Iodometry

IODOMETRY, a method of quantitative volumetric analysis, based on the reaction of reduction of free iodine to iodide ion or oxidation of iodide ion to free iodine: J₂ + 2e ;£ 2J'; (e--denotes an electron). The direction of the reaction depends on the oxidizing ability of the substances reacting with iodine: e.g., with Na₂S₂O₃ the reaction proceeds from left to right, with KMnO₄, K₂Cr₂O₇-from right to left. The degree of oxidizing ability of oxygen-containing compounds depends on the activity of the reaction medium (concentration of ions); by changing it, the direction of the process can be easily changed; for example, in an acidic medium As₂O₆ liberates free iodine from iodides, turning into As₂O₅; in a slightly alkaline or neutral medium As₂O₃ is oxidized by iodine to As₂O₅. The principal reaction in I. is the reduction of J by a solution of sodium thiosulfate (hyposulfite, sodium thiosulfate-Na₂S₂O₃): J₂ + 2S₂O₃' -* -»S₄O₆'-f-2J'. During titration, an alkaline reaction should be avoided, as in this case a side reaction of oxidation of Na₂S₂O₃ to sulfate occurs: S₂O₃' + 4 J₂ + 10OH'-* -» 2SO₄'+8 J'-f-5 H₂O. The indicator in titration is usually a starch solution (10 cm? of 0.2%-solution per 100 cm? of liquid), which gives a blue coloration with iodine in the presence of soluble iodides. The sensitivity of the reaction in aqueous solution at 15°=2.10^-5 n. When titrating to the disappearance of blue coloration, it often reappears after some time. This may depend on 1) the slow course of the iodine reduction reaction; 2) oxidative processes due to atmospheric oxygen (especially in sunlight); 3) the participation of foreign substances in the reaction; nitric acid has a particularly strong influence; present in negligible amounts (e.g., in laboratory air), nitric acid can oxidize significant amounts of iodides. In I., n/10 solutions are most often used, from which solutions of lower concentration can be prepared by dilution. Two solutions are needed: thiosulfate and iodine. To prepare an n/10 solution of Na₂S₂O₃, about 25 g of chemically pure substance (Na₂S₂O₃. .5 H₂O) are dissolved in 1 liter of water and the titer is established 10-14 days after preparation. The thiosulfate solution should be stored in a dark place and the titer should be checked every 2 months. Once the change in titer begins, it quickly progresses; it is best to establish and check the titer of the thiosulfate solution against metallic iodine. For this, iodine is triturated with an equal weight amount of KI and sublimed. Weigh a weighing vial together with the 2-2.5 g KI and 0.5 cm? water contained in it; slightly opening the vial lid, add about 0.5 g of sublimed purified iodine and weigh again. The closed vial is transferred to a wide-mouthed flask containing 200 cm? of 1% KI solution. The vial should be opened under the solution or at the moment of immersion in the liquid. Titrate with thiosulfate solution to a pale yellow coloration, add starch solution and continue titration until the blue coloration disappears. The normality of thiosulfate is calculated by the formula with Q °i2693 (^a '- ^sample of iodine; C-amount of cm? of thiosulfate solution used in titration; 0.012693- amount of iodine contained in 1 cm? of its n/10 solution). The titer of thiosulfate can also be checked with solutions of chemically pure potassium iodate KJO₃ (3.5672 g in 1 liter), or its acid salt KJO₃. .HJO₃ (3.2508 g), or potassium dichromate K₂Cr₂O₇ (4.9083 g) and others. In all the cases listed, the reaction proceeds in an acidic medium and is based on the liberation of iodine from iodides in an amount equivalent to the amount of oxidant taken. A decinormal solution of iodine is prepared by dissolving 12.8-13 g of iodine and 25 g of potassium iodide in 1 liter of water. The solution is not stable, and its titer should be checked from time to time; the solution should be stored in the dark. The titer is established against the thiosulfate solution as described above.--Thanks to the very high sensitivity of the reaction of iodine with starch and the distinctness of color change at the end of titration, I. is considered one of the best methods of quantitative analysis and has found wide application in inorganic, organic, and biological chemistry. In inorganic chemistry, I. is used for the quantitative determination of free halogens, <;(>:; iodides, bromides, sulfurous acid, hydrogen sulfide, chromic acid, many heavy metals. I. is used for establishing the titer of strong acids, etc. In organic chemistry, I. is used in quantitative determinations of aldehyde and ketone groups, for determining acetone, quinone and hydroquinone, antipyrine and many others. In biological chemistry, many microanalysis methods are based on iodometry.