Water Potential Calculator
Calculate water potential (Ψ) from solute potential and pressure potential. The solute potential is computed using the van't Hoff equation Ψs = -iCRT, where i is the ionization constant, C is the molar concentration, R is the gas constant, and T is temperature in Kelvin.
Water potential describes the tendency of water to move from one area to another. It is a central concept in plant biology and AP Biology courses. Water always moves from regions of higher water potential to regions of lower water potential.
The Core Equation
Ψ = Ψs + Ψp
- Ψ = water potential (bar or MPa)
- Ψs = solute potential (always zero or negative)
- Ψp = pressure potential (positive in turgid cells, zero in open systems)
Calculating Solute Potential
Ψs = -iCRT
- i = ionization constant (number of particles in solution). Glucose = 1, NaCl = 2, CaCl2 = 3.
- C = molar concentration in mol/L
- R = pressure constant = 0.0831 L bar / (mol K)
- T = temperature in Kelvin (Celsius + 273.15)
Worked Example
A 0.5 M NaCl solution at 25 degrees C in an open beaker:
- i = 2 (Na+ and Cl-)
- C = 0.5 mol/L
- T = 25 + 273.15 = 298.15 K
- Ψs = -(2)(0.5)(0.0831)(298.15) = -24.82 bar
- Ψp = 0 (open container)
- Ψ = -24.82 + 0 = -24.82 bar
Key Points
- Pure water in an open container has a water potential of 0.
- Adding solute always lowers water potential (makes it more negative).
- Pressure potential is positive when the cell wall pushes back on the contents (turgor pressure).
- Water moves toward more negative water potential.