Reactions iodide-iodate test

The iodide-iodate test reaction is based on a system of two competing parallel reactions [48] ... 

It will be appreciated that iodate is incompatible with both iodide (cf. Section IV.21, reaction 6) and with thiocyanate (Section IV.21, reaction 9) since iodine is liberated in acid solution. Also sulphide is incompatible with both bromate and iodate (oxidation to sulphate occurs), and an arsenite is oxidized by iodate in acid solution. These facts should therefore be borne in mind when interpreting Table V.30. An independent test for iodate (test 11) is provided below this can be performed before the silver nitrate tests. 

Self-Test K.3B When sulfuric acid reacts with sodium iodide, sodium iodate and sulfur dioxide are produced. Identify the oxidizing and reducing agents in this reaction. 

Sei f-Test 12.2B When iodide ions react with iodate ions in basic aqueous solution, triiodide ions, I,, are formed. Write the net ionic equation for the reaction. (Note that the same product is obtained in each half-reaction.)... 

Hypophosphorous acid-starch solution test Iodates are reduced by hypo-phosphorous acid eventually to iodides. The reaction takes place in three stages (with the transitional formation of phosphorous acid) ... 

The selectivity of the test is quite limited, even compared to the specificity seen in the identification test for chlorides. In the identification three criteria have to be fulfilled to qualify for a positive reaction. The unknown should give a white (curdled) precipitate formed upon addition of silver nitrate, which is insoluble in dilute nitric acid but redissolves in ammonia. In the limit test 2.4.4. Chlorides any substance capable of giving a white or weakly colored precipitate in dilute nitric acid will give a response like chloride, and this should be remembered in case of an xmexpected result. For the sake of example the following ions and substances are capable of giving a false positive reaction bromide, iodide, bromate, iodate, sulfite, chlorate, oxalate, and benzoate. In addition to this a variety of more complex organic substances are likely to precipitate, for example, alkaloids. 

As the experiments showed, iodide and iodate were formed in a ratio of about 5 1 upon contact of I2 with the basic aerosol materials. Apparently, it is easier for I2 to disproportionate on the surface than it is for it to undergo a redox reaction with ions in the crystal to form Csl alone. Thermochemical data show that the formation of iodide and iodate would result in a lower free energy of reaction than formation of iodide alone. Formation of iodate alone would give a lower iodine potential than formation of iodide and iodate however, iodate formation seems to be limited by reaction kinetics. The extent of I2 reaction with anhydrous CS2O and CS2CO3 is probably limited only by the surface concentration of iodide and iodate which prevents or delays further interaction between I2 and the host crystal. In tests in which saturated aqueous solutions of these compounds were present, no such limitation was observed, nor had it been expected. 

Upon contact of gas-phase I2 with stainless steel surfaces, a deposited amount of iodine on the order of 0.1 mg /cm was measured, whereas comparable experiments with liquid I2 solution did not result in a measurable deposition of iodine, apparently due to the rapid dissolution of the metal iodides formed (Deane and Marsh, 1990). From the earlier desorption experiments performed by Rosenberg et al. (1969) it was assumed that FeU is the main primary product of this reaction, but that parallel formation of NiU and/or Cris cannot be ruled out all these compounds are readily soluble in water. Experiments using tubes made of steel 1.4541 (Funke et al., 1994) showed an I2 deposition rate constant in dry air at 120 °C of about 6 10" m/s. Under condensing steam conditions at the same temperature a considerably higher value of 1.4- 10 m/s was measured, indicating that under such conditions the U-U conversion on steel surfaces is a fast process. Analysis of the surfaces after completion of the test yielded only a small iodine retention in the condensed steam no iodate could be detected, indicating that no unreacted I2 had passed through the reaction tube and that only metal iodides had been... 

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