io8 PRINCIPLES OF GENERAL PHYSIOLOGY 



MODES OF PREPARATION OF COLLOIDAL SOLUTIONS 



Certain processes have incidentally been given in the previous pages. 



As a general statement, it may be said that the object to be attained is the 

 formation of excessively minute particles of the substance which it is desired to 

 obtain in the colloidal state. In the case of the suspensoid or irreversible class, it 

 will be plain that we cannot take a portion of the dry solid and dissolve it in 

 water in the way usually done in preparing solutions. This can be done with 

 emulsoids in most cases, especially with the proteins ; so that, if a preparation of 

 colloidal silver, for example, is desired in the dry state, such that it can be made 

 into a solution by mere addition of water, the method adopted is to coat the 

 particles with some protein. This is done by adding such a protein to a solution 

 of the suspensoid and then evaporating to dryness. The commercial "collargol" 

 is such a preparation of colloidal silver. 



In the case of the suspensoids, in order to prepare their solutions, the particles 

 themselves must, as a rule, be formed in the liquid which is to be the medium of 

 dispersion. Arsenious sulphide is formed by passing hydrogen sulphide through 

 a solution of arsenious acid and sols of various metals by disintegration with the 

 electric arc (Bredig) or spark (Svedberg) in the water or other liquid. In order 

 that such sols shall be permanent, foreign electrolytes must be removed as far as 

 possible. 



It is indeed sometimes a difficulty in analytical work that precipitates will not deposit 

 because of the absence of electrolytes to cause their aggregation. Sometimes it is possible to 

 add a trace of an appropriate positive or negative trivalent ion, which will produce immediate 

 clearing. 



In traces, certain metals such as lead and copper pass into what seem to be 

 colloidal hydroxides by mere contact with water, conferring certain toxic 

 properties on the water. This is known as " oligodynamic " action, of which 

 more will be said later. 



Metallic hydrosols can be frequently prepared by reduction of their salts with 

 various reagents, such as phosphorus or formaldehyde in the case of gold. 



When a metallic salt is hydrolytically dissociated in water, prolonged dialysis 

 removes the free acid, leaving the colloidal hydroxide. Instances are ferric 

 and thorium hydroxides. In such cases, as also in those where the colloid is 

 formed by double decomposition, an adsorption compound with the salt or 

 precipitant is usually formed. For example, ferric chloride on dialysis gives 

 a series of colloids containing, less and less chlorine in relation to iron, from 

 3 of Cl to 1 of iron, to 1 of Cl to 400 or 500 of Fe, in no stoichiometrical 

 proportion. If dialysis be continued until nearly all the chlorine is removed, 

 the colloid tends to deposit rapidly ; it seems to be stable only when in adsorption 

 combination with a certain amount of the chloride. 



To prepare emulsoids free from salts, as is frequently required, the only wav i* 

 prolonged dialysis. Owing to the peculiarity of adsorption being relatively greater 

 the lower the concentration of the solution of the adsorbed substance, it is a matter 

 of much difficulty to remove the last traces of salts (Bayliss, 1906, p. 181). 



In the case of colloidal dyes, the method of Harrison (1911, p. 17 of reprint) is 

 useful. Precipitate by saturation with ammonium carbonate. This will replace 

 T)ther adsorbed salts. The solution of the dye should be fairly strong, but not too 

 strong, since it will be difficult to filter off the deposit. Probably the use of the 

 centrifuge would be advantageous. Re-dissolve the deposited dye and reprecipitate 

 with ammonium carbonate. Filter again. Repeat the process, if great purity is 

 required. Finally, dry at 110, which drives off the ammonium carbonate, leaving 

 pure dye. It is important to remember that commercial dyes often contain as 

 much as 30 per cent, of sodium chloride or sulphate. 



SUMMARY 



Matter in the colloidal state is in the form of ultra microscopic particles of solid, 

 or droplets of fluid, in suspension or in other manner of dispersion, in another solid, 

 liquid, or gas. It consists, therefore, in a heterogeneous system, in the sense that 



