Sodium iodide solution, electrolysis

Immediately upon connecting the cell to a source of direct current, a deposit of gray metallic zinc appears on the surface of the cathode and bubbles of chlorine gas appear at the surface of the anode. A simple chemical test for chlorine may be made by leading this gas into an aqueous sodium iodide solution, whereupon the solution assumes a yellow color caused by displacement of iodine by chlorine. Accordingly, it is concluded that the products of the electrolysis of a zinc chloride solution are elemental zinc and elemental chlorine, and the next problem is that of explaining the mechanism by which these products may be produced. 

An unusual type of reaction is anodic reduction which can be performed at certain metal anodes. Thus, when magnesium is used as an anode in the electrolysis of benzophenone in pyridine/sodium iodide solution, the anode is consumed and benzopinacol can be isolated from the anolyte 15S). Here reduction by univalent magnesium ion is postulated ... 

In any aqueous solution in which electrolysis is to take place, the electrolysis of water is also possible. For example, consider the electrolysis of a sodium iodide solution, as shown in Figure 18.24 t. For the electrolysis of molten Nal, we can readily predict that 1 is oxidized at the anode and that Na is reduced at the cathode. In an aqueous solution, however, two different oxidation half-reactions are possible at the anode, the oxidation of 1 and the oxidation of water ... 

The electrolysis is best performed as follows A solution of 13-15 g. calcined soda and 10 g. potassium iodide in 100 cc. water and 20 cc. alcohol is placed in a porous earthenware cylinder with platinum anode. The cathode, of nickel, is surrounded by a strong solution of sodium hydroxide. The electrolysis is carried out at a temperature of 70° C., with a current density at the anode of 1 amp. per 100 sq. cm., and is continued for 2-3 hours. After several hours the iodoform crystallizes out, the current yield being from 60-70 per cent. The chief by-product remaining in the mother liquor is sodium iodate. 

Sodium hypoiodite, NaOI.—The hypoiodite has never been isolated, but is formed in dilute aqueous solution by the interaction of sodium hydroxide and iodine, and to a small extent by the electrolysis of an alkaline solution of sodium iodide. The substance is extremely unstable, reduction to iodide and oxidation to iodate taking place simultaneously.5... 

Industrially, iodoform is prepared by electrolysis, at 60- 5 C, of a solution of 60 kg of KI, 20 kg of Na COs, and 80 liters of ethanol per 400 liters of solution. The iodine set free by the current converts the ethanol and the sodium carbonate into iodoform and sodium iodide. In practice, part of the iodine reacts with the caustic alkali formed at the cathode and forms an iodate. Prevention of this reaction by surrounding the cathode with a porous cup brings the yield of iodoform up to about 90 per cent of the theoretical. 

If an electric current is passed through aqueous solutions of sodium chloride, sodium bromide, and sodium iodide, the elemental halogens are produced at one electrode in each case, with hydrogen gas being evolved at the other electrode. If the liquid is then evaporated from the mixture, a residue of sodium hydroxide remains. Write balanced chemical equations for these electrolysis reactions. 

Determination. The most accurate (68) method for the deterrnination of copper in its compounds is by electrogravimetry from a sulfuric and nitric acid solution (45). Pure copper compounds can be readily titrated using ethylene diamine tetracetic acid (EDTA) to a SNAZOXS or Murexide endpoint. lodometric titration using sodium thiosulfate to a starch—iodide endpoint is one of the most common methods used industrially. This latter titration is quicker than electrolysis, almost as accurate, and much more tolerant of impurities than is the titration with EDTA. Gravimetry as the thiocyanate has also been used (68). 

Lead dissolves in liquid ammonia solutions of sodium giving a highly coloured liquid. The formation of pol) plumbides has been demonstrated by electrometrically titrating a solution of sodium in ammonia with one of lead iodide in ammonia the compounds formed are Na Pb and Na Pbg. The electrolysis of such a solution releases Na at the cathode and Pb at the anode. Evaporation gives [Na(NH3) ]4Pbg which loses NHg to leave pyrophoric Na Pbg. These compounds seem to possess a character between that of a true valency compound and an intermediate phase (Zintl, 1929),... 

Procedure, Oxidize in 0-2 n sodium acetate for (III) and in a 2 per cent solution of acetic acid for (IV). Carry out the oxidation for 6 (III) or 4 (IV) min and then add 2 n sulphuric acid to change the pH to 1-0. When the purple colour of iodoadrenochrome appears add potassium iodide. Make the final polarographic electrolysis in an acetate pH 4-5 buffer, so that the quinoid system is reduced at positive potentials (F1/3 = +0-03 V). 

Because the electrolysis started with sodium chloride, the cation in the electrolyte solution is Na. When you evaporate the electrolyte solution at the cathode, you obtain sodium hydroxide, NaOH. Figure 20.21 shows the similar electrolysis of aqueous potassium iodide, KI. 

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