# Ionic Strength (molal based or m-based)

The ionic strength is a quantity representing the strength of the electric field in a solution, and it is equal to the sum of the molalities of each type of ion present multiplied by the square of their charges, as represented by the following equation:

I=1/2 ∑_(i=1)^n▒〖(z_i^2 m_i)〗

Where n is the number of charged species.

For example, a 1.0 molar solution of NaCl has 1.0 moles of Na^+ions and 1.0 moles of Cl^- ions in 1 kg of H2O. Therefore, the ionic strength is 1.0 molal.

I=1/2((z_(Na^+ ) )^2 (m_(Na^+ ) )+(z_(Cl^- ) )^2 (m_(Cl^- ))

I=1/2((1)^2 (1)+(-1)^2 (1)=1

Now, consider a 1.0 molal solution of CaCl2. This solution has 1.0 moles of Ca^(+2) ions and 2.0 moles of Cl^- ions in 1 kg of H2O. Therefore, the ionic strength is 3.0 molar, or it can be said that a 1.0 molal solution of CaCl2 behaves similar to a 3.0 molar solution of NaCl.

I=1/2((z_(Ca^+ ) )^2 (m_(Ca^+ ) )+(z_(Cl^- ) )^2 (m_(Cl^- ))

I=1/2((2)^2 (1)+(-1)^2 (2)=3

# Ionic Strength (mole fraction based or x-based)

In this case the ionic strength is calculated using the mole fraction rather than the molality:

I=1/2 ∑_(i=1)^n▒〖(z_i^2 x_i)〗

Where n is the number of charged species.