Water Potential Calculator
Water potential of a solution or cell from solute potential (van’t Hoff: −iCRT) and pressure potential — direction of water movement between two compartments and the equilibrium condition.
How it works
ψ = ψₛ + ψₚ; ψₛ = −i·C·R·T (R = 0.008314 MPa·L/mol·K); water flows toward the lower (more negative) ψ
Water potential is the free energy of water per unit volume relative to pure water at zero pressure. Solutes lower it (osmotic effect) and turgor pressure raises it, so a plant cell in a dilute solution takes up water until its pressure potential offsets the solute potential. Water always moves from higher to lower ψ, which is why the AP Biology classic — a potato core in sucrose — gains or loses mass depending on which side of the cell’s ψ the bath sits.
Worked example
0.3 M sucrose at 25 °C: ψₛ = −0.74 MPa; with 0.5 MPa turgor the cell’s ψ is −0.24 MPa. Placed in 0.1 M sucrose (ψ −0.25 MPa) it is almost at equilibrium — a fraction more turgor would stop uptake.
Frequently asked questions
Why is solute potential negative?
Solutes reduce the tendency of water to leave, so ψₛ is always ≤ 0; pure water is the zero reference.
What is i for NaCl?
Ideally 2 (Na⁺ and Cl⁻); real solutions ionise slightly less, about 1.8 at moderate concentrations.
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This calculator is for educational purposes. Double-check important results before acting on them.