Ionic Strength Calculation

Ionic Strength_Chem-Eng21.blogspot.com

As it was already signaled in the introduction, all equilibrium calculations done using concentrations are wrong. Ions are charged so they interact in the solution attracting and repelling each other with coulomb forces. These interactions influence ions behavior and don't allow to treat every ion in the solution independently. Whole phenomenon - although investigated for over 100 years - is still not fully understood and described. To be precise in our equilibrium calculations instead of using concentrations we should use ions activities. Activities are not a theoretical construct - they can be measured for every solution. In fact whenever you put pH electrode into a solution you are measuring not [H+] but activity of H+ions. It is enough to add an inert salt to the solution of a known acid to observe pH change that confirms activity concept (and is in accordance with the results of calculations presented below). Most popular method used to calculate ions activities is the one proposed by Debye and Hückel in 1923.

IONIC STRENGTH

●Dissociation of an electrolyte: M_xX_m  Þ      xM^m+  +  mX^x-

●Ionic Strength: m = 0.5 å z_i²C_i

●Activity coefficient: a_i = g _i [X]_I

● Debye-Huckel limiting Law relates activity coefficient to ionic strength

●Mean ionic activity:

a = g C (m^mx^x) ^1/(m+x)

Ionic Strength Calculation, for Examples

1.      What is the ionic strength for a 1.0 M NaCl solution?

I = 1/2(1*1² +1*1²)

I = 1

2.      What is the ionic strength for a solution whose concentrations

are 1.0 M La₂(SO₄)₃ plus 1.0 M CaCl₂?

for this solution the concentrations are:

[La ³⁺] = 2.0 M

[SO₄^2-] = 3.0 M

[Ca ²⁺] = 1.0 M

[Cl ^-] = 2.0 M

I = 1/2 (2*3² + 3*2² + 1*2² + 2*1²)

I = 18

3.    Calculate ionic strength of a 0.02 M NaCl solution:

Answer:  μ = 0.5[(0.02 × 1² ) + (0.02 × 1² )] = 0.02 M

4.    Calculate ionic strength of a 0.02 M BaSO₄ solution:

Answer:  μ = 0.5[(0.02 × 2² ) + (0.02 × 2² )] = 0.08 M

5.    Calculate ionic strength of a 0.02 M Na₂SO₄ solution:

 Answer:  μ = 0.5[(0.04 × 1² ) + (0.02 × 2² )] = 0.06 M

6.    Calculate ionic strength of a solution containing 0.02 M NaCl and 0.02 M BaSO₄:

Answer:  μ = 0.5[(0.02 × 1² ) + (0.02 × 1² ) + (0.02 × 2² ) + (0.02 × 2² )] = 0.10 M