260 
Proceedings of Eoycil Society of Edinburgh. [sess. 
was phenol-plitlialein, which gives a sharp end-point when exactly 
half the sodium in the solution has gone to form sodium chloride 
with the hydrochloric acid. The only precaution to he observed is 
to perform the titration rapidly, for the neutral solution obtained 
becomes alkaline again on standing. 50 cc. of a solution of sodium 
sulphide (corresponding to 48 ‘5 cc. of decinormal hydrochloric acid, 
with phenol-phthalein as indicator) were mixed in a thermostat at 
25° with 5 cc. of a solution containing 1 cc. of ethyl acetate and 
15 cc. of water. 
Time from beginning 
in minutes. 
0 
1 
3 
5 
8 
13 
46 
Titre. 
[4-37] 
3-08 
2-41 
2*10 
1-86 
1-66 
1-65 
From these numbers it may he seen that the reaction was completed 
in 13 minutes : the first titre, given within brackets, was calculated 
from the composition of the mixed solution. A comparative experi- 
ment with an equimolecular solution of sodium hydroxide gave 
numbers closely corresponding to those in the preceding table. The 
electrical conductivity of sodium sulphide solutions was measured 
at 25° with the following results : — 
Dilution. 
16 
32 
64 
128 
256 
Molecular Conductivity. 
276 
287 
298 
303 
(298) 
At greater dilutions the molecular conductivity continued to 
decrease. This is exactly the behaviour of soda solutions, as 
observed by Kohlrausch, who found a maximum for the molecular 
conductivity at a dilution of about 166 1. If we assume that the 
degree of electrolytic dissociation of sodium hydroxide and sodium 
