Osmosis and Reverse Osmosis

If two solutions of different strengths are separated by a membrane that allows molecules of the solvent to pass, but not those of the solute (the dissolved substance), solvent molecules will cross the membrane from the weaker to the stronger solution until the two are of equal strength. The membrane separating them is called ‘differentially permeable’ if it allows water molecules to pass but slows the passage of larger molecules or prevents some of them, or ‘semi-permeable’ if it is completely permeable to molecules of solvent and completely impermeable to those of the solute. Cell membranes are differentially permeable. Membranes that allow the passage of some but not all molecules are now often described as ‘partially permeable’.

The passage of water through a membrane requires energy. Pure water is considered to possess zero energy and a solution to have a negative energy value. Osmosis occurs when there is an energy difference between two solutions and the energy involved, known as the ‘osmotic pressure’ or ‘water potential’, can be measured.

In reverse osmosis sufficient pressure is applied to a solution to overcome the water potential and force water molecules to cross a semi-permeable membrane from the higher to lower concentration. The pressure required is about 25×105 Pa (25 times ordinary sea-level atmospheric pressure).