Electrode Ag/AgCl

As with the saturated calomel electrode, the potential of the Ag/AgCl electrode is determined by the concentration of Cb used in its preparation. 

When prepared using a saturated solution of KCl, the Ag/AgCl electrode has a potential of +0.197 V at 25 °C. Another common Ag/AgCl electrode uses a solution of 3.5 M KCl and has a potential of +0.205 at 25 °C. The Ag/AgCl electrode prepared with saturated KCl, of course, is more temperature-sensitive than one prepared with an unsaturated solution of KCl. 

A typical Ag/AgCl electrode is shown in figure 11.9 and consists of a silver wire, the end of which is coated with a thin film of AgCl. The wire is immersed in a solution that contains the desired concentration of KCl and that is saturated with AgCl. A porous plug serves as the salt bridge. The shorthand notation for the cell is... 

The reference potential of the Ag-AgCl electrode in brackish water must be eorreeted for ehloride ion eontent (i.e., a ehange in ehloride ion eoneentration by a faetor of 10 shifts the referenee potential by about 50 mV in the positive direetion... 

Ag-AgCl electrodes are usually chosen (see Section 16.7 and Table 3-1). Care has to be taken that the electrical connection to the ship is sufficiently low resistance... 

As an example. Fig. 20-7 shows potential and protection currents of two parallel-connected 750-liter tanks as a function of service life. The protection equipment consists of a potential-controlled protection current rectifier, a 0.4-m long impressed current anode built into the manhole cover, and an Ag-AgCl electrode built into the same manhole [10,11]. A second reference electrode serves to control the tank potential this is attached separately to the opposite wall of the tank. During the whole of the control period, cathodic protection is ensured on the basis of the potential measurement. The sharp decrease in protection current in the first few months is due to the formation of calcareous deposits. 

Potential control with zinc reference electrodes presented a problem because deposits of corrosion products are formed on zinc in hot water. This caused changes in the potential of the electrode which could not be tolerated. Other reference electrodes (e.g., calomel and Ag-AgCl reference electrodes) were not yet available for this application. Since then, Ag-AgCl electrodes have been developed which successfully operate at temperatures up to 100°C. The solution in the previous case was the imposition of a fixed current level after reaching stationary operating conditions [27]. 

The impressed current method with metal oxide-coated niobium anodes is usually employed for internal protection (see Section 7.2.3). In smaller tanks, galvanic anodes of zinc can also be used. Potential control should be provided to avoid unacceptably negative potentials. Pure zinc electrodes serve as monitoring and control electrodes in exposed areas which have to be anodically cleaned in the course of operation. Ag-AgCl electrodes are used to check these reference electrodes. 

The films were then soaked in water and removed from the plates. Portions were mounted in glass cells which were filled with potassium chloride solution two Ag/AgCl electrodes were inserted into the limbs of the cells and the unit was placed in a thermostat. The resistance of the films was determined, from time to time, by connecting the cells in series with a known resistance and applying a potential of 1 V to the combination the potential drop across the standard resistance was measured by means of a valve potentiometer. 

To avoid contamination of the solution under study, and to minimise the liquid-junction potential, it is usual to use a salt bridge, but in many cases this can be dispensed with thus if corrosion in a chloride-containing solution is being studied a Ag/AgCl electrode immersed directly in the solution could be used similarly a Pb/PbOj electrode could be used for studies of corrosion in H2SO4. 

In the cells discussed in Sec. 57 the solvent in every case was water. But in this chapter we shall discuss cells placed back to back, where one solution contains a solute dissolved in water, while the other contains the same solute dissolved in ethanol, or in methanol, or in a methanol-water mixture. When, for example, a hydrogen electrode containing IIC1 dissolved in ethanol is coupled to a Ag/AgCl electrode, also containing HC1 dissolved in ethanol, the cell may be written... 

The silver reductor has a relatively low reduction potential (the Ag/AgCl electrode potential in 1M hydrochloric acid is 0.2245 volt), and consequently it is not able to effect many of the reductions which can be made with amalgamated zinc. The silver reductor is preferably used with hydrochloric acid solutions, and this is frequently an advantage. The various reductions which can be effected with the silver and the amalgamated zinc reductors are summarised in Table 10.11. ... 

Photoelectrochemical measurements were carried out by using a three-electrode cell containing the modified electrode as a working electrode, a platinum electrode as a counter electrode, and an Ag/AgCl electrode as a reference electrode. Na2S04 was used as the supporting electrolyte. Photocurrents from the modified electrode were... 

Two aqueous phases separated by a liquid membrane, EM, of nitrobenzene, NB, were layered in a glass tube, which was equipped with Pt counterelectrodes in W1 and W2 and reference electrodes in three phases as in Eq. (1). Reference electrodes set in W1 and W2 were Ag/AgCl electrodes, SSE, and those in LM were two tetraphenylborate ion selective electrodes [26,27], TPhBE, of liquid membrane type. The membrane current, /wi-w2 was applied using two Pt electrodes. The membrane potential, AFwi-wi recorded as the potential of SSE in W2 vs. that in W1. When a constant current of 25 /aA cm was applied from W1 to W2 in the cell given as Eq. (1), the oscillation of membrane potential was observed as shown in curve 1 of Fig. 1. The oscillation of AFwi-wi continued for 40 to 60 min, and finally settled at ca. —0.40 V. 

FIG. 3 Temperature dependence of the standard potential of the saturated calomel electrode (1) and Ag/AgCl electrode (2). 

An example of the experimental setup for the measuring of extracellular action and resting potentials is shown in Fig. 4. All electrochemical measurements can be conducted at constant temperature inside a Faraday cage mounted on a vibration-stabilized table in a laboratory (Fig. 4). Ag/AgCl electrodes were connected to a voltmeter/pFl meter [Cole Palmer Microcomputer pFl-vision Model 05669-20, Fig. 4(a)] with high input impedance or a programmable electrometer/amplifier [Keithley-2000/20, Keithley-6517, or Keithley-6514, Fig. 4(b)]. An IBM-compatible microcompu-... 

The generation and propagation of action potentials and electrical impulses between the tissues in higher plants can be measured by reversible nonpolarizable electrodes [1]. Since both Ag/AgCl electrodes are identical, we decided to call them reference and working electrodes as shown in Fig. 4. The reference electrode (—) was usually inserted in the stem or in a root of a soybean plant, and the upper (working) electrode (-I-) inserted in the stem or a leaf of the plant. 

Ag/AgCl electrodes were inserted in the plant at different positions. Following insertion of the electrodes, the plants were allowed to rest until a stable potential difference was obtained between the measuring and reference electrodes. Insertion of electrodes in plants induces action potentials across the stem and slow fluctuations of the resting potential (Fig. 5). After approximately 1-2 h, the resting potential stabilizes and action potentials induced by wounding disappear (Fig. 6). 

FIG. 5 Potential difference between two Ag/AgCl electrodes in the stem of a soybean plant after insertion of the electrodes. Distance between electrodes was 8 cm. Volume of soil was 0.5 L. The plants were given water every other day and kept at 24°C. Frequency of scanning was 1000 samples per second. 

FIG. 17 Resting and action potentials of a potato plant after (a) pinch along the midrib in the center of the young terminal leaflet by forceps (b) thermal shock of terminal leaflet leaf by a flame. The plants were given water every other day and kept at 20° C. Distance between Ag/AgCl electrodes (a) 5 cm, (b) 5 cm, and (c) 9 cm. (From Ref. 7.)... 

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