Ferrous tetrathionate

Ferrous tetrathionate, FeS406, results in unstable solution on adding ferrous thiosulphate solution in small quantities at a time to ferric chloride —... 

The salt may also be obtained by triturating a concentrated solution of ferrous sulphate with barium thiosulphate,1 but it is less pure, as it contains some tetrathionate as well.2 It results when sulphur is digested with ferrous sulphate solution, and when iron is dissolved in aqueous sulphurous acid.3 This latter reaction is somewhat complicated, ferrous sulphite being first produced, thus —... 

Williamson and Rimstidt (1993) reported the rate of decomposition of ferric thiosulfate to form ferrous iron and tetrathionate ions. The reaction brings two ferric thiosulfate molecules together into an activated complex, which decomposes to form two ferrous ions and a tetrathionate ion. 

Figure 5.5. Schematic diagram showing the steps in the ferric thiosulfate reaction. (a)Two FeSjOj molecules approaching each other. (b)The approximate geometry of the activated complex. (c)The reaction products, which are two ferrous iron ions and a tetrathionate ion.

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