Estimate an ion’s activity coefficient in a dilute aqueous solution at 25 °C from charge and ionic strength.
| log₁₀ γ | – |
| γ | – |
Use this calculator in physical chemistry when you know the ionic strength and want a quick activity coefficient γ before treating concentrations as activities.
The Debye–Hückel limiting law is log₁₀ γ = −0.509 z² √I, then γ = 10^(log₁₀ γ). The constant 0.509 is for water at 25 °C. The law is meant for very dilute aqueous solutions, not seawater, concentrated buffers, or non-aqueous solvents.
Ion charge z: Charge number of the ion, without sign. Use 1 for Na⁺ and 2 for Ca²⁺.
Ionic strength I: Ionic strength in mol/L. For 0.01 M NaCl, I is about 0.01.
log₁₀ γ: Common logarithm of the activity coefficient from the limiting law.
γ: Activity coefficient. A value near 1 means the ion behaves almost ideally.
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