Reference electrodes diaphragm

Reference electrodes are used in the measurement of potential [see the explanation to Eq. (2-1)]. A reference electrode is usually a metal/metal ion electrode. The electrolyte surrounding it is in electrolytically conducting contact via a diaphragm with the medium in which the object to be measured is situated. In most cases concentrated or saturated salt solutions are present in reference electrodes so that ions diffuse through the diaphragm into the medium. As a consequence, a diffusion potential arises at the diaphragm that is not taken into account in Eq. (2-1) and represents an error in the potential measurement. It is important that diffusion potentials be as small as possible or the same in the comparison of potential values. Table 3-1 provides information on reference electrodes. 

Point (a) only concerns simple metal electrodes and needs to be tested for each case. Point (b) is important for the measuring instrument being used. In this respect, polarization of the reference electrode leads to less error than an ohmic voltage drop at the diaphragm. Point (c) has to be tested for every system and can result in the exclusion of certain electrode systems for certain media and require special measures to be taken. 

Measuring electrodes for impressed current protection are robust reference electrodes (see Section 3.2 and Table 3-1) which are permanently exposed to seawater and remain unpolarized when a small control current is taken. The otherwise usual silver-silver chloride and calomel reference electrodes are used only for checking (see Section 16.7). All reference electrodes with electrolytes and diaphragms are unsuitable as long-term electrodes for potential-controlled rectifiers. Only metal-medium electrodes which have a sufficiently constant potential can be considered as measuring electrodes. The silver-silver chloride electrode has a potential that depends on the chloride content of the water [see Eq. (2-29)]. This potential deviation can usually be tolerated [3]. The most reliable electrodes are those of pure zinc [3]. They have a constant rest potential, are slightly polarizable and in case of film formation can be regenerated by an anodic current pulse. They last at least 5 years. 

Reference Electrodes By definition, the normal hydrogen electrode (N H E) is the reference for electrode potentials (see Sect. 2.3.2.1), but practically it is scarcely usable. A reference electrode (RE) has to provide a well-defined potential between the electrolyte and its electric connector, joined with the input of the measuring instrument. Usually, a metal and a slightly soluble salt of this metal is applied (secondary electrode) [76, 77]. The electrolyte in the RE is connected to the electrolyte in the electrochemical cell via a diaphragm, which has to separate both electrolytes, as far as possible without a potential difference (see below). 

Fig. 8 Beaker glass cells (a) undivided, (b) divided. 1 mercury cathode, 2 cathode current feeder, 3 platinum mesh anode, 4 anode current feeder, 5 anolyte tube with, 6 diaphragm, 7 magnetic stirrer, 8,9 PTFE stopper, 10 reference electrode with Luggin capillary,...

Figure 15.1. Potentiometric measurement for pH. V), glass membrane V2, inner buffer solution V3, internal reference electrode relative to internal buffer V4, external reference electrode V5, diaphragm.

Figure 15.2. Design of the combined electrode. 1, Internal reference electrode, usually Ag AgCl 2, outer glass membrane 3, inner glass membrane 4, external reference electrode, usually Ag AgCl 5, diaphragm.

Sensitive to handling Reference electrode is not filled/Top up with electrolyte solution, free of air bubbles. Reference electrodes tilled with the wrong solution/Empty and refill the reference electrolyte. Diaphragm clogged/Clean diaphragm. Measurement of poorly conductive solutions/Measure with different amplifier or add supporting electrolyte. 

Figure 22.8 Schematic representation of an electrolysis cell (three-electrode construction W = working electrode AUX = auxiliary electrode R = reference electrode Q = coulometer D = diaphragm, if necessary CS = current source A = amperometer V = voltmeter).

In addition, other sources of errors can occur owing to imperfections in the glass-membrane construction or due to limitations related to the reference electrodes and the diaphragm. Temperature also plays an important role in pH measurement. However, for these sources of errors reference is made to the literature for more detailed information15. 

There is no fundamental difference between the two half-cells or electrodes in a cell for measuring emf (electromotive force), especially in molten salts. However, it is usual to designate one of the electrodes as reference electrode if it is used for the measurement of an emf series. In many cases a diaphragm is used to separate the two half-cells. 

A number of problems must be considered when designing a cell for electrolysis, such as potential distribution at the working electrode, position of the reference electrode, ohmic resistance in the circuit, mass and heat transfer, the need for a diaphragm, and the necessity of working in a closed system. 

Special problems are found for mercury electrodes, especially in the widely employed H cells the part of the electrode near the diaphragm has a higher current density than the rest of the electrode, and the stirring of the mercury introduces changes in the distance between the tip of the reference electrode and the working electrode, which affects the measured reference potential. 

A stable potential of a reference electrode is very important. Instability may be caused by several factors. One of the components may be unstable in the solvent. Thus, mercurous chloride is unstable in acetonitrile and a calomel electrode is unreliable in this solvent. Evaporation of solvent from a standard solution, the concentration of which determines the potential, changes the potential. A liquid junction potential may change this is observed when an aqueous calomel electrode, connected to the test solution through a diaphragm, is used for measurements of potentials in acetonitrile for periods longer than about 5 min (see later). 

Figure 3. Divided thermostatted beaker type cell (a) mercury pool cathode with current feeder (platinum wire sealed in glass rod), (b) diaphragm, (c) platinum foil anode, (d) Luggin capillary to reference electrode...

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