Crafts et al. — 42 — Water in Plants 



ing point, and boiling point of solutions may be used to calculate the activi- 

 ties of solutions. Such measurements may be used directly to indicate 

 concentrations, conductivities, or activities, or they may be converted to 

 osmotic pressures by appropriate formulae. Because such methods may 

 be more accurate or convenient than direct osmotic pressure measurements, 

 they have been widely used. 



There remains one field of physico-chemical study in which osmotic 

 pressure measurements are of value, that is, in determining the molecular 

 weight of large molecules. Substances such as dextrin, albumin, and many 

 others that form colloidal solutions cannot be studied by the indirect meth- 

 ods mentioned above because the low molecular concentrations render the 

 readings so small, and minute traces of impurities may overshadow the 

 true determinations. Membranes impermeable to such large molecules yet 

 unaffected by the impurities may be readily prepared ; they are sufficiently 

 rigid to withstand the small pressures involved. Many descriptions of such 

 osmotic pressure measurements may be found in the current chemical litera- 

 ture. Although they constitute an appreciable portion of current research, 

 they in no way reflect the immense importance of osmosis and osmotic 

 pressure in biology. In the latter field the study of the mechanism of 

 osmosis as it relates to turgor of cells, translocation of solutes, absorption 

 of water, and the more complex functions of growth and movement seems 

 highly important. 



Summary: — The Abbe Nollet performed the first recorded experiments on 

 osmotic pressure. Traube discovered the copper ferrocyanide membrane, and Pfeffer 

 using such a membrane precipitated in a porous pot made the first reliable quantitative 

 measurements, van't Hoff, realizing the significance of Pfeffer's measurements, de- 

 rived the simple relation PV = n RT relating the osmotic pressure of a solution con- 

 taining n mols of solute in V liters of solution at the absolute temperature T to the 

 constant R. He showed this R to be identical to the gas constant, van't Hoff's law, 

 as given above, applied only to very dilute solutions. 



Many refinements of the osmotic pressure law have been made. Corrections have 

 been made to put the concentration on a weight molar basis, to correct for the forces 

 of attraction between solute molecules and for the space they occupy, for hydration of 

 the solute and association of the solvent and for other factors. The most satisfactory 

 measurements for determining the osmotic pressure of an unknown solution, in addi- 

 tion to direct measurement, which is difficult, are vapor pressure lowering, boiling point 

 raising, and freezing point lowering. The latter three types of determination may be 

 used in some form to determine the physical properties of almost any type of solution 

 and by proper formulae the osmotic pressure may be calculated. 



A fundamental weakness in most considerations of osmotic pressure has been a 

 neglect of the role of the solvent. Molecules of solute and solvent are equally im- 

 portant in determining the osmotic properties of a solution. 



Many gases are non-ideal in their behavior ; most solutions are non-ideal ; all 

 aqueous solutions are non-ideal because water is highly irregular in its role of solvent. 



Solutions may be non-ideal 1) because the two components are so dissimilar that 

 they are incapable of forming an ideal system ; the escaping tendency of each com- 

 ponent is aflfected by the other; 2) because the two components have such attraction 

 that they tend to form compounds (i.e., hydrates). Dissimilarity of components re- 

 sults in positive deviations from Raoult's law and osmotic pressures are found that 

 are less than those calculated. Excessive attraction of components results in negative 

 deviations from Raoult's law and observed values of osmotic pressure are higher than 

 calculated. 



Most osmotic pressure determinations are made by indirect methods ; the freezing 

 point method is probably most useful. Direct determinations have been made on su- 

 crose solutions and a few other solutes at ordinary concentrations. Such determina- 

 tions are laborious. Direct determination is used now only to determine molecular 

 weights of large molecules. Continued study of the mechanism of osmosis seems 

 justified in the field of biology, particularly in the field of cell water relations. 



