This method is the quickest of all, and the least troublesome after the apparatus has been once prepared. It yields fairly accurate results when worked in the manner described below; but if greater precautions are taken the results are exact. It depends on the measurement of the volume of gas given off on treating the weighed sample with acid. The apparatus described, page 52, is used. Weigh out a portion of the mineral which shall contain not more than 0.15 gram of carbonic acid (or 0.4 gram of carbonate of lime) and put it in the bottle. Put in the inner tube 10 c.c. of dilute hydrochloric acid (1—1), cork tightly, and read off the level of the liquid in the burette after adjusting the pressure. Turn the acid over on to the mineral. Run out the water so as to keep the level in the two burettes the same. When effervescence has ceased, rotate the contents of the bottle; finally, adjust the level in the burettes and read off the volume. The increase in volume is due to the evolved carbon dioxide. At the same time read off the "volume corrector."
Some of the carbon dioxide remains dissolved in the acid in the generating bottle, and the quantity thus dissolved will depend on the amount of carbonate as well as on the amount of acid present. Consequently, a measured quantity of acid should be used in each assay and a comparative experiment made with a known weight of pure carbonate of lime which will yield about the same volume of gas. The number of c.c. of gas got in the assay multiplied by 4.7 will give the number of milligrams of pure carbonate of lime that must be taken for the standard. With ordinary work the error rarely exceeds half a c.c.
The following example will illustrate the calculations:—
One gram of a mineral was taken, and yielded 49.0 c.c. of gas. The "volume corrector" reading was 100.4 c.c.
0.2405 gram of pure carbonate of lime was then taken, and treated in the same way; 50.5 c.c. of gas were got. The volume corrector still read 100.4 c.c.
0.2405 gram of carbonate of lime is equivalent to 0.1058 gram of carbon dioxide; then,
| 50.5 : 49.0 :: 0.1058 : x |
| x = 10.26 per cent. |
Estimation of Carbonic Acid in the Air of Mines.—According to a series of analyses by Angus Smith, the proportion of carbonic acid in the air of underground workings varied from 0.04 to 2.7 per cent. by volume. In places where men are working the proportion ought not to reach 0.25 per cent.
A simple method of determining whether a sample of air reaches this limit (0.25 per cent.) is described by Dr. C. Le Neve Foster in the "Proceedings of the Mining Association and Institute of Cornwall" for 1888. The apparatus used is an ordinary corked 8-ounce medicine bottle. This is filled with the air to be examined by sucking out its contents with a piece of rubber-tube. Half-an-ounce of dilute lime-water[121] (tinted with phenolphthalein) is poured in. If, on corking the bottle and shaking, the colour is not discharged, the air contains less than 0.25 per cent. of carbon dioxide. "If the colour fades slowly, and does not finally vanish till after a great deal of shaking, it may be assumed that the percentage of carbon dioxide does not greatly exceed one quarter; whereas, if the disappearance is rapid after a very few shakes, the contrary, of course, is the case." The dilute lime-water is measured out and carried in ordinary half-ounce phials. This method does not pretend to great accuracy, but as a method of distinguishing between good and bad air it is very convenient, and will be found useful.
For determining the actual proportion in the air the following plan is adopted:—Take a bottle which will hold about 50 ounces, and measure its capacity; fill the bottle with the air to be examined, pour in 100 c.c. of lime-water, and shake up for some time; add phenolphthalein, and titrate the remaining calcium hydrate with standard solution of oxalic acid.