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Osmotic pressure can be measured in various ways which are described below:- 1. Mechanical Methods 2. Biological Methods 3. Physical Methods.
1. Mechanical Methods:
(i) By Putting Weights:
The simplest way is to apply adequate pressure (i.e., weight) upon the stronger solution to prevent any rise of volume. That pressure which is just needed to stop the increase of volume of a particular solution is the measure of its O.P. (Fig. 3.4).
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(ii) By a Manometer:
The same thing can be done by connecting the apparatus with a suitable manometer in which the pressure will gradually rise till it equalises with the O.P. of the solution, at which point further rise will stop (Peffer’s method, Fig. 3.5).
2. Biological Methods:
(i) Hamburger’s Red Corpuscle Method:
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Red cells are kept in the unknown solution for some time after which the cell volume is noted. If the cell volume be reduced, the solution is hypertonic than plasma (hence, water has been drawn out), if the cells swell up, the solution is hypotonic (so that water has entered), if no change—the solution is isotonic, if sufficiently hypotonic the red cells will gradually swell up and ultimately burst (Haemolysis).
(ii) De Vris’ Plant Cell Method:
The same principle is followed as above, only plant cells are used instead of red cells and the comparison is made with the cell sap inside. In hypertonic solutions, the cells will shrink, in isotonic— there will be no change, while in hypotonic solutions the cells will swell up and may burst (Plasmolysis).
3. Physical Methods:
By noting depression of freezing point Higher the concentration, lower will be the freezing point, and therefore higher will be the O.P.
By Noting the Vapour Tension:
Higher the concentration, lower will be the rate of evaporation from the solution and higher the O.P.
i. Hill’s Method:
Using Thermopile. Higher the rate of evaporation more will be the fall of temperature and less will be the O.P. Comparison is made with a solution of known O.P.
ii. Burger’s Capillary Glass Tube Method:
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Alternate drops of known and unknown solutions separated by air bubbles are drawn in a standard capillary glass tube and after some time the edges of the solutions noted. The edges will shift according to the rate of evaporation. From these data O.P. can be calculated.
