METHODS OF COLLOID RESEARCH 111 



the amount still in solution remains constant over various dilutions, 

 we may assume that the cellulose forms a solid solution with the sub- 

 stance in question. Adsorption probably exists only if the cellulose 

 takes up almost everything from a very dilute solution and if the 

 absorbing power of the cellulose is markedly decreased with increased 

 concentration of the solution; this condition is frequently observed 

 with dye solutions. Thus we may make shaking experiments with 

 solutions of the concentration, 0.1, 0.2, 0.3, etc., in which 0.1 denotes 

 any arbitrary standard. 



It must be determined first whether an equilibrium exists at all. 

 For this purpose a given quantity of adsorbent is shaken with the 

 solution, for example, with 100 c.c. In a second experiment an 

 equal quantity of adsorbent is shaken with half the quantity, 50 c.c. 

 of a solution twice the strength. It is then diluted to 100 c.c. and 

 shaken again until an equilibrium is reached. If there is an equilib- 

 rium, the final concentration of the solution in the first case is the 

 same as in the second. If there are material differences, the process 

 may nevertheless be considered an adsorption, but it is complicated 

 by other phenomena as explained on page 27 et seq. 



If it is unnecessary to determine constants, the simplest proce- 

 dure is to chart the values found on a rectangular system of co- 

 ordinates (millimeter paper). As ordinate is taken the amount of 

 the material that is being investigated which is taken up by 1 gm. 

 of adsorbent (cellulose, charcoal or the like); as the abscissa, the 

 amount which remains in solution after the adsorption; so that the 

 curve shows the ratio between the amount of substance in the solu- 

 tion and the amount that is adsorbed. It is easy to determine from 

 the characteristic form of the curve whether an adsorption has oc- 

 curred. (See p. 22.) 



The determination of adsorption curves and constants is explained 

 in detail on page 22 et seq. 



It is of greatest importance that the adsorbent be absolutely pure. Many in- 

 vestigators have failed in this and many contradictory results may be attributed 

 to it. The adsorbents are treated with acids, alkalies, alcohol, ether and benzol 

 according as their nature permits (charcoal, diatomaceous earth or kieselguhr, 

 fibrin, etc.)- In view of the fact that these substances are themselves more or 

 less adsorbed, it is necessary to remove them by prolonged constant treatment 

 with large quantities of the dispersing substance, usually water. 



Although temperature and time do not play as important a role as in other 

 physico-chemical processes it is important to keep temperature and time con- 

 stant. In most cases the adsorption balance is reached in about one-half hour 

 so that it is always fairly safe to allow an hour. 



It is usual to shake the adsorbent with the solution, but it must not be over- 

 looked that there are substances which are changed by the mere shaking (see 

 Inactivation by Shaking, p. 34). 



