Chemical Thermodynamics of Tin - Volume 12 - OECD Nuclear ...

II.1 Symbols, terminology and nomenclature 23
standard potential for a particular medium in which the activity coefficients are
independent (or approximately so) of the reactant concentrations [1985BAR/PAR] (the
definition of E ο ' parallels that of “concentration quotients” for equilibria). Therefore,
from
ο RT
E = E ' − ∑ νi
ln ci
(II.34)
nF
E ο ' is the potential E for a cell when the ratio of the concentrations (not the activities)
on the right-hand side and the left-hand side of the cell reaction is equal to unity, and
ο ο RT
Ec' = Ec − ∑ ν
i
ln γc,
i
(II.35) (II.35a)
nF
ο ο RT
*
Ec' = E − c ∑ i
ln( ·
m, i· H2O)
nF ν ξ γ ρ
(II.35b)
ο
⎛mi
m ⎞
* ο
γ
ci ,
= ⎜ (
ο ⎟ ρH2O
ρ ) γmi
,
(II.35c) 1
⎝ ci
c ⎠
where the γ
ci ,
and γ mi , are the activity coefficients on amount concentration and molality
basis, respectively [2008GAM/LOR], related by Eq. (II.35c), ρ H2O
is the density
*
m
of pure water, and ξ is ( i c ), the ratio of molality to molarity (cf. Section II.2). The
i
medium must be specified. Equation (II.35c) has been derived most elegantly by
[1959ROB/STO].
II.1.8 pH
Because of the importance that potentiometric methods have in the determination of
equilibrium constants in aqueous solutions, a short discussion on the definition of “pH”
and a simplified description of the experimental techniques used to measure pH will be
given here. For a comprehensive account, see [2002BUC/RON].
The acidity of aqueous solutions is often expressed in a logarithmic scale of the
hydrogen ion activity. The definition of pH as:
pH = − log a = − log ( m γ )
10 + 10 +
H
H
can only be strictly used in the range of the limiting Debye-Hückel equation (that is, in
extremely dilute solutions). In practice the use of pH values requires extra assumptions
as to the values for single ion activities. In this review values of pH are used to describe
qualitatively the ranges of acidity of experimental studies, and the assumptions
described in Appendix B are used to calculate single ion activity coefficients.
The determination of pH is often performed by potential-difference measurements
of galvanic cells involving liquid junctions [1969ROS], [1973BAT]. A common
+
H
1 *
H2
O
ο
ρ is a variable depending on T, = 1 mol·kg –1 ο
m
, c = 1 mol·dm –3 .
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012