I must have been feeling “osmotic pressure” since I missed posting for the last month. We all know that that excuse is as lame as learning by osmosis from a book under your pillow. Now, let’s review osmotic pressure.
Osmotic pressure is the minimum pressure required to stop solvent from flowing through a membrane into a solution. After Nollet’s experiment with wine and water, others discovered that sugar water was an equal replacement for wine (thus removing the religious problems with wine and water). Still, more work by Pfeffer and others showed that osmotic pressure increased with solute concentration. Van’t Hoff showed that this relation between osmotic pressure and concentration was linear at low concentrations. This paralleled the newly developing theory for a perfect gas. Further investigations showed that solute molecular weight, boiling point, and freezing point could be determined from the measured osmotic pressure.
Since osmotic pressure is the dominant variable in the osmosis experiment (a 1% sugar solution requires an osmotic pressure of 2/3 atmosphere), many have assumed that this is the driving force for flow through a membrane. Others assume that osmotic pressure keeps plants from wilting. We feel that this is incorrect. Osmotic pressure is applied to the solution to stop the flow through a membrane.
Since the magnitude of osmotic pressure is large for high concentration solutions, there have been many efforts to convert this pressure into power generation. Statkraft (Oslo, Norway) has a prototype for a Pressure Reduced Osmosis (PRO) system. We are still waiting for positive results.
© Larry Howlett HTMD Engineering 2021
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