Biomedical Engineering Reference
Calculation of the Percentage (mol/mol) Conversion of SO 3 2- to the
Ion-Pair NaSO 3 - in a Solution of Na 2 SO 3 Whose Total S(IV) Concentration
is in the Range 0.01 to 1.00 mol l -1
[S(IV)]/mol l -1
mol% NaSO 3 -
Reproduced from Wedzicha, B. L., and Goddard, S. J., Food Chem ., 40, 127, 1991. © 1991 Elsevier
Science Ltd. With permssion.
similar to sulfite ion. 1 Multiple charged cations form much more stable ion pairs,
which cannot be disregarded, even in dilute systems. All these interactions tend to
reduce the activity of S(IV) species, they perturb the acid-dissociation equilibria,
and the equilibrium for the formation of S 2 O 5 2- . 20 Surprisingly, little attention has
been paid to these interactions in the past.
It can be seen that the state of sulfur dioxide or sulfite in food is complex, but
all the forms that have been identified thus far are readily and rapidly interconvertible.
Regardless of the chemical form in which S(IV) is added to food (e.g., gaseous SO 2 ,
sodium, or potassium metabisulfites), the actual composition of this preservative
depends on the pH of the food, the concentration of S(IV), the ionic strength and the
presence of non-electrolytes. 21 During analysis, all these forms of S(IV) are converted
either to SO 2 (as in the Monier-Williams distillation technique, or its adaptations), or
to some other well-defined species (e.g., SO 3 2- for ion chromatography). 22
In view of the complex specification of the additive in any given situation, the
convention adopted in this chapter refers to the mixture of sulfur(IV) oxospecies,
in all forms which are readily converted to SO 2 on acidifying, as S(IV). Only where
it is necessary to refer to a given species will the actual name of the species be used.
CHEMICAL REACTIVITY OF S(IV)
Sulfur(IV) oxospecies show two distinct types of reactivity; sulfite ion is an excellent
nucleophile, whereas all the S(IV) species behave as reducing agents.
N UCLEOPHILIC R EACTIONS
The main reason for reactions between S(IV) and food components is the nucleo-
philic reactivity of SO 3 2- , leading to the formation of C-S and S-S covalent bonds.
Sulfite ion is one of the best nucleophiles available, with reactivity similar to that
of the thiolate ion, acting both as a carbon- and a sulfur-nucleophile. 23 Hydrogen
sulfite ion is less nucleophilic. 13 The structure of S 2 O 5 2- involves an S-S bond, which
can be imagined to result from the nucleophilic attack by SO 3 2- on a SO 2 molecule;
this species shows no significant nucleophilic reactivity. Thus, the tendency for the
conversion of HSO 3 - to S 2 O 5 2- at high concentration and low water activity, the
formation of ion pairs involving metal ions and SO 3 2- or S 2 O 5 2- , 24 and the preferred