Ionic strength
Definition:
Ionic strength of a solution is a
function of the concentration of all ions present in a solution. It was defined
by Lewis and Randall to reflect the effect of charges and interionic
interactions on electrolyte activities, and hence on ionic activity
coefficients.
General
formula of molar ionic strength, I:
The ionic strength formula is calculated
as the sum of the molar concentration of each ion multiplied by the valence
squared.
where
the term 1/2 is due to both ions are considered (cation and anion), c is
the concentration in molar units (mol/L) and z is the charge
of each ions. For example, if the ions is sulfate (SO42-)
z =2. This, it can be seen multivalent ion has a bigger contribution.
It is used in theoretical chemistry for
calculating dissociation of salts in heterogeneous systems such as colloids. It
is also used in biochemistry and molecular biology for determining the strength
of buffer solutions that should have concentrations similar to the found in
nature.
Examples:
1. For
a solution of potassium chloride 3 M, calculate the ionic strength.
First,
it is needed to draw the dissociation:
KCl
→ K+ + Cl-
so,
the concentration of each ion is the same as the concentration of the salt, 3
mol/L.
Then,
the equation can be applied:
I
= 1/2 [(3 mol/L)(+1)2 + (3 mol/L)(-1)2] = 3 M
2. For
a solution of potassium chloride 1 M and magnesium sulfate 0.2 M, calculate the
ionic strength.
First,
it is needed to draw the dissociation:
KCl → K+ + Cl-
MgSO4 → Mg2+ + SO42-
Then,
the equation can be applied:
I
= 1/2 [(1 mol/L)(+1)2 + (1 mol/L)(-1)2 + (0.2
mol/L)(+2)2 + (0.2 mol/L)(-2)2 ] = 3.6 M
Non-ideal
solutions:
Because in non-ideal solutions volumes
are no longer strictly additive it is often preferable to work with molality m
(mol/kg of H2O) rather than molarity c (mol/L). In that case, molal
ionic strength is defined as:
i =
ion identification number
z =
charge of ion
