S. W. Johnson on estimation of Carbonic Acid. 111 
Art. XII.—Contributions from the Sheffield Laboratory of 
Yale College. No, XIX. 
1. On the estimation of Carbonic Acid ;* by 8. W. JouNson. 
In the processes usually employed, carbonic acid is deter- 
mined by the loss of weight of an apparatus which contains 
no catbonic acid gas at the beginning and which must be com- 
pletely emptied of this gas at the conclusion of the analysis. It is 
a matter of experience, however, that accurate results are not 
attainable with certainty, inthis way. Nothing short of actual 
boiling for some time will expel all carbonic acid gas from the 
dilute acid liquid. This cannot be done conveniently without 
oss of aqueous vapor. The fact that good results are often 
obtained is due to the compensation of opposite errors, as the 
analyst may convince himself by repeatedly heating and suck- 
ing through air, If the suction go on to just the right extent, 
the loss of the apparatus will exactly correspond to the car- 
bonic acid that was contained in the substance, but further ex- 
haustion of the air will diminish the weight of the apparatus, 
not by complete removal of the carbonic acid, but by loss of 
aqueous. vapor which easily escapes the desiccating material. 
By continued working gn a carbonate of known composition - 
one may soon learn how long to exhaust in order to bring out 
the proper loss, but where the analyst is out of practice, an 
error of 1 to 2 per cent is not unlikely to happen, and the pro- 
cess itself furnishes no means of judging when it will give a 
correct result. 
he writer employs a simple modification of this method 
_ which, under proper conditions, gives very accurate results and 
furnishes to a great extent its own control. The process is 
novel in this particular, viz: the charged apparatus is in the 
- first place filled with carbonic acid gas, the substance is then 
decomposed, and as soon as disengagement of gas ceases, the 
apparatus, still filled with carbonic acid gas, is weighed again. 
In this manner all aspiration is done away with, and the de- 
- siccating material has simply to dry as much gas as is yielded 
_ by the substance under analysis. 
It is, however, essential that the substance under examination 
_ dissolve freely and completely in cold acid ; it is likewise neces- 
a hat the analysis and weighings be conducted in an apart- 
ment not liable to change of temperature. 
The apparatus may consist of a light flask or bottle with 
_.. +#* The substance of this paper was verbally communicated to the National Acad- 
__emy of Sciences in August, 1868. 
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