Frans_M_Everaerts_Isotachophoresis_378342.pdf

88 CHOICE OF ELECTROLYTE SYSTEMS
example, for the reaction of an acid HA with the solvent S, we can have the following
reaction :
HA + S=A- + SH'
During the reaction, the number of ions increases. If the interionic forces between A-
and SH+ are small (high dielectric constants), the equilibrium lies on the right-hand side.
With lower dielectric constants, the equilibrium lies towards the left-hand side. On a purely
electrostatic basis, it can be derived that [2]
where K = equilibrium constant; zA = zB + 1 ; zA and zB are the charges of the acid-base
pair; e = standard electrical unit of charge; r = radius of the ions; k = Boltzmann constant;
T= absolute temperature. This equation gives the relationship between the acid and
base constants in the different solvents, and some examples are discussed below.
The acid constant of NH: in water and methanol
Reaction:
NP4 + S=SW + NH3
In this instance, zA =1 andzB = 0. Substituting the constants in eqn. 5.8 for water and
methanol we obtain
log (k)
KCH, OH
= - 1.24 (22,- 1)= - 1.24
Thus the acid constant of NH; in methanol is about 17 greater than that in water.
The acid constant of acetic acid in water and methanol
Reaction:
CH3 COOH + S ECH3 COO- + SH+
Here zA = 0 and zB = -1. Substitution in eqn. 5.8 shows that the acid constant for acetic
acid in methanol is about 17 smaller than that in water.
It can be seen that the change in the acid constants depends on the type of reaction.
In practice, the differences are much larger because, for example, the dimensions of the
molecules are not known exactly and the electrostatic model is not valid as the influence
of the acid-base behaviour of the solvents is neglected.
Another point in comparison with water is the self-dissociation of methanol:
2 CH3 OH
CH3 0- + CH3 OK2
The dissociation constant of methanol is about (for water it is about 10-14). As
with water, we can define a pH and a pOCH3 value for methanolic solutions. Especially
when choosing the pH of the electrolyte systems and when choosing the counter ions,
the pK values of the substances must be known for the solvent chosen. The way in which