553 
and passes into the aquo salt verj slowlj even at tlie oidinaiy 
teinperature. This hydrolysis is raarkedly accelerated by thepresence 
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 seA'e as a means of determining the concen- 
tration of hydroxide ions in a dilute solntion of a base. Thus if we 
make a solntion of the purpureo salt and add a few drops of a 
dilute solntion of potassium hydroxide, the purple colour changes 
and becomes rosé in a few ininntes ; but with a weak base like 
ammonium hydroxide the colour change takes a long time. This 
explanation may be true in the case of hydrolysis with the corre- 
sponding compounds of chrorniurn and platinum. There is evidence 
to show that in some other cases of hydrolysis by alkali, the action 
of the hj’droxide ions is catalytic. The decomposition of sodium 
chloracetate by alkali is a case in point (Senter, Trans Chern. Soc. 
1907, 91, 473). 
One can get the hydroxides of the cobaltammines in solntion b}' 
treating the corresponding halide with moist silveroxide : 
(NHJ 
L (NOJJ 
Cl -f AgOH = 
Co 
(NOJ, 
OH -f AgCl 
Co 
The solntion slowly decomposes even at the ordinary temperatures. 
The hydroxides of the other members of this, series can also be 
prepared by this doublé decomposition. These hydroxides turn phenol- 
phtalein pink and electric condnctivity measnrements show that they 
are strong bases of the type of sodium hydroxide. 
But one cannot prepare the hydroxide from purpureo cobalt chloride 
Cl, by doublé 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 doublé decom- 
position acts catalytically on the purpureo salt and actually one gets 
- 
in alkaline solntion, (compare ürbain et Sénéchal, Chimie des com- 
plexes, p. 280, “Les seis purpureo ne donnent pas une reaction de 
ce genre”). 
the aquopentammine hydroxide 
(OH),, which is stable 
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 
