Gold anode, platinum

By-Product Recovery. The anode slime contains gold, silver, platinum, palladium, selenium, and teUurium. The sulfur, selenium, and teUurium in the slimes combine with copper and sUver to give precipitates (30). Some arsenic, antimony, and bismuth can also enter the slime, depending on the concentrations in the electrolyte. Other elements that may precipitate in the electrolytic ceUs are lead and tin, which form lead sulfate and Sn(0H)2S04. 

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). 

Moebius An electrolytic process for removing gold and platinum from silver. The crude metal, known as Dore, is used as the anode. The cathodes are of silver or stainless steel. The electrolyte is a diluted solution of silver nitrate and nitric acid. Gold and other metals collect as anode slimes. Invented in Mexico by B. Moebius, first operated there in 1884, and subsequently widely operated in Germany and the United States. See also Balbach, Thum, Wohlwill. 

M. Morita, Y. Iwanaga, and Y. Matsuda, Anodic-oxidation of methanol at a gold-modified platinum electrocatalyst prepared by RE-sputtering on a glassy-carbon support, Electrichim. Acta 36, 947-951 (1991). 

We saw in Section 3.1 that CEC consists in substituting the anode platinum wire by a gold him mechanically squeezed between two flat insulahng substrates and that the physical conflnement favours a quasi-2D gro vth. We can here raise the question, what would happen if we directly use free large surface anodes as substrates We next see two examples, using gold plates and silicon wafers as electrodes. 

At the impure Cu anode, copper is oxidized along with more easily oxidized metallic impurities such as zinc and iron. Less easily oxidized impurities such as silver, gold, and platinum fall to the bottom of the cell as anode mud, which is reprocessed to recover the precious metals. At the pure Cu cathode, Cu2+ ions are reduced to pure copper metal, but the less easily reduced metal ions (Zn2+, Fe2+, and so forth) remain in the solution. 

Bryce and co-workers reported that the crown-annulated TTF derivatives 98 and 99 were used for UV-Vis spectroscopic and electrochemical studies of metal complexation <1996J(P2)1587>. Solution electrochemical studies showed that metal complexation to the crown unit leads to a small anodic shift in the first oxidation potential of the TTF system. Langmuir-Blodgett films of amphiphilic 99 have been assembled on solid substrates by Y-type deposition. Compounds 100-104 were used to prepare self-assembled monolayers on gold and platinum surfaces <1998AM395, 2000JOC8269>. The self-assembled monolayers of 104 were the most stable in this series of TTF-crowns. Electrochemical data for the self-assembled monolayers of 100-104 in MeCN showed two reversible one-electron waves, typical of the TTF system. The self-assembled monolayers of 102-104 exhibited an electrochemical response in aqueous electrolyes, which was observed between 50 and 100 cycles. 

Noble metal impurities in the anode are not oxidized at the voltage used they fall to the bottom of the cell to form a sludge, which is processed to remove the valuable silver, gold, and platinum. 

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. 

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. 

Sulfonamides have been studied for cathodic and anodic activity at rotating disk electrodes of gold and platinum in 0.1 M H2SO4 and 0.1 M Na2C03 solutions. All the sulfonamides display anodic activity. The free (protonated) amino group of the sulfonamides is oxidized by a one-electron reaction to the protonated radical cation imine (73). 

Either a gold or platinum electrode may be used in a PAD, although gold electrodes are more popular. One reason is that dissolved oxygen contributes a cathodic response at a platinum electrode over the entire useful range for anodic detection. Serious oxygen interference at the gold electrode can be avoided by careful selection of detection potential. 

The electrolysis is performed either m a platinum or an alumina cell, this being placed in a temperature-controlled furnace. Electrodes are usually either gold or platinum beads or plates soldered to the lead wires. A carbon anode can also be used ... 

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