Silver® ions, reaction with zinc

Other electrolytes, such as sodium sulfate or potassium nitrate, could be chosen for the salt bridge. Neither of these electrolytes interferes in the cell reaction. Silver nitrate, AgN03(aq), would be a poor choice for the salt bridge, however. Positive silver ions would migrate into the half-cell that contains the cathode. Zinc displaces both copper and silver from solution, so both copper(n) ions and silver ions would be reduced at the cathode. The copper produced would be contaminated with silver. 

Submicrometer uniform crystalline spheres of silver-doped zinc sulfide (ZnS Ag) were prepared by aging 0.04 mol dm-3 Zn(N03)2 and 2.80 X 10-6 to l. 68 X 10-5 mol dm-3 AgN03 with 0.4 mol dm-3 TAA for up to 100 min at initial pH 1.52 and 73°C (15). The authors found that the final particle density decreased with increasing content of silver ions, whereas the total reaction rate was virtually unaffected by the significant difference in the total surface area of the particle. In fact, the final particle diameter increased from 0.3 to 1.1 pm with increase in the content of silver ions from 5.6 to 16.8 pmol dm-3, as shown in Figure 3.1.5. Figure 3.1.6 shows the time evolutions of [Zn2+] and [Ag+] in the solution phase. 

The bonding of CN - to gold and silver is exploited in the extraction of these metals from their ores. The crushed rock containing small amounts of the precious metals is treated with an aerated cyanide solution, and the metals are then recovered from their M(CN)2- complex ions by reduction with zinc. For gold, the reactions are... 

Quite frequently the natural surface of a mineral requires preliminary chemical treatment before it will form the surface film required for collection One of the commonest instances of this is with sphalerite (zinc sulphide), which does not float properly when treated with xanthates. If, however, it is given a preliminary treatment with dilute copper sulphate solution, a very small amount of copper sulphide is deposited on the surface and the ore becomes floatable, the surface being now capable of reaction with xanthates. Such treatment is usually termed activation in general, an activating solution for a sulphide mineral should contain a metallic ion whose sulphide is less soluble than that contained in the mineral for zinc sulphides, silver, copper, mercury, cadmium, and lead salts are all effective activators. 

Reduction of nitrates in alkaline medium Ammonia is evolved (detected by its odour by its action upon red litmus paper and upon mercury(I) nitrate paper or by the tannic acid-silver nitrate test, Section III.38, reaction 7) when a solution of a nitrate is boiled with zinc dust or gently warmed-with aluminium powder and sodium hydroxide solution. Excellent results are obtained by the use of Devarda s alloy (45 per cent Al, 5 per cent Zn, and 50 per cent Cu). Ammonium ions must, of course, be removed by boiling the solution with sodium hydroxide solution (and, preferably, evaporating almost to dryness) before the addition of the metal. 

Cyanides, thiocyanates, hexacyanoferrate(II)s, and hexacyanoferrate(III)s also yield ammonia under these experimental conditions. The reaction is somewhat slower for these anions up to 5 minutes may elapse before ammonia can be detected from hexacyanoferrate(II)s and hexacyanoferrate(III)s. If these are present, or are suspected as a result of the preliminary tests, particularly that with concentrated sulphuric acid, they must first be removed as follows. Treat the soda extract with excess of nitrate-free silver sulphate, warm the mixture to about 60°, shake vigorously for 3-4 minutes, and filter from the silver salts of the interfering anions and excess of precipitant. Remove the excess silver ions from the filtrate by adding excess sodium hydroxide solution and filter off the precipitated silver oxide. Evaporate the filtrate to about half bulk and test with zinc, aluminium or Devarda s alloy. If cyanides alone are present, they may be rendered innocuous by the addition of a little mercury(II) chloride solution. 

The jarosite process separates icon(III) from zinc in acid solution by precipitation of MFe2(0H)g(S0 2 where M is an alkali metal (usuaUy sodium) or ammonium (see Fig. 2) (40,41). Other monovalent and hydronium ions also form jarosites which are found in the precipitate to some degree. Properly seeded, the relatively coarse jarosite can be separated from the zinc-bearing solution efficiently. The reaction is usuaUy carried out at 95 0 by adding ammonia or sodium hydroxide after the pH has been adjusted with calcine and the iron oxidized. The neutral leach residue is leached in hot acid (spent + makeup) with final acidity >20 g/L and essentiaUy aU the zinc, including ferrite, is solubilized. Ammonium jarosite is then precipitated in the presence of the residue or after separating it. If the residue contains appreciable lead or silver, they are first separated to avoid loss to the jarosite waste solids. Minimum use of calcine in jarosite neutralization is required for TnaxiTniiTn recovery of lead and silver as weU as zinc and other metals. 

Thus the electrophiles which attack cyanide ion on nitrogen will be the harder ones, and the ones which attack on carbon will be the softer ones. This fits with the reactions illustrated above. As already seen (pp. 38-39) alkyl halides in simple Sn2 reaction are soft electrophiles thus it is appropriate for cyanide to react from the soft end of the ion (32). When a silver ion is present (other Lewis acids like zinc and mercuric ion behave similarly), the halide ion is assisted in leaving the carbon atom, and in the transition state there is now a greater development of charge on the carbon atom undergoing substitution (33). Car-bonium ions are hard electrophiles, and therefore it is again appropriate that on this occasion cyanide ion should react from the harder end of the ion. 

At first glance, the equation representing the reaction of zinc metal with silver(l) ions in solution might appear to be balanced. 

These considerations show the essentially thermodynamic nature of and it follows that only those metals that form reversible -i-ze = A/systems, and that are immersed in solutions containing their cations, take up potentials that conform to the thermodynamic Nernst equation. It is evident, therefore, that the e.m.f. series of metals has little relevance in relation to the actual potential of a metal in a practical environment, and although metals such as silver, mercury, copper, tin, cadmium, zinc, etc. when immersed in solutions of their cations do form reversible systems, they are unlikely to be in contact with environments containing unit activities of their cations. Furthermore, although silver when immersed in a solution of Ag ions will take up the reversible potential of the Ag /Ag equilibrium, similar considerations do not apply to the NaVNa equilibrium since in this case the sodium will react with the water with the evolution of hydrogen gas, i.e. two exchange processes will occur, resulting in an extreme case of a corrosion reaction. 

H. 8-Hydroxyquinaldine (XI). The reactions of 8-hydroxyquinaldine are, in general, similar to 8-hydroxyquinoline described under (C) above, but unlike the latter it does not produce an insoluble complex with aluminium. In acetic acid-acetate solution precipitates are formed with bismuth, cadmium, copper, iron(II) and iron(III), chromium, manganese, nickel, silver, zinc, titanium (Ti02 + ), molybdate, tungstate, and vanadate. The same ions are precipitated in ammoniacal solution with the exception of molybdate, tungstate, and vanadate, but with the addition of lead, calcium, strontium, and magnesium aluminium is not precipitated, but tartrate must be added to prevent the separation of aluminium hydroxide. 

While the silver and zinc salts were effective Lewis acids for these cyclizations, Kikugawa and coworkers reported that the alkoxynitrenium ions could be generated directly from hydroxamic esters (4) using hypervalent iodine oxidants such as hydroxy(tosyloxy) iodobenzene (HUB) and phenyliodine(lll)bis(trifluoroacetate) (PIFA) . Presumably, with such reagents the reactions proceed through A-(oxoiodobenzene) intermediates (54), which can themselves be regarded as anomeric hydroxamic esters and sources of alkoxynitrenium ions (55) (Scheme 11). 

The reaction is not disturbed by silver or copper, or by iron(III), chromium or aluminium in the presence of ammoniacal tartrate solution if zinc is present, ammonium chloride should first be added cobalt(III) ions represss the sensitivity and should be oxidized to the tervalent state with hydrogen peroxide iron(II) interferes and should be oxidized and alkaline tartrate solution added before applying the test. 

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