Silver chloride electrolytes, table

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. 

Various types of reference electrodes have been considered in Section 20.3, and the potentials of these electrodes and their variation with the activity of the electrolyte are listed in Table 21.7, Chapter 21. It is appropriate, however to point out here that the saturated calomel electrode (S.C.E.), the silver-silver chloride electrode and the copper-copper sulphate electrode are the most widely used in corrosion testing and monitoring. 

Table 19.6 lists Popoff and Neuman s values [3] of the solubility of silver chloride (AgCl) in water containing solvent electrolytes at the concentrations indicated. According to the same authors, the solubility of silver chloride in pure water is 1.278... 

TABLE 19.6. Values of the Solubility of Silver Chloride in Water Containing Solvent Electrolytes at the Concentration Indicated [3]... 

In aqueous solutions, the method of measuring electrode potentials has been well established. The standard hydrogen electrode (SHE) is the primary reference electrode and its potential is defined as zero at all temperatures. Practical measurements employ reference electrodes that are easy to use, the most popular ones being a silver-silver chloride electrode and a saturated calomel electrode (Table 5.4). The magnitude of the liquid junction potential (LJP) between two aqueous electrolyte solutions can be estimated by the Henderson equation. However, it is usual to keep the LJP small either by adding the same indifferent electrolyte in the two solutions or by inserting an appropriate salt bridge between the two solutions. 

As stated earlier, the reference electrode in a cell used for electroanalysis is designed so that its potential is independent of the composition of the test solution. There are several general properties that reference electrodes should have in order to be useful in analysis (1) they should be reversible with an electrode potential which is independent of time and reproducible (2) they should have a small temperature coefficient (3) they should be ideally non-polarizable with negligible effects from the flow a small current through the system and (4) they should be easily constructed. The most commonly used reference electrodes are those based on on the mercury calomel system and the silver silver chloride system. The electrolyte most commonly used in these systems is KCl. Relevant parameters for commonly used reference electrodes are given in table 9.4. 

The calomel electrode consists of mercury and mercury (I) chloride (calomel Hg2Cl2) in contact with a potassium chloride solution of constant activity. In case that the supporting electrolyte in the cell contains perchlorate anions, it is advisable to use NaCl instead of KCl since KCIO4 is sparingly soluble and could precipitate in the diaphragm. In most cases, satmated KCl solution is used however, in such solution already at temperatures above 35 °C, a disproportion reaction takes place. The back reaction by cooling down the electrode is very slow so that a hysteresis of the electrode potential occurs. This is the reason why it is reconunended that the calomel electrode only be used at temperatures in maximum up to 70 °C. In Table 1 electrode potentials of the silver/silver chloride electrode and for the calomel electrode at different temperatures and different concentrations of KQ are given. Note that the electrode potentials differ when other salts than potassium chloride (e.g., NaQ) are used because of the different solubility products. 

The silver/silver chloride reference electrode is a widely used reference electrode because it is simple, relatively inexpensive, its potential is stable and it is nontoxic. As a laboratory electrode such as described in Figure 4.4, it is mainly used with saturated potassium chloride (KCl) electrolyte, but can be used with lower concentrations such as 1M KCl and even directly in seawater. As indicated in Table 4.7, such changes in ionic concentrations also change the reference potential. Silver chloride is slightly soluble in strong potassium chloride solutions, so it is sometimes recommended that the potassium chloride be saturated with silver chloride to avoid stripping the silver chloride off the silver wire. 

Lead-silver is primarily used in seawater and strong chloride-containing electrolytes. PbAg anodes are particularly suitable for use on ships and in steel-water constructions, especially as they are relatively insensitive to mechanical stresses. The original alloy developed by Morgan [8,9] consists of 1% Ag and 6% Sb, with the remainder Pb. It is represented as alloy 1 in Table 7-2. A similar alloy developed by Applegate [10] has 2% Ag and the remainder Pb. Another alloy... 

I. 4-methoxyacetophenone (30 //moles) was added as an internal standard. The reaction was stopped after 2 hours by partitioning the mixture between methylene chloride and saturated sodium bicarbonate solution. The aqueous layer was twice extracted with methylene chloride and the extracts combined. The products were analyzed by GC after acetylation with excess 1 1 acetic anhydride/pyridine for 24 hours at room temperature. The oxidations of anisyl alcohol, in the presence of veratryl alcohol or 1,4-dimethoxybenzene, were performed as indicated in Table III and IV in 6 ml of phosphate buffer (pH 3.0). Other conditions were the same as for the oxidation of veratryl alcohol described above. TDCSPPFeCl remaining after the reaction was estimated from its Soret band absorption before and after the reaction. For the decolorization of Poly B-411 (IV) by TDCSPPFeCl and mCPBA, 25 //moles of mCPBA were added to 25 ml 0.05% Poly B-411 containing 0.01 //moles TDCSPPFeCl, 25 //moles of manganese sulfate and 1.5 mmoles of lactic acid buffered at pH 4.5. The decolorization of Poly B-411 was followed by the decrease in absorption at 596 nm. For the electrochemical decolorization of Poly B-411 in the presence of veratryl alcohol, a two-compartment cell was used. A glassy carbon plate was used as the anode, a platinum plate as the auxiliary electrode, and a silver wire as the reference electrode. The potential was controlled at 0.900 V. Poly B-411 (50 ml, 0.005%) in pH 3 buffer was added to the anode compartment and pH 3 buffer was added to the cathode compartment to the same level. The decolorization of Poly B-411 was followed by the change in absorbance at 596 nm and the simultaneous oxidation of veratryl alcohol was followed at 310 nm. The same electrochemical apparatus was used for the decolorization of Poly B-411 adsorbed onto filter paper. Tetrabutylammonium perchlorate (TBAP) was used as supporting electrolyte when methylene chloride was the solvent. 

The above principles do not apply solely to silver salts and chlorides, but to all solutions. Reference to the table of solubilities on page 364 will inform one whether or not a precipitate will be formed when any two solutions of electrolytes are mixed together. 

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