Oxidation of thiocyanate ion

In the oxidation of the thiocyanate ion the major process forms cyanide and sulphite 

SCN -t-3S2 0 - -4H20 = 7HSO4-fHCN by a second-order reaction , viz. 

Only a more thorough study can show whether the observed deviations in product ratios are consistent with this proposal. They are in general agreement with some other measurements  

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Hydroxyl radical oxidation of thiocyanate ion in acid and neutral solution leads to the formation of a transient species which absorbs strongly at a wavelength of 475nm. Figure 2 shows the growth in absorption of this species at intervals from 20ns to 800ns. 

Example 30 The oxidation of thiocyanate ions by permanganate in acid solution is applied in quantitative analysis. Express the reaction equation. 

In oxidation of thiocyanate ion by nitric and nitrous acids the formation of nitrosyl thiocyanate and some of its properties have been convincingly demonstrated However, its role in the oxidation reaction is not yet entirely elucidated. 

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. 

An interesting application of the electrochemical oxidation of thiocyanate ion is the preparation of alkyl and aryl thiocyanates via anodically generated thiocyanogen. Alcohols have been converted to the corresponding thiocyanates by constant current electrolysis of NaSCN in CH2CI2 containing triphenylphosphite and 2,6-lutidinium perchlorate. The yields were fair to good for the primary and secondary alcohols, but no thiocyanate formation was observed with tertiary ones. Similarly, various aromatic amines and phenols were thiocyanated in a two-step procedure, namely electrochemical preparation of (SCN)2 and subsequent reaction with the substrates k... 

This peroxidase, present in milk, saliva, and tears, is involved in the bacterial defense through the oxidation of thiocyanate ions to the antibacterial species hypothiocyanate OSCN and other higher oxyacids. Thiocyanate, the physiological substrate of LPO, is present in secreted fluids in much higher concentrations than in blood plasma or in the extracellular space of tissues. LPO is a glycoprotein with a molecular weight of 78,000 Da and has a high content of carbohydrates (10%). Despite the absence of an X-ray structure for LPO, physico-chemical studies have shown that the bovine milk LPO-heme is a l,5-bis(hydroxymethyl) derivative of heme b, linked to the protein by esterification with Glu-275 and Asp-125 as in MPO. ... 

Addition of materials capable of reacting with electrons reduces the lifetime of enabling kinetics of its fast reactions to be determined. The powerful oxidant OH (the hydroxyl radical) has only a weak absorption in the ultraviolet, and its reactivity is best measured by a competition method based on its very fast oxidation of thiocyanate ion CNS to yield the intensely absorbing (CNS) ion (Xmax 472 nm). Addition of a second substrate X will provide competition for OH, and the intensity of the absorption of (CNS)2 will be systematically reduced as [X] is increased, enabling a rate constant to be derived. 

Wilson IR, Harris GM (1961) The oxidation of thiocyanate ion by hydrogen peroxide. II. The acid-catalyzed reaction. J Am Chem Soc 83 286-289... 

The method is based on the oxidation of ferrous ions to ferric ions, which form a deep red complex with thiocyanate. 

To examine the oxidation of Fe2+ to Fe3+, in the second experiment, 10 ml solution of 0.1 M ferrous ammonium sulphate was taken separately in four different beakers and sonicated for 15, 30, 45 and 60 min, before transferring the solution to a 25 ml volumetric flask and adding to it 10 ml of 0.01 M KSCN and making upto the mark with deionised water. The absorbance of these solutions was measured at 4-,iax 451 nm. Sonication of ferrous ammonium sulphate solutions oxidised ferrous ions to ferric ions, which in the presence of thiocyanate ions, produced an intense red coloured complex Fe(SCN)63, in proportions to the oxidation of ferrous ions to ferric ions, as could be seen in Fig. 10.1. 

Adsorption of thiocyanate ions on Au electrodes from alkaline solutions has been studied using in situ IR spectroscopy and SERS [83]. Even at negative potentials, adsorption of thiocyanate was observed, which occurred via Au—S interaction. At higher concentration of OH ions, the bond strength between the adsorbed SCN ion and the surface was decreased. At very positive potentials, both the oxidation of thiocyanate to cyanate and the... 

At the same time, it is necessary to take into account that the approach described has a number of exceptions, related for example to the nature of other ligands forming pseudohalide complexes. A series of classic examples of inversion of the bond M — N —> M — S —> M — N have been reported [6,8,11,42-44,59] and are presented in Sec. 2.2.3.5. In this respect, we especially emphasize the capacity of other ligands for soft or hard metals, related with symbiotic [60] and anti-symbiotic [61] effects. Thus, Pearson [61] emphasized that soft ligands, which are placed in a trans position to SCN ion, contribute to N-binding of thiocyanate ions, and hard bases contribute to S-coordination of these ambidentate ligands. Metal oxidation number (Table 1.4) is important in this problem and it regulates soft hard properties of complex-formers. 

The oxidation of thiocyanate is a rather complex process, which may be dealt with in two separate steps. First, sulphate and cyanide ions are formed ... 

Ferric Ion Complexes Other chemical methods based on the oxidation of ferrous ion (Fe ) to ferric ion (Fe ) in an acidic medium and the formation of iron complexes have also been widely accepted. These methods spectrophotometri-cally measure the abihty of lipid hydroperoxides to oxidize ferrous ions to ferric ions, which are complexed by either thiocyanate or xylenol orange (23, 28, 29). Ferric thiocyanate is a red-violet complex that shows strong absorption at 500-510 nm (8). The method of determining PV by coloremetric detection of ferric thiocyanate is simple, reproducible, and more sensitive than the standard iodometric assay, and has been used to measure hpid oxidation in milk products, fats, oils, and liposomes (8, 23). 

X 10 s ) and a SCIST-dependent path k = 3.7 x identical to those of the reaction between light-activated TiOa and the thiocyanate ion, in which the hole was thonght to oxidise the thiocyanate ions directly, in reactions analogous to reactions 5.11-5.13. Oxidation of SCN ions in the pulse radiolytic experiments occurred through the following sequence (Lawless et ai, 1991). 

Other reactions of simple anions that may occur in the absence of coordination can also be observed to occur for the complexed form, with the rate of reaction usually changed significantly as a result of complexation. This is anticipated, since a coordinated ion is bonded directly to a highly-charged metal ion, which must influence the electron distribution in the bound molecule and hence its reactivity. Two well-known examples where the product is ammonia occur through either reduction of nitrite with zinc/acid or oxidation of thiocyanate with peroxide. The former example is exemplified in (6.42) below. 

It has been widely reported that electrooxidation of the thiocyanate ion at pH 4 yields hydrogen cyanide and (or) cyanide and sulfate ions as main products [134]. However, there is a huge gap in terms of the mechanism that takes place between the binding of thiocyanate to the catalyst and the products experimentally found. In order to get insight about the elemental reactions between these processes, it is proposed based on experimental evidence that the oxidation of thiocyanate particularly catalyzed by MPc and MPc like systems, leads to the production of thiocyanate radicals that dimerize to form the pseudohalogen molecule thio-cyanogen (SCN)2 [132, 135-139]. 

Thiocyanates are rather stable to air, oxidation, and dilute nitric acid. Of considerable practical importance are the reactions of thiocyanate with metal cations. Silver, mercury, lead, and cuprous thiocyanates precipitate. Many metals form complexes. The deep red complex of ferric iron with thiocyanate, [Fe(SCN)g] , is an effective iadicator for either ion. Various metal thiocyanate complexes with transition metals can be extracted iato organic solvents. 

Cadmium Hydroxide. Cd(OH)2 [21041-95-2] is best prepared by addition of cadmium nitrate solution to a boiling solution of sodium or potassium hydroxide. The crystals adopt the layered stmcture of Cdl2 there is contact between hydroxide ions of adjacent layers. Cd(OH)2 can be dehydrated to the oxide by gende heating to 200°C it absorbs CO2 from the air forming the basic carbonate. It is soluble ia dilute acids and solutions of ammonium ions, ferric chloride, alkah haUdes, cyanides, and thiocyanates forming complex ions. 

SCN- is the thiocyanate ion). Consider the oxidation number of chromium to be +3 and the coordination number to be 6 in both compounds. Estimate... 

The solution should be free from the following, which either interfere or lead to an unsatisfactory deposit silver, mercury, bismuth, selenium, tellurium, arsenic, antimony, tin, molybdenum, gold and the platinum metals, thiocyanate, chloride, oxidising agents such as oxides of nitrogen, or excessive amounts of iron(III), nitrate or nitric acid. Chloride ion is avoided because Cu( I) is stabilised as a chloro-complex and remains in solution to be re-oxidised at the anode unless hydrazinium chloride is added as depolariser. 

Although Pb(IV) is sufficiently strong an oxidant to oxidise halides, no kinetic data are available. Complexes of Pt(IV) and Au(III) oxidise iodide and thiocyanate ions but the other oxidants are weaker and form stable halo-complexes. However, some simple molecules such as hypophosphorous acid, carbon monoxide and molecular hydrogen are oxidised by the weaker members. 

The oxidation of potassium thiocyanate by AuBrJ is first-order in oxidant and the pH dependence indicates that it is also first-order in thiocyanate ion, which is oxidised much faster than HCNS . The activation parameters are E = 6.4+0.4 kcal.mole and A5 = 26 + 2 eu. 

Cyanates contain the OCN group. Inorganic cyanates that are formed industrially by the oxidation of cyanide salts hydrolyze in water to form ammonia and bicarbonate ion. Alkyl cyanates are insoluble in water and form cyanurates. Alkyl isocyanates contain the OCN radical, are formed from cyanates, and, like cyanates, are readily hydrolyzed. Thiocyanates (SCN group) are formed from cyanides and sulfur-containing materials and are relatively stable. 

The ability of vanadium(II) chloride to facilitate sulfoxide deoxygenation has been discussed (Section IV,C), and it appears that vana-dium(III) sulfoxide complexes may be prepared by air oxidation of van-adium(II) salts in the presence of the sulfoxide. In this manner, [V(Me2S0)6][C104]3 was prepared from vanadium(II) perchlorate (119) and the kinetics of substitution with thiocyanate ion detailed. Care is necessary in handling the pure compound, as it is reported to be sensitive to detonation. A large number of oxovanadium(IV) species have... 

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