Letts & Buaxe—The Carbonic Anhydride of the Atmosphere. 125 
extreme of accuracy was aimed at they were diluted to one quarter 
of the strength.’ 
The starting-point for the preparation of both the alkaline 
and acid standard solutions was decinormal hydrochloric acid, 
standardized with great care. 
89:6 ¢.c. of this, when diluted with pure water to one litre, 
give a solution of which 1 ¢.c.=0'1 cc. CO, at N. T. P2 
To prepare the baryta solution, recrystallized barium hydrate 
was dissolved in distilled water and the solution filtered and 
diluted, or possibly strengthened finally, until a stock was obtained 
which was clear and exactly corresponded volume for volume with 
the acid. The preparation is easy to describe but is tedious to 
execute. 
Titrating vessels.—In order to guard against the error arising 
from access of air, and especially of air charged with carbonic 
anhydride from the breath, we performed all the titrations except 
in our earlier experiments in vacuous vessels. These have the 
1 Pettenkofer employed, in his earlier determinations (‘‘ Chem. Soc. Journ.,’’ 10 
[1858], p. 292), lime-water as absorbent, and titrated it with standard oxalic acid of 
the strength 1 c.c. = 1 mg. CaO., which is equivalent to about 0-4 c.c. CO2 at N.T.P. 
The lime-water did not exactly correspond with the oxalic acid solution—80 c.c. of the 
former being equivalent to from 34-88 c.c. of the latter. He tells us that the turmeric 
paper (which he always used as indicator) ‘‘is so delicate that the addition of 4-6 drops 
of lime-water to the neutralized liquid is sufficient to reproduce this alkaline reac- 
tion.”” This quantity is equivalent to from 0-2—-0°3 c.c., which would correspond with 
about 0°1 c.c. CO2. 
We have arrived at a similar result as regards the relative delicacy of phenol- 
phthalein and turmeric paper as indicators by independent experiments. The turmeric 
paper was prepared acvording to Pettenkofer’s directions, i.e. by first washing turmeric 
with successive quantities of cold water, then extracting with alcohol, soaking pure 
filter paper in the extract, and allowing it to dry in the dark. Such paper is much 
more sensitive than ordinary turmeric paper, faint alkalinity being indicated when a 
drop of fluid is allowed to spread on it, by a transient brown ring, which rapidly disap- 
pears. Its delicacy, as compared with phenol-phthalein, was tested as follows :— 
50 c.c. of distilled water, known to contain no free carbonic anhydride, and to be 
neutral, were placed in a flask, and the weak baryta (1 c.c. = 0.1 ¢.c. CO2) added 
from a narrow burette, until the mixture showed an alkaline reaction with the 
turmeric paper. The point was somewhat uncertain; but independent experiments 
made by each of us gave from 0°4—0°6 e.c. as the quantity of baryta necessary. 
To another 50 c.c. of the water in the same flask a droplet of phenol-phthalein 
was added, and the baryta solution run in until a distinct pink colour appeared. This 
required 0°05 c.c. 
Phenol-phthalein is therefore from eight to ten times as sensitive as turmeric. 
2 Ordinary distilled water is by no means pure as regards carbonic anhydride. This 
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