But 1 c.c. acid = 0·5 c.c. CO₂ at 0° C. and 760 mm. of mercury.
Therefore CO₂ taken up by 25 c.c. of baryta = 3·15∕ 2 = 1·575 c.c.
As 50 c.c. were used the CO₂ absorbed by the baryta = 3·15 c.c. This was present in 3,900 c.c. of air. Therefore the CO₂ = 0·80 per cent.
Correction may be required for variations from the normal pressure of 760 mm. and normal temperature of 0° C., in accordance with ordinary rules.
In Lunge and Zeckendorf’s Method, the air to be examined is pumped through a glass bottle in which is 10 c.c. of a N ∕ 500 solution of Na₂CO₃ containing phenolphthalein as an indicator. The air is pumped by a hand pump through this solution until the phenolphthalein is decolourized. The number of times the ball of the pump has been squeezed indicates the amount of CO₂ present in accordance with a table prepared from separate experiments by Pettenkofer’s method.
Dr. Angus Smith’s plan for the estimation of carbonic acid in air is similar in principle to the last calculations. It is based on the fact that the amount of carbonic acid in a given volume of air will not render turbid a given amount of lime water, unless the carbonic acid is in excess.
Table.—To be used when the point of observation is “No precipitate.” Half an
ounce of lime water containing ·0195 gramme lime.
Air at 0° C. and 760 M. M. Barometric pressure.
| CARBONIC ACID IN THE AIR PER CENT. | VOLUME OF AIR IN CUBIC CENTIMETRES | SIZE OF BOTTLE IN CUBIC CENTIMETRES | SIZE OF BOTTLE IN OUNCES AVOIRDUPOIS. |
|---|---|---|---|
| ·03 | 571 | 584 | 20·63 |
| ·04 | 428 | 443 | 15·60 |
| ·05 | 342 | 356 | 12·58 |
| ·06 | 285 | 299 | 10·57 |
| ·07 | 245 | 259 | 9·13 |
| ·08 | 214 | 228 | 8·05 |
| ·09 | 190 | 204 | 7·21 |
| ·10 | 171 | 185 | 6·54 |
| ·11 | 156 | 170 | 6·00 |
| ·12 | 153 | 157 | 5·53 |
| ·13 | 132 | 146 | 5·15 |
| ·14 | 123 | 137 | 4·82 |
| ·15 | 114 | 128 | 4·53 |
| ·20 | 86 | 100 | 3·52 |
| ·25 | 69 | 83 | 2·92 |
| ·30 | 57 | 71 | 2·51 |
The foregoing table shows how to apply this method. The first and second columns state the ratio of carbonic acid in a quantity of air which will give no turbidity or precipitate in half an ounce of lime water; the third column gives the corresponding size of the bottle in cubic centimetres; and the fourth column gives the same in ounces. Thus different sized bottles, each containing half an ounce of lime water, will indicate with a fair degree of accuracy the ratio of carbonic acid in the air containing them, by giving no precipitate when the bottle is well shaken. For instance, if a pint bottle is used and there is no precipitate with half an ounce of lime water, it indicates that the ratio of carbonic acid does not amount to ·03 per cent.; if an eight-ounce bottle be used, and there is no precipitate, it indicates that the ratio does not amount to ·08 per cent., and so on. The air of a room ought never to contain more than six parts of carbonic acid in 10,000 of air, or ·06 per cent., i.e. a 10½ ounce bottle full of the air shaken up with half an ounce of clear lime water ought to give no precipitate.