PHYSICAL CHEMISTRY IN BRIEF

CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 378

and in the second stage according to the equation

HA−= H++ A^2 −. (11.59)

If we denote the equilibrium constant of the first reactionK 1 and of the second reactionK 2 ,
we have

K 1 =

cH+cHA−γH+γHA−

cH 2 AγH 2 A

1

cst

, (11.60)

K 2 =

cH+cA^2 −γH+γA^2 −

cHA−γHA−

1

cst

. (11.61)

Note:In the concentration range in which the Debey-H ̈uckel limiting relation applies, we

have

γH+=γHA−, γA 2 −=γH^4 +.

Note: WhenK 1 K 2 (the differences in the values of dissociation constants are of the

order of 3 and higher), it is not necessary to consider dissociation in the second and higher

stages. The same applies to diacidic bases. Similarly, we also neglect dissociation in higher

stages in polybasic acids and polyacidic bases.

11.5.6 Dissociation of strong polybasic acids and polyacidic bases

A strong dibasic acid H 2 A dissociates in the first stage completely

H 2 A −→ H++ HA− (11.62)

and in the second stage up to the attainment of equilibrium

HA−= H++ A^2 −. (11.63)

For the dissociation constant of the reaction (11.63) we have

K=

cH+cA 2 −γ±^4

cHA−

1

cst

, (11.64)