ft 



Crafts et al. — 40 — Water in Plants 



the van der Waals equation were applied only to the solute molecules and 

 neglected the solvent. And where corrections did consider the solvent 

 they were formulated in terms of stereotyped and arbitrary behavior as 

 in the corrections for association and hydration in the case of water. 

 HiLDEBRAND (1936) statcs, "Moreover, the concept of osmotic pressure 

 as an effect primarily of the solute, with the solvent simply furnishing 

 space, has obscured the effects of intermolecular forces and the inter- 

 changeability of solute and solvent. . . ." 



Using the modified van der Waals formula 



(P + -W^ • (V - b) = RT (13) 



where P is the osmotic pressure and V is the volume of solution containing 

 one gram molecular weight of the solute, Bancroft and Davis (1928) 

 have calculated osmotic pressures for mixtures of benzene and toluene in 

 which the two materials are considered as interchangeable. 



Although their V values at the high concentrations are smaller than the 

 possible volume into which the liquid solute could be compressed and con- 

 sequently improper for calculating R values, the latter values between mol 

 fraction values of 0.9 and 0.1 are fairly close to the true ones. By adding 

 two more constants to the equation Bancroft and Davis were able to cal- 

 culate R values from mol fractions of 0.99 to 0.01 that agreed very closely 

 with theory. For similar calculations involving ethylene chloride and bro- 

 mide in benzene, see Lewis (1908), Tables V and VI. 



Fundamentally there is no difference between solute and solvent in a 

 solution. In an osmotic system the permeability of the membrane to one 

 constituent designates that constituent as the solvent. However, substitu- 

 tion of a different membrane may reverse the designation. 



Practically, the physical state of the pure constituents at the operating 

 temperature and pressure may determine the more convenient designation ; 

 if one constituent is a solid or a gas it is most conveniently considered as 

 the solute. If both are liquids solubility relations may determine the desig- 

 nation, the least soluble being designated the solute. However, since in 

 liquid solution all constituents assume the liquid state, there is no unique 

 basis for naming the solute and solvent ; in osmotic systems the permeability 

 of the membrane constitutes the only true basis for designation. In bio- 

 logical systems water is usually the solvent; in some situations, however, 

 permeability of membranes to gases in solution may complicate analysis of 

 the osmotic system involved. 



In the example illustrated by Bancroft and Davis, benzene and 

 toluene were selected because they form an ideal solution at the tempera- 

 ture used. In most osmotic systems the solutions are not ideal ; because 

 of the anomalous properties of water, all aqueous solutions are non-ideal. 

 Gases themselves, with a few exceptions, are not ideal in their behavior and 

 the corrections developed to take care of departures from ideality of gases 

 do not apply to two-component systems. While the use of the van der 

 Waals corrections for space occupied by solute molecules and intermolecu- 

 lar forces between them may give fair agreement between calculated and 

 observed values of osmotic pressure for aqueous sugar solutions (Table 

 9) , the agreement is largely coincidental. True correction for non-ideality 

 of such solutions should establish a proper balance between factors of the 

 van der Waals type and adjustments for the polarity of water molecules 

 and their high cohesional interaction forces. 



