Chloride anodes

Polarographic Electrodes. Polarographic electrodes usually contain a platinum or gold cathode, a silver/silver chloride anode, and a potassium chloride electrolyte. Figure 4.3a shows a schematic representation of a polarographic electrode. When the anode of the electrode is polarized by an external power supply, the following reactions take place at the surface of the electrode (Linek et al., 1985 Turner and White, 1999 van Dam-Mieras et al., 1992) ... 

Anodic protection of steel can be achieved by applying a small anodic current using a less expensive metal cathode. However, it is necessary to make sure that the protective, passive film on the surface is not attacked by tmce impurities such as chloride. Anodic protection of steel storage tanks for 50% NaOH is successful at a current density of 1A m , but it is not used with diaphragm-cell caustic because of the presence of salt. 

Although a variety of different anode-cathode combinations for oxygen electrode are available, the platinum with silver/silver chloride is the most used cathode-anode combination. The often found arrangement of these electrodes is annular with the tubular silver/silver chloride anode enclosing the platinum cathode. The electrodes dip into an electrol)de solution (usually a buffered potassium chloride solution) which is held inside an electrode by an ojqrgen permeable membrane. The membrane might be a very thin polypropylene. Polarization of electrodes at 0.6 V is achieved with the help of a mercury cell. 

PO2 can be determined by a means of a PO2 electrode (Clark electrode). In this technique, oxygen diffuses from the blood sample across a gas-permeable membrane into an electrochemical system which consists of a platinum cathode and a silver/silver chloride anode. Reduction of the oxygen occurs at the cathode, resulting in the generation of a current which can be measured, the current being directly proportional to the PO2. This is an example of an amperometric technique... 

Figure 8.4 An Electrochemical Cell with a Silver-Silver Chloride Anode and a Calomel Cathode.

Birss V I and Smith C K 1987 The anodic behaviour of silver in chloride solutions-l. The formation and reduction of thin silver chloride films Electrochim. Acta 32 259-68... 

Sodium hydroxide is manufactured by electrolysis of concentrated aqueous sodium chloride the other product of the electrolysis, chlorine, is equally important and hence separation of anode and cathode products is necessary. This is achieved either by a diaphragm (for example in the Hooker electrolytic cell) or by using a mercury cathode which takes up the sodium formed at the cathode as an amalgam (the Kellner-Solvay ceW). The amalgam, after removal from the electrolyte cell, is treated with water to give sodium hydroxide and mercury. The mercury cell is more costly to operate but gives a purer product. 

The aqueous solution of sodium chlorate(I) is an important liquid bleach and disinfectant. It is produced commercially by the electrolysis of cold aqueous sodium chloride, the anode and cathode products being mixed. The sodium chloride remaining in the solution does not usually matter. There is evidence to suggest that iodic(I) acid has some basic character... 

The component electrochemical reactions are the discharge of chloride ions, Cl , at the anode. 

The purification of the galHum salt solutions is carried out by solvent extraction and/or by ion exchange. The most effective extractants are dialkyl-phosphates in sulfate medium and ethers, ketones (qv), alcohols, and trialkyl-phosphates in chloride medium. Electrorefining, ie, anodic dissolution and simultaneous cathodic deposition, is also used to purify metallic galHum. 

This is essentially a corrosion reaction involving anodic metal dissolution where the conjugate reaction is the hydrogen (qv) evolution process. Hence, the rate depends on temperature, concentration of acid, inhibiting agents, nature of the surface oxide film, etc. Unless the metal chloride is insoluble in aqueous solution eg, Ag or Hg ", the reaction products are removed from the metal or alloy surface by dissolution. The extent of removal is controUed by the local hydrodynamic conditions. 

Fused Salt Electrolysis. Only light RE metals (La to Nd) can be produced by molten salt electrolysis because these have a relatively low melting point compared to those of medium and heavy RE metals. Deposition of an alloy with another metal, Zn for example, is an alternative. The feed is a mixture of anhydrous RE chlorides and fluorides. The materials from which the electrolysis cell is constmcted are of great importance because of the high reactivity of the rare-earth metals. Molybdenum, tungsten, tantalum, or alternatively iron with ceramic or graphite linings are used as cmcible materials. Carbon is frequently used as an anode material. 

Silver reduces the oxygen evolution potential at the anode, which reduces the rate of corrosion and decreases lead contamination of the cathode. Lead—antimony—silver alloy anodes are used for the production of thin copper foil for use in electronics. Lead—silver (2 wt %), lead—silver (1 wt %)—tin (1 wt %), and lead—antimony (6 wt %)—silver (1—2 wt %) alloys ate used as anodes in cathodic protection of steel pipes and stmctures in fresh, brackish, or seawater. The lead dioxide layer is not only conductive, but also resists decomposition in chloride environments. Silver-free alloys rapidly become passivated and scale badly in seawater. Silver is also added to the positive grids of lead—acid batteries in small amounts (0.005—0.05 wt %) to reduce the rate of corrosion. 

The most significant nonferrous use of manganese compounds is for primary batteries, where manganese dioxide is the principal constituent of the cathode mix. In the standard Leclanchn ceU, 2inc and ammonium chloride are mixed to form the electrolyte, a mixture of carbon and MnO forms the cathode, and 2inc acts as the anode (221). The principal ceU reaction is as foUows ... 

Electrolytic Preparation of Chlorine and Caustic Soda. The preparation of chlorine [7782-50-5] and caustic soda [1310-73-2] is an important use for mercury metal. Since 1989, chlor—alkali production has been responsible for the largest use for mercury in the United States. In this process, mercury is used as a flowing cathode in an electrolytic cell into which a sodium chloride [7647-14-5] solution (brine) is introduced. This brine is then subjected to an electric current, and the aqueous solution of sodium chloride flows between the anode and the mercury, releasing chlorine gas at the anode. The sodium ions form an amalgam with the mercury cathode. Water is added to the amalgam to remove the sodium [7440-23-5] forming hydrogen [1333-74-0] and sodium hydroxide and relatively pure mercury metal, which is recycled into the cell (see Alkali and chlorine products). 

The most common fused salt baths are complex mixtures of alkah chlorides, rigorously purified and dried. Fused salt plating must be done under an inert atmosphere. Often argon is used because nitrogen can react with some metals. Inert anodes, eg, Pt-coated titanium or graphite, are used and the plating metal is suppHed by additions of an appropriate metal salt. 

The zinc electrolyte contains ca 60 kg/m zinc as sulfate and ca 100 kg/m free sulfuric acid. It is electrolyzed between electrodes suspended vertically in lead or plastic-lined, eg, poly(vinyl chloride), concrete tanks. The insoluble anodes are made of lead with small amounts of silver. The anodic... 

Other Metals. AH the sodium metal produced comes from electrolysis of sodium chloride melts in Downs ceUs. The ceU consists of a cylindrical steel cathode separated from the graphite anode by a perforated steel diaphragm. Lithium is also produced by electrolysis of the chloride in a process similar to that used for sodium. The other alkaH and alkaHne-earth metals can be electrowon from molten chlorides, but thermochemical reduction is preferred commercially. The rare earths can also be electrowon but only the mixture known as mischmetal is prepared in tonnage quantity by electrochemical means. In addition, beryIHum and boron are produced by electrolysis on a commercial scale in the order of a few hundred t/yr. Processes have been developed for electrowinning titanium, tantalum, and niobium from molten salts. These metals, however, are obtained as a powdery deposit which is not easily separated from the electrolyte so that further purification is required. 

Nickel [7440-02-0] Ni, recognized as an element as early as 1754 (1), was not isolated until 1820 (2). It was mined from arsenic sulfide mineral deposits (3) and first used in an alloy called German Silver (4). Soon after, nickel was used as an anode in solutions of nickel sulfate [7786-81 A] NiSO, and nickel chloride [7718-54-9] NiCl, to electroplate jewelry. Nickel carbonyl [13463-39-3] Ni(C02)4, was discovered in 1890 (see Carbonyls). This material, distilled as a hquid, decomposes into carbon monoxide and pure nickel powder, a method used in nickel refining (5) (see Nickel and nickel alloys). 

For many waste streams, electrical efficiencies are compromised owing to the corrosivity of the solution toward the precipitated metals and/or the low concentrations of metals that must be removed. The presence of chloride in the solution is particularly troublesome because of the formation of elemental chlorine at the anode. Several commercial cells have become available that attempt to address certain of these problems (19). 

---