PentaThionate, potassium

In potassium pentathionate hemitrihydrate, the [S(S20s)J - ion has been found to occur in the trans configuration, with the terminal SO3 groups on opposite sides of the S3 central plane, in contrast to the m configurations found in three earlier studies. The ion thus has an approximate twofold axis, The outer S-S distances are 2.124 and 2.110 A, and the inner 2.021 and 2.036 A. The S-O distances range from 1.408 to 1.457 A. 

Pota.ssium pentathionate, K2SSO6, can be made by adding potassium acetate to Wackenroder s solution and solutions of the free acid HiSsOf, can then be obtained by subsequent addition of tartaric acid. 

Properties.—Like the lower members of the series of thionic acids, free pentathionic acid is known only in aqueous solution a solution of the pure acid is obtained by treating an aqueous solution of the potassium salt with the requisite quantity of tartaric acid for the removal of the potassium in the form of hydrogen tartrate. The solution is denser than water 4 it cannot be concentrated beyond a limit of 50 to 60 per cent, acid without decomposition. 

Potassium amalgam reduces an aqueous pentathionate solution to tetrathionate and thiosulphate. Metallic copper and silver are blackened by pentathionate solutions. 

Particle growth can be tenninated by the addition of KI solution, which reacts with the excess of thiosulfate. However, the excess of potassium iodide may lead to system destabilization due to the removal of the layer of potential determining pentathionic ions, S5062 (see Chapters V and VIII). 

Hertlein (US) in 1896 measured refraction, viscosity, and electrical conductivity of aqueous potassium tri-, tetra-, and pentathionates and decided in favor of uiibranched structures, and so did Martin and Metz (171) in 1924 on the basis of thermochemical measurements. Spacu and Popper (208) in 1939 reported the refraction of sodium tetrathionate Grinberg (186) has later commented on the ionic refraction of tetra thionate as indicating negative charge on the divalent sulfur atoms. However, from Hertlein s measurements the increments per sulfur atom, from tri-to tetra-thionate and tetra- to pentathionate, are about 9 cm3. This is the same as in other series, for example, in the polysulfur dichlorides (72) where the sulfur atoms are hardly negative. 

Until 1950 the only X-ray crystallographic work on polythionates was Zachariasen s structure determination of potassium trithionate (239) published in 1934, a partial structure analysis of thallium trithionate by Ketelaar and Sanders (151) in 1936, and unit cell and space group determinations of rubidium trithionate (161) and potassium tetrathionate (218). Since then, surveys of unit cells and space groups of tetra-, penta-, and hexathionates and selenotrithionates, selenopentathionates, and telluropentathionates have been made, and structure determinations of two tetrathionates, three pentathionates, one selenopentathionate, two telluropentathionates, and two hexathionates have been carried out. 

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