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There are many properties of solutions which find importance in our day to day life. We know that the colligative properties of liquids like lowering of vapor pressure, depression in freezing point and elevation in boiling point finds use in determining the molecular weight of the solute in the solution.

Similarly, the process of osmosis and the pressure related to it, the osmotic pressure also finds importance in industrial, as well as household purposes. Osmosis and reverse osmosis are very important properties of solutions.


Osmosis Definition

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Just as a gas can diffuse into vacant space or another gas, a solute can diffuse from a solution into the pure solvent. If you pour a saturated aqueous solution of potassium permanganate with the help of a thistle funnel into a beaker containing water, it forms a separate layer at the bottom.

Osmosis is a process by which the solvent molecules will pass through a semipermeable membrane from the less concentrated part (solute concentration) to the more concentrated (solute) solution. A semipermeable membrane, usually an animal membrane, allows only the solvent particles and not the solute particles inside it, due to the pore size. Osmosis is also a phenomenon like diffusion.

Osmotic Pressure Definition

The excess pressure which must be applied to a solution to prevent the flow of the solvent into it through a semipermeable membrane is known as the osmotic pressure of the solution.

Osmosis Membrane

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The semipermeable membranes used in the osmosis process can be an animal membrane, paper, etc. It should allow only the solvent particles and not the larger solute particles.

Preparation of semipermeable membrane

Parchment paper and the animal bladder mentioned above are not truly semipermeable. Traube, a German chemist, found that films of certain gelatinous precipitates, especially of cupric ferrocyanide, Cu2 [Fe(CN)6], are perfectly semipermeable to sucrose.

Prefer using copper ferrocyanide membrane for a number of quantitative measurements to calculate the osmotic pressure. The membrane is usually deposited in the walls of a porous pot. The porous pot is first thoroughly washed and dried and then filled with a solution of copper sulphate and immersed in a solution of potassium ferrocyanide.

Diffusion of ions takes place through the pores where cupric and ferrocyanide ions meet in the walls of the pot, a gelatinous precipitate of copper ferrocyanide is formed.

2Cu2+ + [Fe(CN)6]4- Cu2[Fe(CN)6]

The clay of pot gives mechanical strength to the membrane.

Hypotonic Solution

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Consider two solutions in contact with each other through a semipermeable membrane. If solution A has a lower osmotic pressure than solution B, then on comparison, such solutions are hypotonic in nature.
Applying this to red blood cells, if the fluid outside the red blood cells have less osmotic pressure than that of inside the red blood cells, such situation is termed as Hypotonicity or hypotonic solutions.

Forward Osmosis

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Forward Osmosis is a normal osmosis process used in water purification, desalination process, etc. Here, a solution of higher solute concentration is allowed to flow with the process into a solution of lower concentration. It is a new field of research.

Water is separated from the unwanted particles using this process. Unlike reverse osmosis where a lot of outside pressure is applied, this process does not need external pressure source.

Reverse Osmosis System

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The opposite of forward osmosis, reverse osmosis process has been used for a lot of years to purify water for drinking purposes. It is now almost a household name because of many purifiers applying this concept to purify drinking as well as pool water too.

In the reverse osmosis process, as the name implies, the process of osmosis is reversed. Instead of solvent flowing from higher to lower concentrations, it flows from lower to higher concentrations. As it is quite against the osmosis theory itself, we need to use external pressure.

This external pressure makes the solvent molecules to flow in the opposite direction, thereby, basically forcing all solute to be left behind in the area where pressure is being applied. In purification of water, if water can be considered as solvent and the dirt/impurities as solute, all the dirt and impurity are left behind and pure water passes through the semipermeable membrane to the other side, thus purifying water completely.

Examples of Osmosis

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Example: 1
Cell membranes act as semipermeable membranes. For example, if a cell is placed in pure water it will swell. The cytoplasm inside the cell contains dissolved ions and some of the water outside the cell passes through the membrane owing to osmosis.

But, if the cell is placed in a concentrated solution of salt, the cell shrivels. This time the water passes out of the cell into the more concentrated solution around it.

Example: 2
When a camel takes in large quantity of water, its blood cells swell owing to water molecules passing into the cells. After many days without water, the cells shrink as water passes out of the cells into the blood stream.

Importance of Osmosis

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  1. Osmosis is a very important process in all biological systems as well as in purification of drinking water.
  2. Osmosis process is also used to desalinate ocean water for its usage in industrial applications.
  3. Osmosis has also been of great use in determining molar masses, especially those of polymers.
  4. Reverse osmosis finds use in water purification.

Osmosis Pressure

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The hydrostatic pressure set up as a result of osmosis is measure of osmotic pressure of the solution.

To understand osmotic pressure, we can take the example of an experiment. Let us place sugar in a cup which has semipermeable walls. Close the cup with a rubber piston. Now, place this cup into a beaker containing water only. Since the solute particles or the sugar particles are inside the cup, water will flow into the cup to equalize the concentration. But, if we lower the piston on the cup, the pressure is applied to sugar solution.

This pressure will make solvent water to flow out of the cup, as against normal osmosis process. When a proper pressure is applied on the piston, the tendency of the water to enter the cup through the semipermeable membrane will just be balanced by its tendency to flow out as a result of counter pressure from the piston.

High Osmotic Pressure

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We know that osmotic pressure is the pressure applied to a solution to prevent the flow of solvent into it through a semipermeable membrane.

If two solutions are in contact with each other, the solvent will move towards that solution, where the osmotic pressure is low. So, solvent moves from high osmotic pressure region to low osmotic pressure region.

Calculating Osmotic Pressure

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Osmotic pressure is calculated using the formula:
π V = n R T

Solved Example

Question: The osmotic pressure of a solution of potassium chloride containing 0.75 grams in 100cm3 of water was 4.52 x 105 N m-2 and 452 Pa at 25 oC. What is the predicted molar mass of potassium chloride ?
n = $\frac{π V}{R T}$ 

We have

n = $\frac{4.52 \times 10^{5} N m^{-2} \times 100 \times 10^{-6} m^{3}}{8.314 J K^{-1} mol^{-1} \times 298 K}$

Correct answer is 0.018 mole
The formula for osmotic pressure was calculated by Van't Hoff after he combined the two important gas laws
  1. Boyle's law and
  2. Charles' law.
The formula (we can call it as Osmotic pressure equation also) was derived at which was similar to the Ideal gas law equation: PV = nRT
Osmotic pressure: $\pi $ = n R T/V
n = number of moles of solute in the solution.
V = volume of the solution
R = Gas constant
T = temperature in Kelvin.
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Osmosis Potential

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Osmotic potential, denoted as: ψπ, is the potential of water to move through semipermeable membrane from less solute concentration to high solute concentration.
The formula for osmosis potential is :
Osmosis Potential: $\psi $ $\pi $ = -CRT
C = concentration of solutes
T = temperature
R = gas constant


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Tonicity is a measurement gradient used to measure or compare osmotic pressures of two solutions separated by a semipermeable membrane.

Accordingly, there are three types of tonic measurements

Isotonic Solutions
Solutions which have the same osmotic pressure as the same temperature are said to be isotonic. When two solutions having the same osmotic pressure are put into communication with each other through a semipermeable membrane, there will be no transference of solvent from one solution to another.

Hypertonic Solutions
If one solution has a higher osmotic pressure than the other, and are connected together by a semipermeable membrane, such solutions are called as hypertonic solutions. Usually, this is used in connection to the fluids inside and outside red blood cells.

If the fluid inside the red blood cell has lower osmotic pressure and that outside the red blood cells have higher osmotic pressure, such solutions are said to be hypertonic solutions.

Hypotonic Solutions
If one solution has a lower osmotic pressure than the other, the solution is hypotonic.
Diffusion is a process where solute particles move from more concentrated part of the solution to less concentrated part of the solution. No semipermeable membrane is involved.

Osmosis, on the other hand, occurs when solvent passes from low solute concentration to high solute concentration. And, semipermeable membrane is involved here.
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More topics in Osmosis
Diffusion and Osmosis Colloid Osmotic Pressure
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