Osmosis is a process by which a solvent (the liquid that dissolves another substance) in solution passes through a barrier. The solvent may pass through the barrier, but the solute (the substance dissolved in the solvent) either does not go through it, or passes through much more slowly than the solvent. The solvent will pass through the barrier until the concentration of solvent is the same on both sides of the barrier. The barrier is a membrane that is either permeable, allowing solvent and solute molecules to pass through, or semipermeable, allowing only solvent molecules to pass through. The pressure of the water passing through the membrane is called osmotic pressure.
The process was first investigated by a French physicist, Abbé Jean Antoine Nollette (1700-1770), in 1748. Nollette covered a glass tube containing sugar water with a piece of paper. He placed the tube, paper end down, into the water. The level of liquid in the tube rose. The pure water passed through the paper faster than the sugar water could. More experiments on osmosis followed. René Dutrochet (1776-1847) investigated the phenomena in the 1820s and 1830s. In the 1840s and 1850s Thomas Graham and Justus von Liebig researched osmosis but could not develop a suitable theory to explain it. Graham did distinguish between those substances that passed through parchment, which he called crystalloids, and those that did not, which he called colloids. Graham's additional research led to the process of dialysis, used today in artificial kidney machines.
The next major advance in the field came in 1877 when Wilhelm Pfeffer (1845-1920), a German botanist, studied osmotic pressure. Again the test subject was sugar water. The sugar solution was placed in a porous clay vessel, which in turn was placed in a container filled with pure water. Using a manometer Pfeffer measured the osmotic pressure and discovered was inversely proportional to the volume of a solution and directly proportional to absolute temperature or PV = kT, where P is pressure, V is volume, and T is absolute temperature. The constant k was later used in defining the universal gas constant by Jacobus Henricus Van't Hoff and was used in other gas laws. It was determined that the osmotic pressure a solute displays is the same pressure it would exert as a gas at the same volume and temperature.
Hugo van Mohl (1805-1872), a German botanist, continued on Pfeffer's path and was the first to describe cell division. He also provided the first lucid explanation for osmosis. Pfeffer's clay pot was a semipermeable membrane, as are the membranes surrounding most animal and vegetable cells. Studies in osmosis led to studies in cell physiology and solution purification.
Reverse osmosis is the process of applying a pressure greater than osmotic pressure on a solution. This reverses the process. In the end, a water-sugar water system would consist of pure water on one side of the barrier and a concentrated solution of sugar water on the other. This method is sometimes used for desalination, the process of removing salt from salt water to make it potable. The method is also used by hikers to remove harmful microorganisms from stream and lake water. Reverse osmosis is sometimes referred to as ultra-or hyper-filtration. The process is used to purify many liquids from milk to polio vaccines. Recently osmosis has been used to dehydrate fruit. Fruits slices are blanched; a sugar solution is pumped over the fruit, which is kept at 140°F (60°C) and the fruit slices lose most of their component water. They are then rinsed and dried briefly. The total process is faster than conventional dehydration, which takes about seven hours.
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