56 PHYSICAL CHEMISTRY IN BIOLOGY. 



KOELICHEN x has studied a specially pretty case of the catalytic 

 action of OH ions in the decomposition of diacetonalcohol into acetone: 



The reaction is reversible, and from the following table it is seen that 

 the velocity constant for various concentrations of the same catalyst 

 remains the same as well as by using different bases. 



r^tali/af Cone, of the Velocity 



Catalyst - Catalyst. Constant. 



Piperidine ............... 0.1090 0.038 



Triethylamine ........... . 4900 . 036 



Ammonia ............... 0.5500 0.038 



Tetraethylammonium / . 0760 . 037 



hydroxide ........... 1 . 0076 . 037 



Sodium hydroxide ...... ^ g.086 



By this a rule which VAN'T HOFF and OSTWALD 2 proved by thermo- 

 dynamic means, is substantiated, namely, that the equilibrium at constant 

 temperature does not change with the quantity and kind of catalyst 

 when the catalyst is not changed by the reaction. 



Among other kinds of ions which act as catalysts we must mention (1) iodine 

 ions, which decompose H 2 O 2 in proportion to their concentration, 3 and (2) cyan- 

 ions, which transform benzaldehyde into benzoin according to the following equa- 

 tion : 



2C 6 H 5 .COH = C e H 5 .CO.CH (OH) .C 6 H 6 .< 



The above-treated catalytic processes all occur in homogeneous sys- 

 tems, i.e., the systems which by mechanical means cannot be separated 

 into different constituents. In heterogeneous systems with phases which 

 can be separated from each other by mechanical means, catalytic reac- 

 tions can also occur, and indeed, in such cases the substances taking 

 part in the reaction and the catalyst occur in different phases. Such a- 

 reaction is the union of detonating gas, the synthesis of SO 3 (from 

 SO 2 and O), and the decomposition of H 2 O 2 by platinum. In case the 

 system is two-phased, and the reaction takes place only at the boundary 

 between both phases, or in the one we can differentiate two simple 

 limits : 



1. The accumulation of the bodies which are necessary for the 

 reaction at the proper place takes such a short time that in comparison 

 with the real chemical reaction it can be neglected. In these cases 

 the reaction velocity behaves similarly to a homogeneous system. 5 



1 Zeitschr. f. physik. Chem., 33, 129, 1900. 



2 Van't Hoff, Vorlesungen, 1, 211. 



3 Walton, Zeitschr. f. physik. Chem., 47, 185, 1904. 



4 Stern, ibid., 50, 513, 1905. 



5 Goldschmidt, Zeitschr. f. physik. Cbem., 31, 235, 1899. 



