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Colloid Osmotic Pressure

Solutions are homogenous mixtures of solute and solvent. On the basis particle size, solutions can be classified as true solution, colloidal and suspensions. In a true solution, the size of particles is too small therefore they are invisible by naked eyes. They are transparent solutions such as sugar in water. On the contrary, the particle size in suspensions is too large compared to true solutions. Due to large particle size, they are not stable and settle down at the bottom of the container. These particles are visible by naked eyes. The particles of colloids have an intermediate size of both types of solutions. These solutions are not transparent but particles are too large to settle at the bottom. Particles of colloids are not visible by naked eyes.

You must have heard about diffusion of gases from high concentration to low concentration. Just like gases, solutions also show movement of particles. The diffusion of particles of solution through a semi-permeable membrane is known as osmosis. The pressure which opposes the movement of particles through a membrane is known as osmotic pressure. It is mainly used in the purification of water and also in the study of the transport of ions in biological systems. Let’s discuss the osmotic pressure in colloids.

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Process of Osmosis

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To understand the process of osmosis, let us consider a pure solvent and solution separated by a membrane which permits the passage only to the solvent molecules, and not the solute particles. Only the solvent will diffuse through the membrane into solution. A membrane which is permeable to solvent and not to solute is called a semi permeable membrane.
The flow of solvent through a semipermeable membrane from pure solvent to solution, or from a dilute solution to concentrated solution, is termed as Osmosis.
It must be clearly understood that the diffusion of solvent molecules through a semipermeable membrane is taking place in both directions. So, the solvent molecules are passing from solvent to solution and then back from solution to solvent. But, since the diffusion from solvent to solution or from dilute to concentrated solution, is more rapid, the net flow of the solvent is from low to high concentration.

There are many kinds of semipermeable membranes. Animal and vegetable membranes are not completely semipermeable. All semipermeable membranes have fine holes or capillaries in their structure. These allow passage to solvent molecules but not to larger solute molecules.

Osmosis Equilibrium

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Osmotic flow takes place in cell membranes too. Wherever osmosis is taking place, the movement of particles through semipermeable membrane will continue, till an equilibrium is established and the concentration of the solvent molecules are same on both.

Example - 1

The egg experiment : The outer hard shell of two eggs of the same size is removed by dissolving in dilute hydrochloric acid. One of these is placed in in distilled water and the other in saturated salt solution. After a few hours, it will be noticed that the egg placed in water swells and the one in salt solution shrinks.

Equilibrium is being established in both cases.
  1. In the first case, water diffuses through the skin, a semipermeable membrane, into the egg material, which swells.
  2. In the second case, the concentration so the salt solution being higher than the egg, the material shrinks.
Thus, Osmosis equilibrium results, whenever there are two parts of solution, separated by a semipermeable membrane.

Example - 2

Crystals of many salts, example: ferrous sulphate, nickel chloride , cobalt nitrate and ferric chloride are placed in a solution of water glass (sodium silicate). The layers of metallic silicates formed on the surface of crystals by double decomposition are semipermeable. The water from outside enters through these membranes which burst and form a silica garden. Here too, until equilibrium is established, the process continues, and then stops at equilibrium.

Osmosis and Osmotic Pressure

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The hydrostatic pressure built up on the solution which just stops the osmosis of pure solvent into solution through a semipermeable membrane is called Osmotic pressure.

Osmosis phenomenon can be counteracted not only by the hydrostatic pressure but also by the application of central pressure on the solution. The external pressure may be adjusted as to prevent the osmosis of pure water into solution. Thus, Osmotic pressure can also be defined as
"Osmotic pressure may be defined as the external pressure applied to the solution in order to stop the osmosis of solvent into the solution separated by a semipermeable membrane"

This definition can be illustrated by means of Pfeffer's method of determination of Osmotic pressure. The external pressure on the solution is applied with the help of the piston and the progress of osmosis is shown by the movement of the liquid in the flow indicator. The pressure required just to arrest the movement of the liquid level in the flow indicator is equal to the osmotic pressure of the solution.

Colloidal Osmotic Pressure

Semipermeable Membrane

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The phenomenon of osmosis can be demonstrated by fastening a piece of animal bladder or cellophane over a thistle funnel. A concentrated aqueous sugar solution is placed inside the thistle funnel which is then immersed in water. The osmosis takes place through the semipermeable membrane from water to the sugar solution. The flow of water into the funnel shows up as the solution is seen rising in the tube remarkably.

Calculate Osmotic Pressure

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From the experimental studies of Osmotic pressure, Van't Hoff showed that for dilute solutions.
  1. The osmotic pressure of a solution at a given temperature is directly proportional to its concentration.
  2. The osmotic pressure of a solution at a given concentration is directly proportional to the absolute temperature. These findings of Van't Hoff resembled closely to the Gas laws.
So, accordingly, Van't Hoff modified the gas law expressions to:

Boyle's- Van't Hoff's law

If $\pi $ is the osmotic pressure and C its concentration, $\pi $ $\propto $ C, if the temperature is constant. If the concentration of the solute is expressed in moles per liter and if volume of the solution is V, that contains 1 mole of solute:

C = $\frac{1}{V}$ or $\pi $= $\frac{1}{V}$

Charles'- Van't Hoff's law

If T is the absolute temperature, then,

$\pi $ $\propto $ T, if temperature is constant.

Van't Hoff's equation for Osmotic pressure of all solutions

So, with respect to the above two laws, the osmotic pressure (π) of a dilute solution is inversely proportional to the volume(V) containing 1 mole of the solute and is directly proportional to the absolute temperature (T).

That is

$\pi $ $\propto $ $\frac{1}{V}$ ---------------(1)

 $\pi $ $\propto $T --------------(2)

Combining equations (1) and (2),

$\pi $ $\propto $ $\frac{T}{V}$

$\pi $ V = R'T

This equation is parallel to the general gas equation, PV = RT, as the value of R' calculated was similar to the gas constant R.

With the use of n moles of solute dissolved in V liters of solution, the equation can be written as:

$\pi $ V = n RT

Examples of Osmotic Pressure

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Calculate the osmotic pressure of a 5% solution of glucose (molecular weight = 180) at 18o C.


$\pi $ V = n RT

Osmotic pressure , $\pi $ = ?

V, volume= 100ml = 1/10 liters.
n, number of moles of glucose = 5 / 180 (weight /molecular weight)
R, gas constant = 0.821 lit.atm/deg mole.
T, temperature = 18o C = 18 + 273 = 291 K

Plugging the values in the equation:

$\pi $ x 1/10 = 5/180 x 0.821 x 291

$\pi $, Osmotic pressure of the given solution = 6.64 atm

Reverse Osmosis

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The osmosis taking place from the solution to pure water by application of pressure greater than the osmotic pressure, on the solution is termed as Reverse osmosis.

When a solution is separated from pure water by a semipermeable membrane, osmosis of water occurs from water to solution. This osmosis can be stopped by applying pressure equal to osmotic pressure on the solution. This osmosis can be stopped by applying pressure equal to the osmotic pressure, on the solution.

If the pressure greater than the osmotic pressure is applied, osmosis is made to proceed in the reverse direction to the ordinary osmosis. And this is reverse osmosis. This method is widely used in the purification of water.
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