Thiosulphate complexes

Because of the strong complexes thiosulphate forms with some metal ions, it is very possible that these metal-thiosulphate complexes undergo a complex-decomposition mechanism (Section 3.3.3). However, one early study on the formation of PbS on boiling Pb and thiosulphate in water found that PbS formed more readily when excess thiosulphate was present [5], which suggests that decomposition of thiosnlphate to sulphide might be the dominant pathway under the conditions of that stndy. 

This precipitate dissolves in thiosulphate to form a thiosulphate complex, which, in common with other metal thiosulphate complexes, decomposes when heated to the metal snlphide (see Sec. 3.3.3). Besides direct decomposition of the thiosnlphate complex, another possibility suggested in this study is formation of snlphide ion by alkaline hydrolysis of thiosulphate [Eqs. (3.20) to (3.24)] and reaction with Hg to form HgS. The substrates were glass precoated with a very thin film of CD PbS (presnmably this improved adherence and/or homogeneity). 

Decomposition of a thiosulphate complex of Pb, a possible reaction being... 

Cupric thiosulphates.—Complex cupric alkali thiosulphates have been prepared by Dutoit.16... 

Simmons, W., Gonnissen, D., and Hubin, A. 1997. Study of the initial stages of silver electrocrystallisation from silver thiosulphate complexes Part I. Modelling of the silver nuclei formation during the induction period. Journal of Electroanalytical Chemistry 433, 141-151. 

Several other compounds that, like thiocyanate and dithiol, have sulphur as the ligand atom, are used in spectrophotometric methods for Mo. Some examples are thioglycolic acid (mercaptoacetic acid, thioacetic acid) [70,71], dithiocarbamates [72-74], rubeanic acid [75], and aminobenzenethiol [76]. Molybdenum has been determined also as the thiosulphate complex [77]. 

Silver (Ag, at. mass 107.87) occurs in its compounds in the (I)- oxidation state. So far, silver(II) is only of limited value in spectrophotometry. Silver(I) -sulphide and -halides are sparingly soluble. Ammine, cyanide, and thiosulphate complexes of silver are formed. In the presence of excess of Cf or SCN , traces of silver form soluble complexes. 

Kullberg [74KUL] found it impossible to study the aqueous Cu -SeCN system since a brown precipitate was immediately formed upon addition of selenocyanate ions to a copper(ll) solution. Toropova [56TOR] prepared CuSeCN(s) by adding KSeCN to a solution of copper(l) sulphite or thiosulphate complexes, and tried to study the complex formation by measuring the solubility of CuSeCN in selenocyanate solutions. The experiment failed since the complexes initially formed were found to decompose into elementary selenium and copper(l) cyanide compounds. 

No experimental information is available on aqueous thiosulphate complexes of thorium. 

Uthe et al 2 have described a rapid semi-micro method for determining methylmercury in fish and crustacean and aquatic mammal tissue. The procedure involves extracting the methylmercury into toluene as methylmercury bromide, partitioning the bromide into aqueous ethanol as the thiosulphate - complex, re-extracting... 

Silver chloride is readily soluble in ammonia, the bromide less readily and the iodide only slightly, forming the complex cation [Ag(NH3)2]. These halides also dissolve in potassium cyanide, forming the linear complex anion [AglCN) ] and in sodium thiosulphate forming another complex anion, [Ag(S203)2] ... 

The film is now fixed by washing in sodium thiosulphate ( hypo ) solution when the unchanged bromide is dissolved to form the complex ion... 

Discussion. Silver can be extracted from a nearly neutral aqueous solution into nitrobenzene as a blue ternary ion association complex formed between silver(I) ions, 1,10-phenanthroline and bromopyrogallol red. The method is highly selective in the presence of EDTA, bromide and mercury(II) ions as masking agents and only thiosulphate appears to interfere.8... 

Linear S-Au-S (but non-linear Au-S-C) is found in PhAs+Au(SCN)2 [96] related phosphine complexes (R3P) AuSCN have been made (section 4.10.3). Sulphate and thiosulphate bind through sulphur Na3Au(S203)2.2H20 has linear 2-coordinate gold in contrast to tetrahedral coordination of silver by sulphur and oxygen. 

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. 

The work with iodide (preceding sub-section) was extended to thiosulphate , and isosbestic points and second-order kinetics were again obtained with the various Pt(IV) complexes (Table 8). Two 8203 ions are consumed per mole of Pt(IV) reduced, suggesting tetrathionate to be the product of oxidation, viz. 

We could not find any study of Bi(III) ions in aqueous solutions except that Wang et al. [132] obtained nanorods of bismuth sulphide by sonicating an aqueous solution of bismuth nitrate and sodium thiosulphate in the presence of complexing agents such as ethylenediamine tetraacetic acid, triethanolamine and sodium tarta-rate. Similar results were found when thioacetamide was used in place of sodium thiosulphate as a source of sulfur. However, the results improved with higher yield... 

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. 

SAQ 8.9 Iodide reacts with thiosulphate to form elemental iodine. If the reaction solution contains a tiny amount of starch solution, then this I2 is seen by eye as a blue complex. The data below were obtained at 298 K. Determine the order of reaction, and hence its rate constant k. 

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