DIFFUSION 85 



added to a molecular dispersion, hut varies with the salt used. 

 Some salts cause au increased and some a decreased osmotic pres- 

 sure to develop. In general, electrolytes belong to the latter class, 

 the extent of their depressing influence depending on the nature 

 of the ions composing the electrolyte. As regards the anions, 

 there is a defmite order of increasing depressing power as one passes 

 along the series 



CNS < I< Br < NOo < CI < SO4 



Similarly the cations may be arranged as follows : alkali metals << 

 alkaline earths << heavy metals. 



The hydrogen ion concentration of the whole colloidal system 

 has a very great influence on the nature of the osmotic change 

 produced by any added salt. 



{}}) Diffusion. The large size of the colloidal particles, especially 

 those of hydrophilic sols, prevents their rapid diffusion through 

 still w^ater. In fact, coloured colloidal substances can readily be 

 arranged in the order of increasing size of the dispersed phase by 

 noting the rate with which the colour passes into water in a test 

 tube. 



Diffusion of Salts. — Dissolved substances diffuse easily into or 

 out of gels, the rate of diffusion depending on the concentration 

 of the diffusing substance and on the nature of the colloid. Resist- 

 ance to the passage of the diffusant varies from gel to gel according 

 to their structure and viscosity. The obstructive power of a gel 

 may be altered by alterations of temperature, which alters both 

 the kinetic energy of the diffusing salt and the viscosity of the 

 gel. Some colloids, like albumin, develop a definite semi-solid 

 structure when heated, while others of a gelatin nature become 

 more liquid. 



The rate of diffusion may be altered by the addition of certain 

 substances to the gel. A gel, after treatment with • sodium 

 sulphate, glucose, alcohol, glycerol, etc. (dehydrating agents) 

 offers considerable resistance to the diffusion of electrolytes. Urea, 

 iodides, and chlorides, on the other hand, cause acceleration of 

 the rate of diffusion. These added substances cause alteration 

 in the relative amounts of water held by dispersoid and dispersant 

 and so produce alterations in the more liquid phase. The degree of 

 continuity of liquidity is a causative factor in the velocity of 

 diffusion. 



The Cause of Diffusion. — What is the force that drives the 

 solute to all parts of a solution or of a sol or of a gel so that except 

 at interfaces it is equally distributed throughout the mass ? Just 

 that force which causes a gas to diffuse equally through a container 



