Peroxodisulphate ion

The rate equation derived from this mechanism is in accord with most of the observed features, but it predicts that with excess substrate the second-order rate coefficient should decrease during a run, whereas the observed rate coefficient always increases during a run, irrespective of whichever reactant is in excess. Whalley et al. suggest that incomplete dissociation of peroxodisulphate in the solvent might be responsible for the discrepancy. Another discrepancy is pointed out by Wilmarth and Haim, but these authors agree with Whalley et al. in concluding that the initiation step is reaction (91) rather than the spontaneous fission of the peroxodisulphate ion. 

Studies on the kinetics and mechanism of the oxidation of ascorbic acid show the rate to be independent of the concentration of organic substrate. A chain mechanism is proposed with reaction between the radical anion of ascorbic acid and the peroxodisulphate ion. Although the influence of neutral salts is negligible and that of acid slight, the presence of allyl acetate strongly inhibits the rate, suggesting the formation of radical ions. In the reaction with lactic acid, a similar rate expression is observed although the description of the mechanism is different. 

Additional papers on the oxidation of L-ascorbic acid have appeared as follows with the peroxodisulphate ion (SoOp )... 

Peroxodisulphate salts in air Lab method using mobile phase ion chromatography 79... 

Finally, there is an extremely violent reaction during the action of ammonia on ammonium peroxodisulphate in the presence of Ag- ions. 

One drop of dilute silver nitrate has to be added to speed up the reaction. Silver ions act as catalysts the catalytic action is due to the transitional formation of silver(III), Ag3+. Halides must be absent they can be removed easily by evaporating the solution with concentrated sulphuric acid until fumes of sulphur trioxide appear. After cooling, the solution can be diluted and the test carried out. The excess of peroxodisulphate can be decomposed by boiling ... 

Ammonium or potassium peroxodisulphate Solid (NH4)2S208 or K2S208 is added to a dilute solution of manganese(II) ions, free of chloride. The solution is acidified with dilute sulphuric acid, and a few drops of dilute silver nitrate (which acts as a catalyst) are added on boiling a reddish-violet solution is formed, owing to the presence of permanganate ... 

Benzidine acetate test (DANGER THE REAGENT IS CARCINOGENIC) A neutral or weakly acetic acid solution of a peroxodisulphate converts benzidine into a blue oxidation product. Perborates, percarbonates, and hydrogen peroxide do not react. Chromates, hexacyanoferrate(III) ions, permanganates, and hypohalites react similarly to peroxodisulphates. 

Sodium bismuthate This reagent, in the presence of dilute nitric acid, converts cerium(III) ions into cerium(IV) in the cold. A similar result is obtained by heating with ammonium peroxodisulphate or with lead dioxide and dilute nitric acid (1 2). In all cases, the solutions become yellow or orange in colour. 

In the acid-catalysed process, dominant for pH less than 2, the products isolated may be oxygen (below 0.5 Af acid), peroxomonosulphuric acid, a mixture of the two, or may contain detectable amounts of hydrogen peroxide or ozone. The formation of hydrogen peroxide (at 98 °C in 2 M HjSO ) has been examined and shown to be catalysed by a number of metal ions. Under all conditions examined the oxygen liberated arises entirely from the peroxide . Exchange between S-labelled sulphate and peroxodisulphate proceeds very slowly, if at all, in this region . Further, it appears probable that this particular decomposition path does not involve intermediates which react at appreciable rate with cerium-(III) It has been suggested that the initial, overall, reaction is... 

Oxidations of thiocyanate ion by nitrous and nitric acids and by peroxomono-sulphate and peroxodisulphate are considered under the oxidants (p. 293 and 350). Oxidation by hydrogen peroxide and by iodine will be considered here. 

Peroxodisulphuric acid, H2S2O8, is a strong acid whose second pK is below zero (Kolthoff and Miller ). Under the conditions normally employed in peroxodisulphate oxidations (aqueous solution, pH >1) the ion 8208 is the dominant species. The ion is a powerful two-electron oxidising agent with a redox potential of — 2.01 V. In the majority of its reactions the primary step is the formation of sulphate radical-ions, either by spontaneous fission of the peroxide bond, or by attack on a substrate X, i.e. 

The highly reactive sulphate radical-ion may attack the solvent or a substrate present in the solution. The spontaneous decomposition of peroxodisulphate in aqueous solution has been widely studied it is, in effect, the oxidation of the solvent by peroxodisulphate. It will be considered here because the steps involved are of importance in the mechanisms of oxidation of organic compounds by peroxodisulphate. The decomposition has the stoichiometry... 

Many of the reactions of peroxodisulphate are catalysed by silver ions, and although the reactions then involve higher oxidation states of silver as oxidants, it is convenient to consider them along with the uncatalysed oxidations. Silver ions also catalyse the decomposition of peroxodisulphate in a second-order reaction, viz. 

The possible types of chain mechanisms for peroxodisulphate oxidation have been classified by Wilmarth and Haim according to the dominant initiation and termination steps, and the relative importance of sulphate radical-ions and hydroxyl radicals in the propagation steps. Some of the rate equations corresponding to the different types of mechanisms are the same, so the observation of a particular rate equation does not always permit a unique mechanism to be inferred. In certain cases the nature of the chain initiation step can be deduced from the effect of a free-radical scavenger on the reaction rate. Thus in the oxidation of 2-propanol, the addition of allyl acetate reduces the rate to that observed for the spontaneous decomposition of peroxodisulphate, indicating that the chain initiation step is the same as the rate-determining step of the spontaneous decomposition, viz. the fission of peroxodisulphate into sulphate radical-ions. 

Later work showed this mechanism to be incorrect. Wiberg showed that 804 when present in the reaction mixture does not give labelled peroxodi-sulphate, as required by the reversible first step of Levitt and Malinowski s mechanism. Furthermore, allyl acetate inhibits the reaction and reduces the rate of consumption of peroxodisulphate to that observed in the absence of 2-propanol. Wiberg proposed a chain mechanism involving sulphate and hydroxyl radicals. In a thorough study of the reaction, Ball et aV showed that all previous studies were complicated by the catalytic effects of trace amounts of metal ions (most likely cupric ions) and inhibition by dissolved oxygen from the atmosphere. In the absence of oxygen there is no catalysis by cupric ions, and the rate equation is... 

Dogliotti and Hayon generated sulphate radical-ions at room temperature by the flash photolysis of peroxodisulphate solutions, and measured the rate at which they attack 2-propanol, i.e. the rate of reaction (21). They found kj = 8.5 3.0xl0 l.mole.sec" at pH 4.4. 

In a study of the silver ion-catalysed oxidation of 2-propanol in 50 % acetic acid, Venkatasubramanian and Sabesan found first-order kinetics with respect to peroxodisulphate and zero-order with respect to the alcohol. They propose a chain mechanism involving either silver(II) or silver(III) ions. 

The reaction is faster than the spontaneous decomposition of peroxodisulphate, so Bartlett and Cotman proposed a chain mechanism involving sulphate radical-ions and the CH2OH radical. Kolthoff et al. showed that allyl acetate inhibits the reaction, and reduces the rate to that observed in the absence of methanol. They pointed out that if the inhibition is explained on the basis of Bartlett and Cotman s mechanism, the predicted rate equation does not include the methanol concentration. This difficulty was resolved by Edwards et al., who showed that in the absence of oxygen the reaction is zero-order with respect to methanol. They proposed the following mechanism (similar to that originally proposed by Bartlett and Cotman)... 

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