NEGATIVE OSMOSIS 113 



static effect on account of the small ionic radius of the hydrogen 

 ion. It is important to note that certain salts of biological 

 interest have a marked electrostatic value very dilute solutions 

 of oxalates, phosphates, and citrates and of the tetra-valent ion 

 Fe(CN) 2 attract water violently. On the other hand, the effect 

 of the anion may be masked by the opposite electrostatic effect 

 of the cation. As the valency of the cation increases, the attrac- 

 tive force of the anion decreases. Calcium chloride, for instance, 

 has little more action than distilled water, because the calcium 

 almost neutralises two positive charges on the two anionic charges 

 (cf. Hydrophilic property of Ca, Chap. VIII.). 



The value of this electrical force has been determined by Loeb 

 in a very neat manner. Inside a collodion bag he placed an M/128 

 solution of KC1 and outside the bag an M/64> solution of sugar. 

 These solutions are approximately isotonic, i.e. movement of 

 water through the membrane by osmotic forces is thus eliminated. 

 He found that water did diffuse from the sugar solution to the 

 KC1 solution. This transport of water must be due to the 

 electrical pull of the KC1. He then raised the concentration of 

 the sugar outside the bag till its osmotic pressure just balanced 

 the attractive forces of the KC1. The sugar solution was now M/8. 

 Therefore the electrical forces which are at work correspond to 

 an osmotic pressure which is the difference between the osmotic 



7M 7 x 22 '4 



pressures of an M/8 and an M/64 solution of sugar = = 



> 64 64 



2-4 atmos. (approx.). 



These electrical forces also account for negative osmosis the 

 passage of water from a more to a less concentrated solution. 

 As far back as 1835, Dutrochet observed that water diffused out 

 of a pig's bladder filled with a dilute solution of oxalic acid, into 

 pure water. Early investigators tried to explain this on the 

 assumption that there was a greater imbibition of water on the 

 acid side of the membrane and a lesser on the side in contact with 

 the pure water. In 1914 negative osmosis was observed taking 

 place through a porcelain filter and, therefore, the imbibition 

 theory becomes untenable. Loeb has shown that negative 

 osmosis occurs when neutral salts as well as acids and alkalies 

 in certain well-defined concentrations are separated from water 

 by a membrane capable of taking up either a positive or a negative 

 charge. At these concentrations the repelling action of the ion 

 with the same sign of charge as that of water becomes greater 

 than the attractive action of the ion with the opposite charge. 



