Bisulfite-sulfite distribution

Figure 7-7. Bisulfite-sulfite and bicartonate-carbonate distributions as a function of pH.

In aqueous phase several species of S(IV), hydrated sulfur dioxide, bisulfite and sulfite are formed according to the Association properties of S(IV)-oxides in aqueous solution. The distribution among the protonated and non protonated species is determined by the pH-value. 

Simenson (1940), and the thermodynamic data for the system SO2-H2O are given by Plummer (1950). In water sulfur dioxide exists as the dissolved gas, as undissociated sulfurous acid, H2SO3, as the bisulfite ion, HSOj , and as the sulfite ion, SO3—. Sulfurous acid is a rather weak dibasic acid its first ionization constant is 1.7 X 10 , and the second is 5 X 10 . At 25° C. the corresponding pK values are 1.8 and 5.3, respectively. The calculated distribution of the various ionic species in aqueous solution of sulfurous acid in the range of pH of 0 to 8 is shown in Fig. 1. At pH s over 9.5 only SO3 ions exist, in the range of pH 9.5 to 4.5 both SO3 and HSOs occur, and at pH 4.5 and lower, SO3— no longer exists in appreciable concentrations. 

Equilibrium conditions for reactions 7-2 and 7-3 (sulfite/bisulfite distribution) and for reactions 7-14 and 7-15 (carbonate/bicarbonate distribution) are defined by die curves of Figure 7-7 (Head, 1977). Equations for calculating equilibrium constants for reactions 7-1 through 7-5 and 7-8 as a function of temperature, at zero imi stmgth, have been compiled by Pasiuk-Bronikowska and Rudzinski (1991). 

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