Instant Notes: Analytical Chemistry

(ii) Complexation equilibria. The reaction between an acceptor metal ion M

and a ligandL to form a complex ML is characterized by an equilibrium

constant. This is discussed further in Topic C7, but a simple example will

suffice here:

M(aq) +L(aq) [ML(aq)

Kf=(aML)/(aM. aL)

For example, for the copper-EDTA complex at 25oC: Kf=6.3¥ 1018

(iii)Solubility equilibria. If a compound is practically insoluble in water, this

is useful analytically because it provides a means of separating this

compound from others that are soluble. The technique of gravimetric

analysishas been developed to give very accurate analyses of materials by

weighing pure precipitates of insoluble compounds to give quantitative

measurements of their concentration. For the quantitative determination of

sulfate ions, SO 42 - , the solution may be treated with a solution of a soluble

barium salt such as barium chloride BaCl 2 , when the following reaction

occurs:

Ba^2 ++SO 42 - [BaSO 4 (s)

Conversely, if solid barium sulfate is put into water:

BaSO 4 (s) =Ba^2 ++SO 42 -

The solubility product,Ksp, is an equilibrium constant for this reaction

Ksp =a(Ba^2 +). a(SO 42 - ) =1.2¥ 10 -^10

bearing in mind that the pure, solid BaSO 4 has a=1. This means that a

solution of barium sulfate in pure water has a concentration of sulfate ions

of only 1.1¥ 10 -^5 M. The concentration of the barium ions is the same.

(iv)Redox equilibria. When a species gains electrons during a reaction, it

undergoes reductionand, conversely, when a species loses electrons it

undergoes oxidation. In the total reaction, these processes occur simultane-

ously, for example:

Ce^4 ++Fe^2 +=Ce^3 ++Fe^3 +

The cerium is reduced from oxidation state 4 to 3, while the iron is oxidized

from 2 to 3. Any general ‘redox process’may be written:

Ox1 +Red2 =Red1 +Ox2

The equilibrium constant of redox reactions is generally expressed in terms

of the appropriate electrode potentials (Topics C5, C8), but for the above

reaction:

K=(a(Ce^3 +). a(Fe^3 +))/(a(Ce^4 +). a(Fe^2 +)) =2.2¥ 1012

Summary

For ionic equilibria in solution, which are widely used in analytical chemistry,

a large equilibrium constant for the reaction indicates that it will proceed

practically to completion. If the equilibrium constant is of the order of 10^10 ,

then the ratio of products to reactants will be much greater than 1000 to 1. For

example:

C1 – Solution equilibria 59