbing it. Gelatinous and protein containing solutions should not be allowed to dry out on the glass surface as they would inhibit response.
As it is clear from above equations that potential produced is temperature dependent (each pH unity change represents 54.2 mV at 0°C and 61.5 mV at 37°C). This effect is predictable and can be compensated for. The pH meter will thus have a temperature compensation dial that must be correctly set before the meter is calibrated.
Calibration will necessitate the use of two solutions of widely differing pH. Usually calibration is first performed with buffer of pH 7, followed by a pH 4 buffer (if the sample is expected to be a acid) or a pH 9 buffer (if the sample is expected to be basic). Once the pH electrode is calibrated, it can simply be immersed in the solution to be measured and a rapid and accurate measurement estimate of pH can be made.
Type # 2. Ion Selective and Gas Sensing Electrodes:
The glass pH electrode is really a kind of ion-selective electrode (ISE) that is sensitive to H+. Similar potentiometric electrodes have been developed which are responsive to other ions, e.g., Na+, NH+4, Cl– and NO–3. The active material within these devices may be glass, an insoluble organic salt, or an ion exchange material.
Glass is the active material within the pH electrode, but modified aluminium silicate glasses can also be used to produce a variety of monovalent cation responsive electrodes. Insoluble inorganic salts like silver sulphite can be used to produce electrodes responsive to Cu2+, Pb2+ and Cd2+, whereas lanthanum fluoride may be used to produce electrodes responsive to F–.
Ion selective electrode responds to the activity of particular ion. However, if the instrument is calibrated with a standard of known concentration then, provided the ionic strength of solution are similar, the concentration of test solution will be recorded. If some of the ions are not free and exist in complex form or an insoluble precipitate, these electrodes will give a much lower reading, then with a method that detects all of the ions present. Generally used ion selective electrodes are Ca2+, K+, and NO–3.
An electrode may be ion selective but not ion specific. As with glass electrodes, these can be fouled by proteins forming a surface film. A reference electrode is also needed with these ISE so that the varying potential of these ISE can be compared with the steady potential produced by reference electrode.
Gas Sensing Electrodes:
They are used generally to estimate the concentration of gas by its interaction in a thin layer surrounding an ion sensitive electrode, commonly a pH electrode. Carbon dioxide, sulphur dioxide, ammonia can all be measured by their dissolution in a thin layer surrounding the pH electrode, and measuring the resultant pH of the layer.
Miniaturisation and Applications of Ion Sensitive Electrodes:
Miniaturisation of ion selective electrodes has been achieved by modification of field effect transistor to respond to specific ions. Such ion selective field effect transistors (ISFETs) are likely to have great clinical value. Multifunctional ISFETs are already available which are used to measure pH, Na+, K+, and Ca2+.
Type # 3. The Clark Oxygen Electrode:
It consists of a platinum cathode and silver anode, both immersed in same solution of saturated potassium chloride and separated from the test solution by a oxygen permeable membrane. When a potential difference of—0.6 V is applied across the electrodes such that platinum cathode is made negative with respect to silver anode, electrons are generated at anode and are then used to reduce oxygen at cathode.
The oxygen tension at cathode drops and so to make this deficit more oxygen moves towards cathode. Since the rate of diffusion of oxygen from the membrane is the limiting step in the reduction process, the current produced by the electrode is proportional to the oxygen tension in the sample.
These electrode reactions may be summarized as under:
At silver anode 4Ag + CI‑ → 4AgCl + 4e–
At platinum cathode O2 + 4H+ + 4e– → 2H2O
Operation of Rank Oxygen Electrode (Clark Electrode):
These allow the sample to be placed in upper reaction chamber by an oxygen permeable and ion impermeable membrane. Teflon is the usual choice, though cellophane, polythene, silicon rubber and cling film have been used with varying degree of success. Care must be taken that membrane must not become contaminated.
Thinner membranes give more response but are more fragile. The membrane covers the electrodes and allows oxygen to diffuse towards them whilst preventing other reaction elements to reach electrode and poison them. The electrodes are maintained in electric continuity with potassium chloride solution.
Oxygen electrode is mounted above a stirring motor, which is able to rotate a magnetic follower (flea) when inserted into the reaction vessel which is important as the platinum cathode reduces oxygen to produce the electric current. A correct set-up will show reduction in current when stirrer is switched off due to depletion of oxygen in the potassium chloride filled electrode chamber.
Resumption of stirring will result in a return of current (oxygen tension in potassium chloride) to its previous level prior to the stirrer being switched off. Since both solubility and rate of diffusion is affected by temperature, therefore some form of temperature control is necessary for better results which are done by circulating water bath.
Calibration of the instrument should be carried out at the same temperature as that of experiment. Many chemicals are adsorbed onto the surface of the membrane and reaction vessel; hence it is important that the apparatus is thoroughly cleaned after each experiment.
Applications of Rank Oxygen Electrode:
Due to their ability to give continuous trace, oxygen electrodes have largely replaced manometric techniques in the study of reactions involving oxygen uptake and evolution.
i. Mitochondrial studies:
The study of respiratory control and effect of inhibitors on mitochondrial respiration and the measurement of phosphorylation: oxidation (P: O) ratios are best done by oxygen electrodes.
ii. The sites of action of electron transport inhibitors can also be determined using an oxygen electrode.
iii. Micro-organism that uses oxygen as the terminal electron acceptor of respiratory electron transport can be studied using oxygen electrode and the effect of electron transport inhibitors determined.
iv. Enzyme assays:
Enzymes are readily studied using Clark oxygen electrode, provided oxygen is involved in the reaction. Glucose oxidase, D-amino acid oxidase, and catalase are examples whose properties can be studied in this way.
Probe Type Clark Electrode:
These rely on same principle of operation as the rank electrode. However, the cathode and retaining membrane are arranged at the end of the probe to enable insertion into a liquid phase. It has disadvantage that it does not have stirring arrangements. It has a variety of uses.
Measurement of oxygen in bulk liquids:
Oxygen concentrations are routinely monitored in fermentation processes, sewage and industrial waste treatment and in inland, coastal and oceanic waters. This involves the variation of the Clark electrode called flush top sensor.
Clinical uses:
Early clinical use of oxygen electrode is to measure heart-lung machines during open heart surgery. They are also used for testing patients who were treated with oxygen. Small samples of blood are taken from patient and oxygen content is measured in a small Clark type pO2 electrode.
Type # 4. The Leaf Disc Electrode:
Whilst the rank oxygen electrode is ideally suited to many applications requiring a measurement of oxygen in aqueous samples, a leaf disc electrode such as the Hanasatech LD2 is of more use if gaseous oxygen measurement is required. Since the measurement of oxygen evolution is one of the easiest ways of following photosynthetic process in leaves, this instrument has found much biological application.
This device measures oxygen amperometrically using the same principle as the rank electrode. However, instead of being a liquid -filled reaction vessel, the reaction chamber is designed to allow a leaf to be held in place and provided with saturating carbon dioxide (or bicarbonate as a source of carbon dioxide). Illumination is usually provided by an array of light-emitting diodes (which produce little heat) and the oxygen emitted by leaf during photosynthesis can be measured.
Calibration of this electrode is bit complex as compared to rank electrode. A zero oxygen signal can be produced by passing nitrogen through the reaction chamber. Once this is stopped and air is passed through the chamber, signal corresponding to 21% of the oxygen can be determined. However, in closed chamber system, the amount of oxygen is related to the oxygen concentration and to volume of the chamber.
In practice, because the leaf disc itself may reduce the effective volume of the chamber, calibration involves injecting known volumes of air into the chamber and measuring the voltage response to obtain the effective volume of the chamber and hence a precise calibration of the electrode.
The leaf disc electrode has been used extensively for the study of the relationship between photosynthetic oxygen evolution under saturating carbon dioxide and the intensity of illumination, enabling calculation of quantum yield, the inclusion of probes to measure emitted fluorescence from the leaf disc at the same time as the oxygen evolutions measurement are made has resulted in a device that provides variety of information.
Applications of these devices are diverse, ranging from studies of micro-propagated plants to those plants suffering from atmospheric pollution. Though leaf disc electrode is clearly designed for whole leaf studies, photosynthetic rates of microalgae have also been studied using theses electrodes.