Thiosulphate reactions

Three thiosulphate ion reactions which may profit from discussion together involve exchange of oxygen with water (1), exchange of sulphur with bisulphide ion (2) and the displacement reaction with water (3), viz. 

In the displacement reaction (3), sulphite is not an intermediate. The reaction was studied between 250-280 °C in water and in aqueous buffer solutions . Like (1), the reaction shows first-order dependence on thiosulphate and hydrogen ion concentrations, without any indication of general acid catalysis. Minor side reactions form polysulphide and also sulphite. 

In reactions (1) and (3) attack of water probably occurs at the inner sulphur atom in a bimolecular process (though unimolecular sequences are not excluded). In the S-exchange reactions (2) and (4), attack is at the outer sulphur atom. Process (2) has an activation energy some 40 kcal.mole less than that for (1). This can be rationalized in terms of Edwards and Pearson s four-parameter equation, viz. 

Decomposition of acidified solutions of thiosulphate forms sulphur dioxide, colloidal sulphur (S8,Ss), polythionic acids and hydrogen sulphide. The reaction kinetics have been examined several times ° and profitable studies of the conditions under which colloidal sulphur is formed were made 3,714, 716-718. acceptable interpretation was first given by Davis , in terms of the steps (7)-(15). These involve successive steps of nucleophilic displacement by thiosulphate, viz. 

Alternative reactions, displacing HS instead of sulphite, can lead to the observed formation of polythionic acids. Assuming the value of K to be about 100, then ks and ki2 are reported as 5x 10 l.mole sec and (2.3 0.3)x 10 sec , respectively at 25.0 °C. It was presumed that fcg-fcu, were all similar in value. 

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The mechanism of the thiosulphate reaction is not clear. Lokhande, in his studies, has suggested an internal reduction involving the reaction... 

L.6 Chlorite-Thiosulphate Reaction without Iodine Species.105... 

IIIL) Orban, M., Epstein, I. R. Systematic Design of Chemical Oscillators, Part 13 1982-1 Complex Periodic and Aperiodic Oscillations in the Chloride Thiosulphate Reaction. J. Phys. Chem. 86, 3907-3910... 

Figure 28C represents a titration with two platinum wire electrodes and involving iodine in solution being titrated with sodium thiosulphate (I2 by 8203 ). Here the thiosulphate reaction... 

Hydrolysis of the esters (107) and (108) involves concomitant cleavage of both C-O and P-O bonds, the relative contributions of such fragmentations depending on experimental conditions. The reaction at carbon is favoured by an increa.se in temperature and is accelerated (60 times) by thiosulphate. Reaction at the P-O bond is favoured relative to that at the C-O bond by the addition of an organic. solvent (acetone) to the aqueous medium. Only P-O cleavage occurs in THF, in which the process is accelerated by fluoride anion (100 times). ... 

Fig. 9. The results of the residual polarization experiments for the reacting system of Mu with thiosulphate (reaction 29) in aqueous solution. Pp is the diamagnetic polarization which is the sum of the diamagnetic polarization in neat solvent and that formed on reaction 29. The number at left of each curve is the concentration of thiosulphate in mol/dm. From these results, using Eq. 18 and theoretical residual polarization as in Fig. S-A, the reaction rate constant is derived.

Descending wave front > velocity of ascending wave front > velocity of wave front due to R-D process. They [38] have reported similar observations in chlonite-thiosulphate reaction system at lower concentrations but a reversal of behaviour at higher concentrations. 

The reaction of cyanide ion with tetrathionate in acetonitrile occurs with a 2 1 stoicheiometry, and it is proposed that the SOaCN" ion may be formed. This species, or its conjugate acid, has been postulated as an intermediate in the hydrolysis of thiacyanogen and in the sulphur dicyanide-thiosulphate reaction. A study of the analytical determination of microamounts of hexathionate has included some kinetic work on the reaction... 

However, the sulphide ion can attach to itself further atoms of sulphur to give polysulphide ions, for example Sj , Sj , and so these are found in solution also. Further, the sulphite ion can add on a sulphur atom to give the thiosulphate ion, S203 which is also found in the reaction mixture. 

The ability of the solid chlorates(V) to provide oxygen led to their use in matches and fireworks. Bromates(V) and iodates(V) are used in quantitative volumetric analysis. Potassium hydrogen diiodate(V), KHflOjlj, is used to standardise solutions of sodium thiosulphate(Vf) since in the presence of excess potassium iodide and acid, the reaction... 

The reaction provides a method of estimating copper(Il) since the liberated iodine may be titrated with sodium thiosulphate ... 

Introduce a solution of 15 g. of the diazo ketone in 100 ml. of dioxan dropwise and with stirring into a mixture of 2 g. of silver oxide (1), 3 g. of sodium thiosulphate and 5 g. of anhydrous sodium carbonate in 200 ml. of water at 50-60°. When the addition is complete, continue the stirring for 1 hour and raise the temperature of the mixture gradually to 90-100°. Cool the reaction mixture, dilute with water and acidify with dilute nitric acid. Filter off the a-naphthylacetic acid which separates and recrys-talhse it from water. The yield is 12 g., m.p. 130°. 

In order that the reaction may proceed rapidly it is important to shake the mixture thoroughly after adding the iodine solution. When this is done the iodine compound is formed completely within one minute. With thymol it affords thymol di-iodide. In order to make sure that any iodine wnich may have entered into the hydroxyl-group is again liberated, care should be taken that a little hydriodic acid is always present hence the addition of the potassium iodide solution before the exc ess of iodine is titrated back with thiosulphate. Titration can only be regarded as complete when the blue coloration does not return in 10 minutes. 

The NH2 groups can be diazotized and reduced in the presence of thiosulphates and different metal ions. The effect of some metal ions, namely Fe ", Sn, Cu +, and Co on the graft yield of cotton modified with aryl diazonium groups via its reaction with 2,4-dichloro-6-(p-nitroaniline)-5-triazine in the presence of alkali and followed by reduction of nitro group was studied [4]. 

Alternative procedure. The following method utilises a trace of copper sulphate as a catalyst to increase the speed of the reaction in consequence, a weaker acid (acetic acid) may be employed and the extent of atmospheric oxidation of hydriodic acid reduced. Place 25.0 mL of 0.017M potassium dichromate in a 250 mL conical flask, add 5.0 mL of glacial acetic acid, 5 mL of 0.001M copper sulphate, and wash the sides of the flask with distilled water. Add 30 mL of 10 per cent potassium iodide solution, and titrate the iodine as liberated with the approximately 0.1M thiosulphate solution, introducing a little starch indicator towards the end. The titration may be completed in 3-4 minutes after the addition of the potassium iodide solution. Subtract 0.05 mL to allow for the iodine liberated by the copper sulphate catalyst. 

When thiosulphate solution is added to a solution containing iodine the overall reaction, which occurs rapidly and stoichiometrically under the usual experimental conditions (pH < 5), is ... 

The intermediate reacts with thiosulphate ion to provide the main course of the overall reaction ... 

In this reaction 1 mole of bromate yields six atoms of bromine. Bromine is very volatile, and hence such operations should be conducted at as low a temperature as possible and in conical flasks fitted with ground-glass stoppers. The excess of bromine may be determined iodometrically by the addition of excess of potassium iodide and titration of the liberated iodine with standard thiosulphate solution ... 

Aravamudan and Venkappayya75 oxidized dimethyl sulphoxide in acetate buffer of pH 4 to 4.5 and with a reaction time of only 1 min. They then added potassium iodide and acid and titrated with thiosulphate the iodine liberated by unused reagent. They reported that cerium(IV) and Cr(VI) were much less effective oxidizing reagents for the sulphoxide. A very similar procedure was used by Rangaswama and Mahadevappa76 to determine dimethyl sulphoxide and numerous other compounds with chloramine B. 

Bohme77 employed excess monoperphthalic acid in diethyl ether to oxidize dibenzyl and benzyl ethyl sulphoxides. Reaction time was 24 h at - 15 to + 10 °C, after which he added potassium iodide and water and titrated the iodine set free with thiosulphate. Dickenson78 oxidized dimethyl sulphoxide in malt, wort or beer with Na2S2Os. In... 

The Co(III) complexes Co(NH3)6 " and Co(NH3)sOH bring about oxidation of stannate(II) ion in strongly basic solution . The rates were found to be independent of the concentration of the Co(III) complex. It is proposed that stannate(Il) exists as a dimer, and that the monomer is the reactive species, the rate being close to half-order in stannate(II). Cyanide and thiosulphate catalyse the reaction but Co(CN)g is immune to attack by stannate(II) ion. The experimental difficulties encountered in this study preclude a full analysis as regards mechanism. 

These and similar results can be explained if the simultaneous reduction of hydrogen peroxide is due to an induced reaction. To show the characteristic features of this reaction some results are presented in Table 19 and Table 20. The procedure for these measurements was as follows. The solution of peroxy compounds given in columns 1 and 2 was made up to 20 ml and the pH was adjusted to the given value. Then potassium thiocyanate solution was added and, after the reaction time noted, the process was quenched by adding potassium iodide solution (0.3 g KI). After 5 sec the solution was acidified with 1 ml 2 iV sulphuric acid then using, molybdate catalyst solution, the iodine liberated was titrated with standard thiosulphate. 

Among the most important indirect methods of analysis which employ redox reactions are the bromination procedures for the determination of aromatic amines, phenols, and other compounds which undergo stoichiometric bromine substitution or addition. Bromine may be liberated quantitatively by the acidification of a bromate-bromide solution mixed with the sample. The excess, unreacted bromine can then be determined by reaction with iodide ions to liberate iodine, followed by titration of the iodine with sodium thiosulphate. An interesting extension of the bromination method employs 8-hydroxyquinoline (oxine) to effect a separation of a metal by solvent extraction or precipitation. The metal-oxine complex can then be determined by bromine substitution. 

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