578 
and passes into the aquo salt very slowly even at the ordinary 
temperature. This hydrolysis is markedly accelerated by the presence 
of hydroxyl (OH’) ions. The greater the concentration of the hydroxide 
ions, the greater is the acceleration. The study of the reaction velocity 
of this hydrolysis may serve as a means of determining the concen- 
tration of hydroxide ions in a dilute solution of a base. Thus if we 
make a solution of the purpureo salt and add a few drops of a 
dilute solution of potassium hydroxide, the purple colour changes 
and becomes rose in a few minutes; but with a weak base like 
ammonium hydroxide the colour change takes a long time. This 
explanation may be true in the case of bydrolysis with the corre- 
sponding compounds of chromium and platinum. There is evidence 
to show that in some other cases of hydrolysis by alkali, the action 
of the hydroxide ions is catalytic. The decomposition of sodium 
chloracetate by alkali is a case in point (Senter, Trans Chem. Soc. 
1907, 91, 473). 
One can get the hydroxides of the cobaltammines in solution by 
treating the corresponding halide with moist silveroxide : 
(NH), re. OE, 
| co an Cl + AgOH = | co Arn OH + AgCl 
The solution slowly decomposes even at the ordinary temperatures. 
The hydroxides of the other members of this series can also be 
prepared by this double decomposition. These hydroxides turn phenol- 
phtalein pink and electric conductivity measurements show that they 
are strong bases of the type of sodium hydroxide. 
But one cannot prepare the hydroxide from purpureo cobalt chloride 
| co i Cl, by double decomposition with silver oxide. The 
explanation becomes simple on the light of the catalytic effect of 
hydroxide ions on the hydrolysis of purpureo salts into the aquo 
compounds. The hydroxide ions set free by the double decom- 
position act catalytically on the purpureo salt and actually one gets 
nto (OH),, which is stable 
in alkaline solution, (compare URBAIN et SÉNÉCHAL, Chimie des com- 
plexes, p. 280, “Les sels purpureo ne donnent pas une réaction de 
ce genre’). 
the aquopentammine hydroxide | co 
Summary and Conclusion. 
1. The principle of the preparation of the cobaltammines is guided 
by the law of mass action and thus depends on the concentration 
