Direct titration with iodine

In most direct titrations with iodine (iodimetry) a solution of iodine in potassium iodide is employed, and the reactive species is therefore the tri-iodide ion 13. Strictly speaking, all equations involving reactions of iodine should be written with 13 rather than with I2, e.g. 

Theory Iodine in aqueous solution acts as an oxidizing agent which forms the basis of assay methods involving direct titration with iodine. Thus, we have ... 

The following pharmaceutical substances can be assayed by direct titration with iodine as stated in Table 7.1. 

Table 7.1 Substances Assayed by Direct Titration with Iodine...

In reactions where there is no change in the number of charges on the ions the ionic strength does not change and the reaction rates are somewhat simpler. A reaction of interest in this connection is the oxidation of the arsenite ion by the tellurate ion.18 The reaction was chosen because it goes with conveniently measurable velocity at the temperature of boiling water and the reaction can be followed easily by direct titration with iodine. More reactions of this type would undoubtedly be available for study at the higher temperatures if suitable analytical methods could be found. The reaction is... 

A similar procedure may also be used for the determination of antimony(V), whilst antimony (III) may be determined like arsenic(III) by direct titration with standard iodine solution (Section 10.113), but in the antimony titration it is necessary to include some tartaric acid in the solution this acts as complexing agent and prevents precipitation of antimony as hydroxide or as basic salt in alkaline solution. On the whole, however, the most satisfactory method for determining antimony is by titration with potassium bromate (Section 10.133). 

Gold is estimated quantitatively by methods similar to those employed in its qualitative detection. It can be precipitated with a standard solution of ferrous sulphate or potassium oxalate,10 and the excess estimated by titration with permanganate or precipitated with stannous chloride in alkaline solution, the excess of tin being ascertained by titration with iodine.11 Other methods are direct titration with stannous... 

If the arsinic acid is sjmringly solutfie its sodium salt may be used, and with nitro-compounds tlie reaction is conducted in the cold to minimise rciduction of tlio nitro-group by the thiol-compound. The thioarsinites arc rccrystallised from hot water or dilute acetic acid, and may be estimated rajndly by direct titration with standard iodine solution ... 

Even when errors in true free sulfur dioxide content due to dilution or oxidation are avoided, the direct iodometric determination of sulfurous acid and sulfites is subject to error. Eolthoff and Menzel (1929) point out that an appreciable error occurs when sulfite solutions are titrated with iodine owing to oxidation by air and point out that correct results can be obtained only when the sulfurous acid or sulfite solution is allowed to flow into a solution of iodine. The titration of sulfurous acid or bisulfites is... 

Determined through direct titration with starch indicator and iodine. 

A range of iodine concentrations was used with controlled potassium iodide concentrations and equilibrated over known weights of carbon. To know extents of adsorption of iodine by the carbon, the latter was centrifuged from the solution and the equilibrated iodine desorbed from the carbon by shaking the carbon with benzene and titrating the benzene with sodium thiosulfate solution. Total iodine concentrations ([I2] + [I ]) were measured by direct titration, with sodium thiosulfate, of the solution as separated from the carbon. Concentrations of I2 were calculated from the equilibrium constant for the reaction [I2 + 1 I ] taken as 870 L mol at 298 K. 

Sodium thiosulfate cannot be directly titrated with potassium permanganate because a mixture of tetrathionate and sulfate ions is formed. The titration is indirect. Potassium iodide is oxidized by permanganate ions in sulfuric acid medium. The liberated iodine is titrated with thiosulfate. Let s recall that the used thiosulfate solution must have been quite recently standardized. 

The direct iodometric titration method (sometimes termed iodimetry) refers to titrations with a standard solution of iodine. The indirect iodometric titration method (sometimes termed iodometry) deals with the titration of iodine liberated in chemical reactions. The normal reduction potential of the reversible system ... 

Discussion. One of the most useful titrations involving iodine is that originally developed by Winkler18 to determine the amount of oxygen in samples of water. The dissolved oxygen content is not only important with respect to the species of aquatic life which can survive in the water, but is also a measure of its ability to oxidise organic impurities in the water (see also Section 10.103). Despite the advent of the oxygen-selective electrode (Section 16.36) direct titrations on water samples are still used extensively.19... 

In determinations of sulphurous acid and sulphites, excess of standard 0.05M iodine is diluted with several volumes of water, acidified with hydrochloric or sulphuric acid, and a known volume of the sulphite or sulphurous acid solution is added slowly and with constant stirring from a burette, with the jet close to the surface of the liquid. The excess of iodine is then titrated with standard 0.1M sodium thiosulphate. Solid soluble sulphites are finely powdered and added directly to the iodine solution. Insoluble sulphites (e.g. calcium sulphite) react very slowly, and must be in a very fine state of division. 

The end point of the reaction is conveniently determined electrometrically using the dead-stop end point procedure. If a small e.m.f. is applied across two platinum electrodes immersed in the reaction mixture a current will flow as long as free iodine is present, to remove hydrogen and depolarise the cathode. When the last trace of iodine has reacted the current will decrease to zero or very close to zero. Conversely, the technique may be combined with a direct titration of the sample with the Karl Fischer reagent here the current in the electrode circuit suddenly increases at the first appearance of unused iodine in the solution. 

By this method the weighed dry product is dissolved in methanol and titrated with the Karl Fischer solution until the color changes from brown to yellow. The visual observation can be replaced by an ammeter, which shows an steep increase in current, when the titration is terminated (dead-stop-titration). The samples can be two to four times smaller than for the gravimetric method. To avoid the visual observation completely, iodine can be produced by electrolyzation and the water content is calculated by Coulomb s law. Such an apparatus (e. g. Fig. 1.97.1 and 1.97.2) is available commercially. The smallest amount of water to be detected by such instruments is 10 pg. Wekx and De Kleijn [1.84) showed, how the Karl Fischer method can be used directly in the vial with the dried product. The Karl... 

If the ether is evaporated under reduced pressure (Note 5), crystalline monoperphthalic acid is obtained. It is more convenient, however, to use the ether solution directly (Note 6). Its peracid content is determined by adding to 2 cc. of the solution 30 cc. of 20 per cent potassium iodide solution and titrating the iodine after ten minutes with 0.05 N thiosulfate solution. The yield is 60-65 g. (65-70 per cent of the theoretical amount based on the phthalic anhydride) (Note 7). 

Hydrogen sulphide is sometimes estimated volumetrically by treating a solution containing less than 0-04 per cent, with excess of standard iodine and then after a short period titrating the residual iodine in the usual manner with sodium thiosulphate or arsenious oxide accurate direct titration is not possible 3... 

When a reducing analyte is titrated directly with iodine (to produce 1 ), the method is called iodimetry. In iodometry, an oxidizing analyte is added to excess I to produce iodine, which is then titrated with standard thiosulfate solution. 

Quadrivalent cerium salts may be volumetrically determined by arsenious acid in aqueous sulphuric acid solution using a manganese salt as catalyst, with potassium iodate present as a promoter.2 Excess of arsenious acid is used and back-titrated with permanganate. If nitric acid is present in place of sulphuric acid, an alkali chloride and a trace of iodine are used to promote the action of the manganese salt. Direct potentiometric titration with arsenious acid may also be employed. 

Potassium iodide solution in excess is added to the sample. Analyte such as chlorine liberates iodine from KI under acidic condition. The liberated iodine is directly titrated against standard Na2S20, or PAO. The concentration of the iodine liberated is proportional to the concentration of the analyte in the sample. The reaction of chlorine with potassium iodide is as follows ... 

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