Semipermeable membrane and its mechanism

Semipermeable membrane and its mechanism

Introduction of Semipermeable membrane

The membrane through which the only solvent molecules can pass. But not the solute molecules is known as a semipermeable membrane (SPM). Many plants and species have a natural semipermeable membrane. Gallbladder, cellophone, colodian, the cell membrane of some plants and animals are the few examples of the semipermeable membrane.

To measure the osmotic pressure, a few synthetic semipermeable membranes are used. Some of the synthesized semipermeable membranes are copper ferrocyanide. And also calcium phosphate, cellulose nitrate etc. Semipermeable membrane of copper ferrocyanide is most widely used. Since it can resist high pressure of about 150 atm and also easy to prepare.

Preparation of copper ferrocyanide membrane

There are two types of the method by which copper ferrocyanide membrane can be prepared i.e

1. General Method:

It was first prepared by Traube.

The porous pot in which SPM is to be prepared is first thoroughly cleaned by washing it with acid, water, alkali and finally with distilled water. The air bubbles in the pores of the pot are removed by forcing water through the pot under pressure.

The pot is then filled with 2.5% solution of copper sulfate. And also then placed in a container with a solution of potassium ferrocyanide of the same strength.

Copper ions go out of the pot. While ferrocyanide ions come into the pot. They meet somewhere in the walls of the pot to form a membrane of copper ferrocyanide.

2CuSo4 + K4[Fe(CN)6] → Cu2 [Fe(Cn)6] + 2K2So4

2Cu² + Fe(Cn)6 → Cu2[Fe(CN)6]

copper ferrocyanide

Semipermeable Membrane and its mechanism

2. Electric method:

The semipermeable membrane prepared by the above method takes more time. So another method, electrical method, also known as morse and Horn method is used. The semipermeable membrane prepared by this method is stable and more compact.

In this method, potassium ferrocyanide solution is filled in a container. A porous cylindrical pot and copper plate A are placed in the container as shown in fig. Platinum cylinder C acts as cathode and copper plate A acts as the anode.

Copper sulfate solution is filled is porous pot. And electrodes are connected with a battery and electrical bell B. On passing current both types of ions start to diffuse. Copper ions move towards the cathode. And ferrocyanide ions move towards the anode.

They meet almost at the center of the wall of the pot to form a compact. And hence stout membrane of copper ferrocyanide. When the membrane is being formed, the electrical resistance of the cell increases and finally reaches a maximum value. On completion of a membrane, the current stops, and the bell ceases ringing. Now the porous pot is washed with distilled water. This is necessary to remove all other ions. And is placed in distilled water. This membrane is quite stout and compact.

2Cu² + [Fe(Cn)6] → Cu2[Fe(Cn)6]

Mechanism of Semipermeable membrane 

The mechanism of working of the semipermeable membrane can be understood by the following theories-

  1. The Sieve Theory- Traube (1867) suggested that semipermeable membrane acts like a molecules sieve. Where the molecules bigger than the solvent is stopped. While the solvent molecules filter through it. Thus solvent flows from a region of higher solvent concentration to one of lower concentration. But this theory fails in the case when solute particles are smaller than solvents particles.
  2. The Solution Theory- This theory was developed by Thermite in 1855. According to this theory, the membrane is permeable to those substances which dissolve in it. And is impermeable to those materials which are insoluble in it. For example, calcium nitrate is soluble in water. But insoluble in phenol. And thus a layer of phenol slipped between calcium nitrate. And water acts as a semipermeable membrane. This theory is wider than the sieve theory.
  3. The Vapour-Pressure Theory- This theory was forwarded by Calander in 1903. And according to this theory, the semipermeable membrane is a vapor sieve. Means the vapor of solvent molecules can pass through the membrane. As the vapor pressure of the pure solvent is higher than that of the solution the solvent molecules. And hence can pass through the membrane to the solution side. This theory explains the phenomenon of osmosis.

In this particular article Semipermeable Membrane and its mechanism, we have discussed basic information about semipermeable membrane. Preparation of copper ferrocyanide membrane, and mechanism of the semipermeable membrane in the easiest way possible.

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