SED & PREDOM / Ionic Strength

The Debye-Hückel model is based on electrostatic theory, and it only considers long-range interactions between ions:

log γi =

−A Zi2 √I  /  (1+åBγ √I )

where I is the ionic strength; γ_i is the activity coefficient of an ion “i” with electric charge Z_i;  A and B_γ are solvent parameters (A=0.509 for water at 25°C); and å is related to the ionic size.

The Debye-Hückel equation is only useful in very dilute solutions. For most real applications activity coefficient models including short-range ion interactions must be used.