Debye and Hückel Equation. The Debye and Hückel equation is used to calculate the activity coefficient (γ) of a reactant in a dilute solution of known ionic strength when the system is not ideal by employing an equation of the following form.
 

where (z_i) is the ion charge number,

(q) is the elementary charge,
(k) is the Debye screening length,
(e_r) is the relative permativity of the solvent,
(e_o) is the permativity of free space,
(k_B) is the Boltzman Constant,
(T) is the solution temperature,
(I) is the solution ionic strength,temperature,

and (A) is 1.172 mol^-1kg^1/2

Further Information on this Topic can be found in the books of R.P.W. Scott by following the listed links