Inputs
Enter values and compute equilibrium
Formula used
This calculator uses ideal Donnan equilibrium for a 1:1 electrolyte with an impermeant fixed species in compartment A.
- Electroneutrality: A: c+A − c-A + zfCf = 0, and B: c+B = c-B.
- Electrochemical equality: For each ion, RT ln(ci,A/ci,B) + ziFΔψ = 0.
- Donnan relations: c+A = c+Be−α, c-A = c-Beα, where α = FΔψ/(RT).
- Mass balance: Total permeant moles are conserved using your volumes and initial concentrations.
- Osmotic estimate: Δπ ≈ RT(ΣCA − ΣCB), including the fixed species as an osmole in A.
Assumes ideal solutions, constant volumes, and a membrane permeable to both permeant ions.
How to use this calculator
- Choose an input mode: salt-only for quick setups, or custom for full control.
- Enter temperature, compartment volumes, and the fixed charge concentration and valence.
- Provide initial concentrations. Use mol/L and liters for consistent results.
- Press Compute. Results appear above the form under the header.
- Download a report using the CSV or PDF buttons in the results box.
Example data table
Example inputs and typical outputs (rounded). Try these values to validate your setup.
| Case | T (degC) | VA (L) | VB (L) | Cf (mol/L) | zf | Salt A (mol/L) | Salt B (mol/L) | Δψ (mV) | c+A (mol/L) | c+B (mol/L) |
|---|---|---|---|---|---|---|---|---|---|---|
| Demo 1 | 25 | 1.0 | 1.0 | 0.05 | -1 | 0.15 | 0.15 | ~ -5 to -25 | ~ 0.18 to 0.22 | ~ 0.14 to 0.17 |
| Demo 2 | 37 | 2.0 | 1.0 | 0.10 | -2 | 0.10 | 0.15 | More negative | Higher | Lower |
Outputs depend on charge balance, volumes, and fixed-charge strength.