112 



COLLOIDS IN BIOLOGY AND MEDICINE 



A second disadvantage of the shaking is that the adsorbent is thereby still 

 further broken up and its surface thus permanently increased. When large 

 quantities of colloid are in solution, there is a counterbalancing error in that the 

 adsorbent becomes coated with a layer of colloid which thus diminishes the ac- 

 tive surface. Though these errors are small in the case of adsorbed crystalloids, 

 in the case of true colloids they become quite considerable. To eliminate these 

 two disadvantages, H. WISLICENUS and W. MUTH* have developed a method 

 which they call the siphon (or filter) process. In this method a solution of con- 

 stant strength comes repeatedly in contact 

 with the adsorbent. The process is as fol- 

 lows: a tube is filled with washed clay or 

 other adsorbent and in connection with a 

 separatory funnel, forms a siphon. The 

 solution to be studied is poured into the 

 funnel and very slowly filters through the 

 adsorbent. The apparatus (see Fig. 23) is 

 entirely practical. In the strict scientific 

 sense, however, equilibria are not obtained 

 with it. 



FIG. 23. Apparatus for adsorption 

 analysis. (H. Wislicenus.) 



Before determining the content of 

 the solution after adsorption, the ad- 

 sorbent must be removed. Filtration is 

 rarely suitable because the filter-paper 

 itself adsorbs. In any event the filter 

 used should be very small, and the 

 quantity of fluid to be filtered as large 

 as possible. Centrifugation is the 

 most practical method. The fluid 

 may be poured or pipetted from the 

 adsorbent which has been deposited. 

 The determination of the content of the solution before and after 

 adsorption varies so much in accordance with the nature of the sub- 

 stance under investigation, that it is hardly possible to formulate 

 general rules. The simplest procedure, when it is possible, is to de- 

 termine the weight of a given volume after evaporation, or the solu- 

 tions may be titrated. In other cases suitable physical or biological 

 methods must be employed (animal experiment, hemolysis, agglu- 

 tination, etc.). 



To determine the electric charge of a colloid by adsorption, we 

 choose for adsorbent, a suspension of a substance having the most 

 pronounced electrical charge. Electropositive iron oxid or alumina 

 gel removes electronegative colloids from solution. Electronega- 

 tive diatomaceous earth (kieselguhr), kaolin or* mastic suspensions 

 (obtained by dropping an alcoholic solution of mastic into water) 

 attract electropositive colloids. As has been said, the charge of 



