Page 88 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 FUNDAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

The extent of hydrolysis of depends upon the characteristics of the
metal ion, and is largely controlled by the solubility product of the metallic
hydroxide and, of course, the stability constant of the complex. Thus iron(II1)
is precipitated as hydroxide (K,,, = 1 x in basic solution, but nickel(II),
for which the relevant solubility product is 6.5 x IO-'', remains complexed.
Clearly the use of excess EDTA will tend to reduce the effect of hydrolysis in
basic solutions. It follows that for each metal ion there exists an optimum pH
which wili give rise to a maximum value for the apparent stability constant.
(b) The effect of other complexing agents. If another complexing agent (Say
NH,) is also present in the solution, then in equation (q) [Mn+] will be reduced
owing to complexation of the metal ions with ammonia molecules. It is
convenient to indicate this reduction in effective concentration by introducing
a factor fi, defined as the ratio of the sum of the concentrations of al1 forms of
the metal ion not complexed with EDTA to the concentration of the simple
(hydrated) ion. The apparent stability constant of the metal-EDTA complex,.
taking into account the effects of both pH and the presence of other complexing
agents, is then given by:

2.28 ELECTRODE POTENTIALS
When a metal is immersed in a solution containing its own ions, Say, zinc in
zinc sulphate solution, a potential difference is established between the metal
and the solution. The potential difference E for an electrode reaction
Mn+ +ne= M
is given by the expression:

where R is the gas constant, T is the absolute temperature, F the Faraday
constant, n the charge number of the ions, a,.+ the activity of the ions in the
solution, and Ee is a constant dependent upon the metal. Equation (32) can
be simplified by introducing the known values of R and F, and converting
natural logarithms to base 10 by multiplying by 2.3026; it then becomes:
0.000 198 4T
E = EQ+ log a~.+
n
For a temperature of 25 OC (T = 298K):
0.0591
E = E~+--- log a,.,
n
For many purposes in quantitative analysis, it is sufficiently accurate to replace
a,.+ by cM.+, the ion concentration (in moles per litre):
0.0591
E = Ee+--- log CM.+
n
The latter is a form of the Nernst equation.