Arsenious acid reaction with iodine

It is striking that reactions of arsenic(III) with 1-equivalent oxidizing agents take place fairly slowly, while reactions with 2-equivalent partners e.g. with iodine, are fast. To interpret this behaviour it was suggested by Waters that arsenious acid has the tautomeric forms... 

The induction period of the reaction may be curtailed 2 by (1) the presence of an excess of iodic acid, (2) an increase in the concentrations of the reactants, (3) the addition of a trace of arsenic acid, (4) the addition of a mineral acid and (5) exposure to sunlight. On the other hand, the period may be prolonged by the addition of mercuric chloride or by violent shaking. The proportion of the iodine liberated increases with the arsenious acid concentration and passes through a maximum. The iodine appears on the surface of the solution even though the latter may be covered with benzene (or occasionally it appears at a nucleus on the glass). The reduction of periodate to iodate by means of arsenite is a bimolecular reaction and is of the first order with respect to both components.3 At 25° C. it proceeds according to the velocity equation... 

The reaction between sodium arsenite and chloroacetic acid is very rapid, as may easily be demonstrated by titrating i-cc. portions of the solution with N/10 iodine, according to the usual volumetric method for arsenious acid, before and after the reaction with chloroacetic acid. The excess of sodium arsenite is necessary, as was learned by means of an iodometric study of the reaction. 

The time required for a definite small amount of iodate to be consumed will be measured by determining the time required for the iodine produced by the reaction (as I3) to oxidize a definite amount of a reducing agent, arsenious acid, added at the beginning of the experiment. Under the conditions of the experiment, arsenious acid does not react directly with iodate at a significant rate but reacts with iodine as quickly as it is formed. When the arsenious acid has been completely consumed, free iodine is liberated, which produces a blue color with a small amount of soluble starch that is present. Since the blue color appears rather suddenly after a reproducible period of time, this series of reactions is commonly known as the iodine clock reaction. 

Arsenites can be detected in the presence of arsenates by method (a) or (b) a) Reduction of iodine to iodide. The reaction of arsenious acid with iodine, and of arsenic acid with iodides, is reversible ... 

The reaction is quite similar to that of iodine with arsenious acid. The hydrogen carbonate buffer is used to displace the reaction toward the right. The standard potential of the couple p-quinone/hydroquinol is E° = 0.70 V. Resorcinol (meta-diphenol) and pyrocatechol (ortho-diphenol) do not give any reaction of analytical interest with iodine. 

The determination of arsenic in these derivatives is imperatively required by the pharmacopeia. To achieve it, the molecule is first mineralized by the sulfonitric mixture, which has strong oxidizing properties. The organic arsenic of the initial sample becomes mineralized and transformed into arsenic acid H3ASO4. The latter is brought in the presence of a solution of iodide ions in concentrated hydrochloric acid. In these conditions, As(+V) quantitatively oxidizes iodide ions to give iodine. The reaction evolves in the direction As(+V) As(+III). The liberated iodine is titrated later with an arsenious acid solution in hydrogen carbonate medium. Therefore, at this time, the reaction As(+III) As(+V) is used. 

We notice that it is not necessary to achieve the reaction in neutral or weakly alkaline medium as with iodine for arsenious acid to be sufficiently reductant. We also notice that arsenious acid is oxidized before chloride ions. This is normal because of the standard potential values of the different couples brought together. 

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