What is Osmotic Pressure

Osmotic pressure is a function of dissolved substances. As in Osmosis, water will pass through a semi-permeable membrane from the lower concentration compartment into the higher concentration compartment. Osmotic pressure is simply what makes osmosis process occurs. The force of this flow is measured by measuring how much pressure must be applied to the higher salt concentration side in order to stop osmosis. This pressure then must be the force of osmosis. This pressure is called osmotic pressure. Osmotic pressure is a function of the number of collisions of water molecules with either side of the membrane. The more dissolved substances, of any kind, there are, the fewer the water collisions.

A very rough rule of thumb is that for every 100 mg/L or ppm of TDS (Total Dissolved Solids) the osmotic pressure is roughly 1 psi (0.07 bar). If pure water is separated from a 1000 mg/L TDS solution by a semi-permeable membrane. It will take around 10 psi of pressure on the 1000 mg/L side in order to stop osmosis. Therefore, the osmotic pressure forcing water from the pure water side into the 1000 mg/L side is 10 psi. While the greater water collisions are from the pure water side of the membrane, the reason for water movement left to right is due to the higher salt concentration on the right hand side. 

osmotic pressure is what makes osmosis process occurs

Amount of osmotic pressure generated, therefore, is directly proportional to the amount of total dissolved solids (TDS) in the solution. Since every 100 mg/L creates around 1 psi (0.07 bar) of osmotic pressure, we simply have to divide the TDS by 100 (move the decimal to the left two places) in order to calculate the approximate pressure.

Now what happens when we have salt solutions on both sides of a semi-permeable membrane? Net osmotic pressure becomes important. The net pressure is the difference between the pressure of each of the solutions separated by a semi-permeable membrane. If two 1000 mg/L salt solutions separated by semi-permeable membrane, roughly 10 psi of osmotic pressure is being exerted by each solution. In this case the pressures are equal and opposite in direction. Substracting one from the other, the net osmotic pressure is zero. This is when the osmosis process reaches equilibrium.