Experimental Results of Osmotic Measurements

The van Hoff equation has been carefully tested experimentally from two aspects. One requirement is that the osmotic pressure of dilute solutions is proportional to the temperature. Data by Morse on cane sugar solutions show that the relationship holds in very dilute solutions, so that the purely statistical character of the phenomenon is evident. 

The second requirement of van t Hoff s equation, that osmotic pressure is proportional to concentration, is also well satisfied by dilute cane sugar solutions. 

Direct measurements of the lowering of vapor pressure by isothermal distillation or similar methods present not inconsiderable difficulties. Numerous experiments of Hess, his co-workers and others have shown that osmotic measurements by these methods are attended with great difficulties and must be carried out very carefully if they are to give trustworthy results. 

In the high polymer domain there remains, therefore, only the direct method of using semi-permeable membranes, which has actually been developed successfully by Herzog and Spurlin and a number of observers in recent years. In this way we may use either the static elevation methody in which equilibrium between osmotic and hydrostatic pressure serves as the criterion, or a dynamic method which was first designed by Van Campen and later developed by C. G. Boissonnas, Obogi and Broda, and K. H. Meyer. In the latter the osmotic pressure, which causes a streaming of the pure solvent through the semi-permeable membrane is compensated by a suitable counter-pressure in such a way that the motion of the liquid practically ceases. 

The chief difficulty in both cases is always the suitable choice of the 

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