The spontaneous flow of solvent through a semipermeable membrane from a solution of lower concentration towards a solution of higher concentration is known as Osmosis.
Semi Permeable Membranes
Three types of membranes can be used to separate a solution of a non-volatile substance from a pure solvent.
I. The membranes through which all particles of a solution are released on the other side are called permeable membranes. Link
II. Such membranes, through which no particles of any type of solution are able to come out on the other side. are called impermeable membranes. Link
III. There are also some such membranes between the above-mentioned types of membranes, from which only solvent particles or molecules can be released but solutes cannot be removed. These are called semi permeable membranes. These types of membranes have superfluous holes, only the molecules of the solvent can cross through.
Most examples of semi permeable membranes are natural,
Example: egg membrane, membranes of animal cells etc. Of the parchment membranes, some membranes are permeable and some membranes are semi-permeable.
Some chemical compounds, such as calcium phosphate and copper ferrocyanide also form semi permeable membranes. Semi permeable membrane of copper ferrocyanide is the best among Semi permeable membranes of chemical compounds.
Method of making Semi permeable membrane of copper ferrocyanide: Copper ferrocyanide is deposited on the walls of a porous pot to make Semi permeable membrane of copper ferrocyanide.
After thoroughly cleaning the porous pot, fill the copper sulphate solution in it and place this vessel in another ordinary pot containing potassium ferrocyanide solution. Diffusion of ions from pores of the porous pot results in the following reaction.
2Cu2+ + [Fe(CN)6]4- → Cu2[Fe(CN)6]
As a result of the above reaction, gelatin-like precipitate of copper ferrocyanide accumulates in the walls or pores of the porous pot and forms a semi permeable membrane.
For the best quality of the membrane, it is necessary that the porous pot has a similar shape and does not have air bubbles.
Before starting the experiment to expel air from the holes, the porous pot is completely immersed in distilled water for a long time and water also flows under the pressure of the holes.
The membrane constructed by the above method is not very strong. Modern methods or method of electrodynamics are used to make a more robust semi permeable membrane of copper ferrocyanide. This method is similar to the above method.
The difference is that in the modern method, copper sulphate flows power using an anode of copper in solution and potassium ferrocyanide solution using a cathode of platinium.
Under the influence of copper, copper ion porous pot is attracted from inside and ferrocyanide ion from outside to inside and deposits of copper ferrocyanide accumulate in the holes of the vessel. The membrane obtained by this method is stronger and can bear higher pressure.
If pure solvent is placed on one side of a semi permeable membrane and the solution on the other side, then the molecules of solvent reach the solution through the membrane and try to produce the same concentration on both sides.
If a solution with less concentration is placed on one side of the semi permeable membrane and more concentrated solution on the other side, the solvent molecules move from the less concentrated solution to the more concentrated solution and try to make the concentration on both sides equal. This action continues until the concentration on both sides is equal.
“The spontaneous flow of solvent through a semipermeable membrane from a solution of lower concentration towards a solution of higher concentration is known as Osmosis.”
Egg has two covers. The outer cover is mainly made of calcium carbonate and the inner shell is in the form of a transparent semi permeable membrane.
When the egg is put into dilute HCl, its outer cover comes off and the inner cover is not affected.
After removing the outer cover and placing one egg in distilled water and the other egg in a saturated solution of salt for a few hours, the following inspection is obtained –
The egg placed in distilled water swells after some time. The reason for this is that the concentration inside the egg is high and the outside is pure solvent. Hence water molecules enter the egg and the egg swells.
The egg placed in a saturated solution of sodium chloride shrinks after some time. The reason for this is that the concentration outside the egg is high.
In the process of osmosis, the solvent molecules move from a less concentrated solution to a more concentrated solution. Hence water molecules flow out from within the egg. Therefore, the egg volume decreases and the egg shrinks.
Difference between Diffusion and Osmosis
If a layer is made of copper sulphate solution in a beker and the second layer of water by carefully pouring water over it, initially these two layers look different, but after some time these layers look different. And the entire fluid of the beker forms a homogeneous mixture.
This is caused by diffusion. In the process of diffusion, the particles of solute move from the solution of the more concentrated solution to the solution of less concentration and the particles of solvent go from the solution of less concentration to the solution of more concentration.
The main differences between diffusion and osmosis are the following.
· The presence of a semi permeable membrane is necessary in the action of osmosis while diffusion occurs without any membrane.
· In the action of osmosis the flow of molecules is only in one direction while in the action of diffusion the flow of molecules is on both sides.
· In the process of osmosis, only solvent molecules move from a low concentration to a more concentrated solution, whereas in the process of diffusion, both solute and solvent molecules move in opposite directions.
The osmotic pressure of a solution can be defined based on the following usage.
A thistle binds a semi permeable membrane at the top end of the funnel. By reversing the thistle funnel, fill the potassium parmagnate solution in it and immerse it in a beker filled with pure water.
As a result of the action of osmosis, the solvent molecules start moving from the beker to the funnel through the semi permeable membrane. Therefore, the surface of the solution in the funnel tube starts to rise.
After some time the top of the solution of the solution in the funnel shuts down. It is an equilibrium in which the hydrostatic pressure in the tube becomes equal to the pressure in the solution that causes osmosis. This pressure is called osmotic pressure.
First Definition: The osmotic pressure is equal to the fluid static pressure that is generated when the solution is separated from the solvent by a semi permeable membrane.
In fact hydrostatic pressure is not an accurate measurement of osmotic pressure because its osmotic pressure changes as the funnel solution in the funnel attenuates at equilibrium. Hence it is necessary to amend the above definition of osmotic pressure. The revised definition of osmotic pressure is given based on the following usage –
A character is divided into two parts by a semi permeable membrane. Both these parts have piston. The solution of one part of these and the solvent is filled in one part, as soon as the action of osmosis starts, the piston of the solution starts to rise.
To prevent this action, so much pressure is produced on the piston that osmosis does not occur. The deaf pressure that is applied to stop the action of osmosis is called osmotic pressure.
Second Definition: To prevent osmosis when a solution is separated from the solvent by a semi permeable membrane, at least the external pressure exerted is called the osmotic pressure of that solution.
According to modern belief, the vapor of the action of osmosis leads to a solution through the holes in the semi permeable membrane.
This action occurs until the vapor pressure of the solvent on both sides is equal. The vapor pressure of the pure solvent is greater than the vapor pressure of the solution.
Now if the pressure on the solution is externally so that the solvent and the vapor pressure of the solution are equal, the action of osmosis will stop.
Third Definition: The minimum external pressure, which is applied on the solution, reduces its vapor pressure equal to the vapor pressure of the solvent is called osmotic pressure.
Methods of measuring osmotic pressure
The osmotic pressure is determined by the following methods –
- Pfeffer’s method
- De-vries’ Method
- Morse and Frazer’s Method
- Plasmolysis Method
- Townsend’s porous disc method
- Berkley and Hartley’s Method
The first 5 methods take longer to measure osmotic pressure and there is also a possibility of a semi permeable membrane bursting. In 1909, berkley and hartley presented the most appropriate method, removing both these shortcomings. Which is described below –
Berkley and Hartley Method
In this method a thin membrane of copper ferrocyanide prepared by the method is applied in the holes of a porous pot. This membrane acts as a semi-permeable membrane.
The porous pot fills this vessel in a cylindrical vessel, which is made of metal, by filling it with pure water. The solution whose osmotic pressure has to be found, fills it in this cylindrical vessel, there is a piston and pressure measuring device.
The porous pot has a tube in which the floor of water is noted before starting the experiment. When osmosis starts, the water starts going through the membrane towards the solution, which causes the water level in the tube to fall downwards.
Now with the help of piston, put so much pressure on the solution that the bottom of the water in the thin tube stops falling and the floor remains undone.
The pressure exerted by the piston on the solution is equal to the osmotic pressure, which is read by the pressure gauge. In this method there is no direct pressure on the membrane, so high osmotic pressure can also be measured.