Gold anodes

Cyanides. Salts of the complex ion, [Au(CN)2] , can be formed directiy from gold, ie, gold dissolves ia dilute solutions of potassium cyanide ia the presence of air. Additionally, a gold anode dissolves ia a solution of potassium cyanide. The potassium salt can be isolated by evaporation of the solution and purified by recrystallization from water (177). Boiling of the complex cyanide ia hydrochloric acid results ia formation of AuCN [506-65-01]. Halogens add oxidatively to [Au(CN)2] to yield salts of [Au(CN)2X2] which are converted to the tetracyanoaurates usiag excess cyanide (178). These last can also be prepared directiy from the tetrahaloaurates. 

The mud or slime from the silver separation is processed to form impure gold anodes. These anodes are then electroly2ed to yield purified gold and to separate platinum and palladium for subsequent recovery (31). 

Notes 1, Gold anode 2, silver anode 3, mercury anode 4, mercury cathode for reagent see footnote J 5, trace of mercury(II) acetate dissolved in acetic acid/methanol mixture added as catalyst, t A, amperometric I, indicator P, potentiometric. 

B) an impure silver cathode and a pure gold anode... 

The complex cyanide, Na[Au(CN)2] is made by dissolving gold in a dilute solution of sodium cyanide in the presence of air or by dissolution of a gold anode in a solution of sodium cyanide during electrolysis. The solution is evaporated to separate the complex, Na[Au(CN)2], which is purified by recrystallization from water. Potassium cyanide may be used instead of sodium cyanide to prepare gold(I) cyanide. 

J. Tafel found that while nitric acid is reduced only to hydroxylamine q.v.) by mercury or well-amalgamated electrodes, a copper cathode reduces it to ammonia and at the same time has no action on hydroxylamine. A. Brochet and J. Petit studied the electro-reduction of nitric acid by an alternating current. T. H. Jeffery described the electrolysis of nitric acid with a gold anode, and obtained from the anode liquor crystals of aurinitric acid, HAu(N03)4.3H20. R. Ihle s observations on the oxidation-potential of nitric acid have been discussed in connection with nitrous acid (q.v.). He found that if the cone, of the nitric acid be expressed by... 

F. H. Jeffery observed that in the electrolysis,of 15 per cent, orthophosphoric add with a gold anode, a little of that metal passes into soln. For the affinity constant, vide sulphuric acid. 

Platinum and gold are regarded as unattackable metals because they are nearly always in the passive state. A gold anode dissolves in neutral or acid chloride solutions at very low c.d. s to form Au " + ions at a potential of about + 1.2 volts as the current is increased, however, the potential rises rapidly to + 1.7 volts, the electrode ceases to dissolve and chlorine is evolved instead. Platinum is actually less noble than gold, but it is rendered passive much more easily, especially in solutions of oxyacids if the electrolyte contains ammonia or hydrochloric acid, however, platinum suffers appreciable anodic attack, particularly if the metal is in a finely-divided form. 

When a gold anode is rendered passive in hydrochloric acid solution the conditions appear to be somewhat different from those described above experiments show that the gold dissolves in the tervalent state to form AuCli" ions in solution. There is consequently a limiting c.d. determined by the maximum rate of diffusion of chloride ions to the anode (cf. p. 459) when this rate is exceeded the potential must rise so as to permit another process to occur. The gold then becomes covered with a layer of oxide, produced either by reaction of Au++" ions with water, or by the direct action of oxygen or hydroxyl radicals, and ceases to dissolve. ... 

Hi quality gold nanocubes can also be prepared by electrochranical methods in the presence of the aiqxopriate surfactant and solvent [86,87]. As illustrated in Figure 11.30 below [87], gold for nanopaiticle growth is supplied through oxidation of a gold anode. Reduction takes place at an inert platinum cathode in the presence of the surfactant. 

Several derivatives of phenothiazine have been found to be oxidizable at a gold anode in dilute sulfuric acid [290]. The product proved to be the sulfoxide rather than the A-oxide. 

Figure 14 Scanning electromicrograph of a single reservoir in a prototypical MEMS drug delivery device. (A) The gold anode is in place over the small square end of the reservoir. (B) After passage of a small electrical potential the gold anode has dissolved. Abbreviation MEMS, microelectromechanical systems.

Reservoir-Based MEMS Drug Delivery System, Fig. 3 An electrocliemically actuated microcliip, (A) a cutaway portion of a microcliip (17 x 17 mm x 310 pm) containing reservoirs filled with the chemical to be released. Release from a particular reservoir is initiated by applying an electric potential between the gold anode... 

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