Osmosis and Osmotic Pressure

Diffusion and osmotic pressure are connected as will be seen in due course and so it is appropriate to consider the process of osmosis subsequent to diffusion. When two substances separated by a membrane diffuse into one another the process is called osmosis. Many years ago Abbé Nollet filled a jar with alcohol and covered it with a section of a pigs bladder (the semi permeable membrane) and then immersed it in water. The water entered the jar faster than the alcohol left the jar and some pressure was built up in the jar (the osmotic pressure) and bladder bulged outward. On reversing the liquids the bladder bulged inwards. This was probably the first reported example of the development of osmotic pressure. In a similar type of experiment Graham placed a solution of a mixture of salts and colloids in a tray with a parchment base and floated it in pure water. He found that the salts passed rapidly through he parchment into the water whereas the colloids remained in the tray. This process of separation he gave the term dialysis.