60 



Chemistry. — ''Colloids and the phase )-ide." Bj Dr. E. H. Buchner. 

 (Communicated by Prof. A. F. Holleman). 



(Communicated in the meeting of April 25, 1913.) 



The question, whether tlie phase rule can be applied to .systems, 

 in which a coUoïd occurs, and if so, in what way, lias been discussed 

 more then once. Some investigators have concluded, that it is not 

 allowed; the greater part, ho\ve\er, think, that — under certain 

 restrictions — it can be done; systems, in which irreversible reactions 

 occur, must, for instance, be excluded'). The question has been 

 examined most closely by Pavlov^) and Jonker^). Both consider a 

 colloid a system of two phases, but, whilst Jonker applies the usual 

 rule, Pavlov introduces a new independent variable, namely the 

 specific surface of a phase, i.e. the ratio of its surface to its quantity. 

 He gives the phase rule then naturally the form 



without applying it practically or giving any experimental proof of 

 its exactness. It is certainly easy to see, that such a proof will be 

 impossible, since the number of degrees of freedom is found to be 

 just as great if one applies the ordinary phase rule, provided that 

 the colloid is taken as one phase. If we take e.g. a solution of 

 gelatine in equilibrium with its vapour at a certain temperature, 

 71 = 2, and according to Pavlov ;■ =r 3 ; the system has therefore 

 one degree of freedom. If, on the other hand, we consider the 

 solution one phase, and apply the familiar rule, then it follows 

 from F=72-{-l — r{l, because the temperature is fixed), that in 

 like manner F^=l, because now ?^ is taken ^ 2. Thus it will 

 never be possible to decide by experimental evidence, which of the 

 two formulae has to be applied; this must be deduced from consi- 

 derations a priori: we should ask ourselves whether really a new 

 independent variable must be introduced, and whether a colloid 

 actually consists of two phases. As to these questions, Jonker has 

 adopted the view, that neither the surface-tension, nor any other 

 quantity must be taken into consideration, and that therefore the 

 ordinary rule may be maintained. Taking a colloidal solution as 

 composed of two phases, one will find — ceteris paribus — one 

 degree of freedom less than in an ordinary solution, and Jonker thinks 

 this to be confirmed by the absence of the depression of the freezing 

 point and the other quantities connected therewith in typical colloidal 



i; Henri, Z. phys. Gliem. 51, 31. 



2) Z. phys. Ghem. 75, 48. 



3) Kolloid-Zeitschrift 8, 15. 



