Salts of Dithionic Acid

Potassium sulfite forms no hydrates, but the pyrosulfite forms the compound K2S205-%H20, which is probably, the double salt K2S205-4KHS03. In solution, the pyrosulfite is converted to the hydrogen sulfite. 

Because of the small solubility of the pyrosulfite, this compound crystallizes from solutions containing as little as 1.2 mols SO2 per mol K2O. Excess SO2, therefore, is to be avoided when the pure sulfite is being prepared. Like the sodium compounds, the hydrogen sulfite solutions have high vapor pressures of SO2 at the boiling point, but the loss of SO2 during evaporation can never reduce the ratio of SO2 to K2O to a point where the pyrosulfite wifi, not crystallize in pure form. This indicates that the mother liquor from the pyrosulfite precipitation can be evaporated to increase the yield. 

Foerster, Broschb, and Norberg-Schultz Z. physik. Chem., 110, 435 

International Critical Tables, Vol. 4, p. 236, McGraw-Hill Book Com- 

Dithionates may be prepared by the electrolytic or chemical oxidation of sulfurous acid and sulfites. Chemical oxidation is accomplished by the action of chlorine, iodine, hydrogen peroxide, or oxygen in an acid medium by the action of chromates or permanganates in neutral solution -  

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Dithionites. Although the free-dithionous acid, H2S2O4, has never been isolated, the salts of the acid, in particular zinc [7779-86-4] and sodium dithionite [7775-14-6] have been prepared and are widely used as industrial reducing agents. The dithionite salts can be prepared by the reaction of sodium formate with sodium hydroxide and sulfur dioxide or by the reduction of sulfites, bisulfites, and sulfur dioxide with metallic substances such as zinc, iron, or zinc or sodium amalgams, or by electrolytic reduction (147). 

The dithionate, like the sulphate of the corresponding ammino-salts, is very sparingly soluble in water, and the other salts may be obtained from it by decomposing it with ammonium salts of the acid. They decompose on treatment with concentrated hydrochloric acid thus ... 

Salts of Thionic Acids.—A third class of thio salts are those derived from the various thionic acids of which sodium thiosulfate and barium dithionate may be... 

For the constitution and estimation of dithionic acid and its salts, see later (pp. 221-4.). 

Dithionous acid, H2S2O4, is not obtainable as a pure compound, but several salts of the acid are known. The acid formally contains sulfur(III), and this suggests reduction of sulfites as a method of preparation. Such a process is illustrated in the following equation ... 

Although we show the structure of dithionous acid in Table 15.8, only its salts are known and these are powerful reducing agents. Dithionite is prepared by reduction of sulfite in aqueous solution (equation 15.93) by Zn or Na amalgam and possesses eclipsed structure 15.48. 

In dithionic acid and dithionates, 8205 , the oxidation state of the 2 8 atoms has been reduced from VI to V by the formation of an 8-8 bond (Table 15.18, p. 705). The free acid has not been obtained pure, but quite concentrated aqueous solutions can be prepared by treatment of the barium salt with the stoichiometric amount of H28O4 ... 

The acid is known only in aqueous solution and in the form of its salts, the dithionates or hyposulphates, the latter name now being infrequently used. 

The Dithionates or Hyposulphates.—These salts, like the nitrates, are all soluble in water only normal salts are known.4 They may be obtained by neutralising dithionic acid solution with the hydroxide of the base, and also by double decomposition between barium dithionate solution and the sulphate of the base, or between manganese dithionate solution and the hydroxide of the base 5 also by methods on pp. 206-8. 

Disulfur dinitride, 6 126 Disulfur pentoxydiehloride, 3 124 analysis of, 3 126 Dithiocarbamates, of selenium(II) and tellurium(II), 4 91 Dithionic acid, salts of, 2 167 Dithionite ion, formation of, from sulfur dioxide by sodium hydride, 5 13... 

This conclusion is drawn from the observed fact that the dithionic acids, on the electrolysis of their alkali salts, actually give acid supersulphides which correspond with the superoxides ... 

The oxidation state of sulfur in H2S206 is formally +5. This acid is not obtained as a pure compound, although a sizable number of dithionate salts are known. The structure of the dithionate ion is... 

In acidic solutions, dithionic acid decomposes to sulphate and sulphur dioxide at rates which are readily measured (Table 26) . The reaction is of first order at defined acidity and the rate increases, but not proportionately, with acid concentration. The authors have favoured an interpretation in terms of undissociated H2S2O6. Salt effects and solvent deuterium effects are in general agree-... 

The dithionate salt of the (+) and (-)-[(Co(NOsartacn)]3+ trication was resolved using aqueous sodium-antimony (-1-)-tartrate as an eluents. The reduction of [Co(NOsartacn)]Cl3 2H20 complex with zinc dust in aqueous hydrochloric acid followed by treatment with hydrogen peroxide led to the formation of the [Co(AMHsartacn)]4+ aminosarcophaginate. This complex was separated into optically active forms by the same procedure [142]. 

The inertness of the dinuclear complexes is greatest in slightly acidic solutions, which therefore have been employed for the reprecipitation reactions. Apparently the chromium systems are much more labile toward bridge breaking than are the cobalt systems. In aqueous solution the meso-[(en)2Cr(OH)2Ci(en)2] cation (I) enters into a rapidly established (t 1 min. at room temperature) equilibrium with the mono-ju-hydroxo complex [(OHXen)2Cr(OH)Cr(en)2-(HaO)] (n) The equilibrium constant K = [II]/[I] is 0.83 in 1 Af NaC104 at 0°. The salts (dithionate, bromide, chloride, and perchlorate) of the di-p-hydroxo cation are less soluble than the respective salts of the mono-/i-hydroxo cation. It is therefore possible to precipitate the pure salts of the di-/i-hydroxo cation from the equilibrium mixture following the procedure given above. 

Dithionic acid appears in a solution of sulphurous acid oxidised by finely divided MnOg. From this, baryta precipitates all the sulphur-containing anions except dithionate, and, when the excess of Ba + has been precipitated by H2SO4, only 112820 is left in the solution. Although conductance measurements show the acid to be dibasic, acid salts are unknown. Dithionates are soluble in water and, unlike salts of the higher thionic acids, are not decomposed by sulphites and sulphides. 

Pyrazolin-5-ones form complexes with both inorganic and organic compounds much more readily than do the 2-pyrazolin-5-ones. The most extensive series of complexes is that formed with a variety of metallic salts. Antipyrine (2,3-dimethyl-l-phenyl-3-pyrazolin-5-one) forms a series of complexes with salts of divalent, trivalent and tetra-valent metals. Two molecules of antipyrine form a complex with one molecule of copper, cadmium, cobalt and zinc salts.266,866,1116 Complexes prepared from metallic nitrates are usually hydrated.1322 There also exists a series of complexes in which three molecules of antipyrine form a complex with one or two molecules of metallic salts. Such complexes form with two molecules of simple ferric salts272 or with one of complex iron cyanides.608 Nitrates of thorium, lanthanum, cerium and samarium also give such complexes.841 This ratio also occurs in some antipyrine complexes with cadmium and zinc thiocyanate.266 A number of salts of rare earths and iron which have complex anions such as thiosulfate, thiocyanate, dithionic acid and complex iron cyanides form complexes in which six molecules of antipyrine are present.405,408 608,841,950 Stannic chloride forms salts containing three or four molecules of antipyrine and hydrochloric acid.46... 

Salts of the thiolate anions represented in Equation (10.49) combine with phosgene with the loss of the sulfur, rather than the loss of the oxygen atom [317]. Reaction between COClj and the anion of the corresponding dithionic acid, however, gives the readily isolated product of Equation (10.50) [317]. 

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