Indirect Gravimetric, or Determination by Loss.—Take a Geissler's carbonic-acid apparatus (fig. 81) and place in the double bulb some strong sulphuric acid. Put into the other bulb, the stopcock being closed, 3 or 4 c.c. of nitric acid diluted with water. Leave the apparatus in the balance-box for a few minutes and weigh. Introduce into the flask (through A) about 1 gram of the powdered substance and again weigh to find the exact amount added. Allow the acid to run gradually on to the carbonate, and when solution is complete, heat and aspirate. Cool and again weigh; the loss in weight is the carbonic acid.
For Example:—
| Weight | of apparatus | and acids | 85.494 | grams |
| " | " | marble | 86.879 | " |
| ——— | ||||
| Equal | to marble taken | 1.385 | " | |
| Weight | of apparatus | and marble | 86.879 | grams |
| " | " | minus carbonic acid | 86.2692 | " |
| ——— | ||||
| Equal | to carbonic acid | 0.6098 | " |
1.385 : 100 :: 0.6098 : x
x = 44.03 per cent.
The substance contains 44.03 per cent. of carbonic acid; a duplicate experiment gave 43.73 per cent.
This method is quicker, but less exact, than the direct gravimetric determination.
VOLUMETRIC METHOD.
This, which is of somewhat limited application, is based upon the determination of the quantity of acid required to decompose the carbonate. It consists in adding to a weighed quantity of the mineral a known amount of standard solution of acid which is in excess of that required to effect the decomposition. The quantity of residual acid is then determined by titrating with standard solution of alkali. This method has been described under Lime.