CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 388
- The acid concentration is very low,c= 10−^6 in order. At very low concentrations a weak
acid dissociates completely. The same procedure can be applied as in the Example in
section11.6.4. - The product of the acid dissociation constant and its concentration is low,K c < 10 −^12.
In this case the set of equations (11.48) and (11.53) has to be solved.
11.6.8 pH of a weak monoacidic base
A weak monoacidic base dissociates up to the attainment of a chemical equilibrium between
the ions and undissociated base molecules, see11.5.4. We have
pH =−1
2
log(
Kw^2 (cst)^2
K′c γ±^2)
+1
2
log (1−α), (11.94)wherecis the initial concentration of the base andαis the degree of dissociation for which it
follows from equation (11.57)
α=−K′+
√
K′^2 + 4K′c
2 c, K′=
K cstγBOH
γ±^2, (11.95)
whereKis the dissociation constant of the base.
If the dissociation degree is low,α < 0 .1 in order, relation (11.94) may be simplified to
pH =−1
2
log(
Kw^2 (cst)^2
K′c γ^2 ±). (11.96)
Relations (11.94) and (11.96) apply on condition that the effect of hydrogen ions formed in
consequence of water dissociation can be neglected.
11.6.9 pH of weak polybasic acids and polyacidic bases
In weak dibasic acids and diacidic bases, whenK 2 K 1 , it is enough to consider solely
dissociation in the first stage. To calculate the pH of a weak acid and a weak base, we use
relations (11.93) and (11.96), respectively.