TriThionate, potassium

Against this latter view is the fact that little or no sulphuric acid is formed unless the mixture is boiled for a long time. In alkaline solution, alkali sulphide and trithionate react to form thiosulphate,1 but alkaline solutions of sulphate and sulphide do not.2 It would appear, therefore, that the correct explanation lies in the reversible equation (6) (p. 195). This receives support from the fact that when lead thiosulphate is boiled with water it yields, in the first instance, lead sulphide and lead trithionate.3 Conversely, lead sulphide on digestion with potassium trithionate yields lead thiosulphate.4 Further, weakly alkaline solutions of sodium thiosulphate itself yield, on boiling, sodium sulphide and sodium trithionate, with only a trace of sulphate if boiled with sodium plumbite, lead sulphide is precipitated and sodium trithionate remains in solution. 

Trithionic Acid, H2S3Og.—Formation.—(1) From Thiosulphates Potassium thiosulphate in concentrated aqueous solution reacts with sulphur dioxide forming potassium trithionate 3 the reaction is sometimes represented as... 

A similar formation of trithionate can be effected by recrystallising a mixture of potassium thiosulphate and potassium hydrogen sulphite in aqueous solution.5 It is probable that the method by which potassium trithionate was first prepared depended on the same reaction.6 Potassium hydrogen sulphite solution was warmed with sulphur for several days, with the result that sulphate, thiosulphate and trithionate were obtained, the formation of the last-named in all probability occurring by way of the thiosulphate. 

Potassium trithionate, K2Sa06.—The trithionate is prepared by the action of sulphurous acid on potassium thiosulphate, and separates in rhombic crystals, of density 2-804 at 20° C. Its heat of formation is given as 405-85 Cal.7 and 416-0 Cal.8 It dissolves readily in water, forming a neutral solution. 

Ammonium trithionate, (NH4)2S306.—An aqueous solution of potassium trithionate reacts with hydrofluosilicic acid and ammonia to form the trithionate, which is precipitated from its aqueous solution by alcohol as a very deliquescent, unstable salt.15 It is also formed, along with the tetrathionate, by the action of sulphur dioxide on ammonium thiosulphate 16... 

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. 

Potassium trithionate is orthorhombic bipyramidal, space group D -Pnma with four formula units per unit cell. From Zachariasen s (239) structure analysis, carried out in the early thirties by trial and error methods, the trithionate ion has mirror plane symmetry in the crystals, lying with all three sulfur atoms and two of the oxygen atoms in the mirror plane. The rubidium salt (161) is isomorphous with the potassium salt. The third trithionate for which data are available, the thallium salt, is monoclinic prismatic, space group Ca ,6-C2/c with four formula units per unit cell (151). Here, the trithionate ion has a twofold axis as symmetry element. 

The first structure determination of a tetrathionate, the barium salt, was published in 1954, 20 years after that of potassium trithionate. 

The decomposition of the alkali pyrosulphites under the influence of heat follows a very complicated course.1 At 150° C. the potassium salt yields trithionate and sulphate, the ionic reaction being ... 

From Sulphites Potassium hydrogen sulphite, when kept in aqueous solution for a long time with exclusion of air, undergoes spontaneous change with formation of sulphate and trithionate.3 The change, which is commonly represented as... 

Trithionate is also produced when sulphur dioxide is passed into a mixture of solutions of potassium sulphide and potassium hydrogen sulphite 4... 

Sulphur chloride or dichloride can convert potassium sulphite into trithionate,5 the equations being... 

In the oxidation of alkali sulphides or polysulphides by potassium permanganate solution at the ordinary temperature, trithionic acid has been found amongst the reaction products, in addition to sulphuric acid and sulphur.7... 

Properties.—Trithionic acid is the least stable of the polythionic acids. The aqueous solution of the free acid, which is generally obtained from a cold concentrated solution of the potassium salt by the addition of a suitable acid, such as hydrofluosilicie or perchloric acid, which will remove the metal as a sparingly soluble salt, slowly decomposes, even at the ordinary temperature, with formation of sulphur, sulphur dioxide and sulphuric acid ... 

Sulphur dioxide when passed through a solution of a thiosulphate gives rise not only to tetrathionate but also to trithionate and penta-thionate.12 Under suitable conditions, however, with a high concentration of sulphur dioxide, the tetrathionate separates. The reaction is accelerated by traces of potassium arsenite.13... 

The tetrathionates are converted by nascent sulphur into penta-thionates, whilst by potassium sulphite solution they are reduced to trithionate.7 The latter reaction explains the erroneous assumption that trithionate could be directly produced by the action of iodine on an aqueous mixture of potassium thiosulphate and sulphite8 (see p. 210). With equivalent quantities of tetrathionate and sulphite, the reaction leads to the equilibrium 9... 

In presence of sulphurous acid a similar reaction takes place. The trithionate forms rhombic crystals isomorphous with those of the corresponding potassium salt, the density being 2-845.4... 

---