Debye–Hückel Limiting Law
The theoretical relation predicting activity coefficients of ions in very dilute solution from their charge and the ionic strength.
Formula / equation
log γ = −A z² √I
Unit
Dimensionless
In depth
The model treats each ion as surrounded by a diffuse atmosphere of opposite charge that lowers its effective activity. The limiting law is accurate only below about 0.01 M; extended forms that include ion size are needed at higher concentrations.
Examples in the real world
For a singly charged ion at I = 0.001 M in water at 25 °C the law predicts γ ≈ 0.96.