Grafts et al. — 26 — Water in Plants 



diminished intensity of bonding force per molecule of water. As the water 

 content approaches saturation capillary condensation is suggested to ac- 

 count for the loosely held water. However, these three mechanisms are 

 so closely integrated that the vapor pressure : water content curves are 

 smooth. Babbitt (1942) shows such a curve in his Figure 3. The rela- 

 tion of binding energy to vapor pressure in terms of relative humidity is 

 shown in his Figure 7. 



GoRTNER (1938) has reviewed the literature on bound water and has 

 described thirteen methods for measuring it. Though results vary, all 

 methods show appreciable quantities of water that fail to obey the laws of 

 dilute solutions. Deviations from laws of concentrated solutions are not 

 always so great. 



Any molecular species which dissolves in water, when added to a water 

 mass, tends by means of its own valence forces to satisfy those of the 

 water. When added, not only the water molecules in the immediate vicinity 

 but statistically all water molecules in the system are affected, as evidenced 

 by a change in vapor pressure and a corresponding lowering of activity. 

 With continued addition of solute there results a smooth and progressive 

 reduction in activity of the water. Provided there is no change in state, 

 the initial addition of solute to the solvent results in reduced activity also 

 of the solute. The case of a single undissociated solute is relatively simple 

 to analyse. With dissociation of the solute molecules three activities are 

 involved, and with the addition of two or more solutes a high degree of 

 complexity occurs with the possibility pointed out by Chandler (1941), 

 that sufficient decrease in two or more activities may actually allow increase 

 in a third without disturbing the thermodynamic balance. This phenomenon 

 may possibly explain negative values for bound water that are found oc- 

 casionally. 



As one deals with more complex situations involving both colloidal 

 and crystalloidal solutes, accurate determination of the activities of all 

 constituents becomes impossible. Hence many of the measurements on 

 so-called "bound" water do not necessarly prove its existence but simply 

 reflect complex intermolecular reactions that are incapable of analysis. On 

 the other hand, there are many examples of the hydration of colloids in 

 which small but measurable amounts of water are held by intense binding 

 forces. The use of the term "bound" to describe such water is very con- 

 venient. 



Hydration: — The relations of the solvent to the solute in aqueous 

 solutions has received much study and theories of hydration enter into most 

 considerations of the properties of solutions. In Armstrong's (1908) 

 theory of the polymeric isomerism of water the presence of solutes is sup- 

 posed to affect materially the dissociative change 



(HoOni^nHoO {12) 



When a compound such as HCl dissolves, complexes are assumed to be 

 formed as follows: 



H H 



HoO , HCl , and H2O : CIH 



\ \ 



CI OH 



The first two are active, the last inactive so long as it remains closed. 



