[10] Prepared by gradually heating the pure crystallised carbonate to redness. From being uniform in composition, and easily procured or prepared, they are much employed; preference being usually given to the soda-salt.
[11] The crystals must be free from attached water, but not the least effloresced.
Oper. A determined amount of the acid under examination is accurately weighed into the flask A (see engr.); and if it be a concentrated acid, or a solid, it is mixed with or dissolved in 6 or 8 times its weight of water. The little glass tube (e) is then nearly filled to the brim with pure bicarbonate of soda, in powder, and a fine silken thread is tied round the neck of the tube, by means of which it can be lowered down into the flask (A), so as to remain perpendicularly suspended when the cork is placed in the latter; the cord being held between the cork and the mouth of the flask. The flask (B) is next about half filled with oil of vitriol, and the tubes being arranged in their places, as represented in the engr.; and time having been allowed for the mixture of acid and water to cool completely, after the increase of heat caused by mixing, the whole apparatus is very accurately weighed. The cork in the flask (A) is then slightly loosened, so as to allow the little tube containing the bicarbonate of soda to fall into the acid, and is again instantly fixed AIR-TIGHT in its place. The evolution of carbonic acid now commences, and continues until the acid in the flask (A) is neutralised. When this takes place, which is easily seen by no bubbles being emitted on shaking the apparatus, the flask (A) is put into hot water (120° to 130°
Fahr.), and kept there, with occasional agitation, until the renewed evolution of gas has completely ceased. The little wax stopper is then taken off the tube (a), the apparatus taken out of the hot water, wiped dry, and suction applied, by means of a perforated cork, or a small india-rubber tube, and the mouth, to the end of the tube (d), until the sucked air no longer tastes of carbonic acid. The whole is then allowed to become quite cold, when it is replaced in the balance (the other scale still containing the original weights), and weights added to restore the equilibrium.
(A) A wide-mouthed flask, capable of holding 21⁄2 to 8 oz., containing sample for trial (f). (B) Ditto, capable of holding 11⁄2 to 2 oz., partly filled with oil of vitriol (g). (a, c, d) Tubes fitting air-tight in the flasks by means of the corks (i) and (j). (b) Piece of wax fitting air-tight on the end of a. (e) Small tube capable of holding about 1 drachm of powdered bicarbonate of soda or potash. (h) Open end of the tube (d). (k) Silk cord fastened to the tube (e).
The loss of weight represents the exact quantity of dry carbonic anhydride, or anhydrous carbonic acid gas, that has been expelled from the bicarbonate of soda, by the action of the acid in the sample examined.
The quantity of real acid it contained is then deduced by the following calculation:—One equivalent of gaseous carbonic anhydride, or anhydrous carbonic acid (= 44) bears the same proportion to one equivalent of the acid in question, as the amount of the carbonic anhydride expelled does to the amount of the acid sought. Thus, suppose a dilute sulphuric acid expels 3 gr. of carbonic anhydride, the arrangement is—
44 : 49 :: 3 : 3·349
Consequently the sample operated on contained 3·5 (nearly) grains of true sulphuric acid.