How to Calculate Water Potential
Water potential (represented by the Greek letter Psi, $\Psi$) is a measure of the potential energy in water. In biology and botany, it predicts which way water will flow through cell membranes. Water always moves from an area of higher water potential to an area of lower water potential.
This calculator automates the standard equations required for AP Biology and environmental science. For other complex academic calculations, check out our standard Math Calculators.
The Main Formula: $\Psi = \Psi_p + \Psi_s$
Total water potential is the sum of two main components:
- Pressure Potential ($\Psi_p$): The physical pressure exerted on water. In an open beaker, this is always 0. In a plant cell, the cell wall exerts positive pressure (turgor pressure).
- Solute Potential ($\Psi_s$): Also called osmotic potential. Adding solutes (like salt or sugar) to pure water lowers its potential, meaning $\Psi_s$ is always negative (or zero for pure water).
The Solute Formula: $\Psi_s = -iCRT$
If you don't know the solute potential, you must calculate it using the properties of the solution:
- $i$ (Ionization Constant): How many particles the solute breaks into (e.g., Sucrose = 1, NaCl = 2).
- $C$ (Molar Concentration): The concentration in moles per liter.
- $R$ (Pressure Constant): Always $0.0831 \text{ L} \cdot \text{bars} / \text{mol} \cdot \text{K}$.
- $T$ (Temperature): Must be in Kelvin (Celsius + 273.15).