ACIDIMETEY 



23 



of water has been previously introduced, and likewise a small test-tube, B con- 

 taining the exactly weighed quantity of acid to be examined. All the joints 'being 

 perfectly air-tight, if the Florence flask bo now carefully tilted on one side, a portion 

 of the acid in the test-tube will, of course, flow down upon the bicarbonate of soda and 

 a corresponding quantity of carbonic acid gas being evolved will depress the water in 

 the glass cylinder, causing an overflow from the tube B, which should bo held over a 

 basin, and progressively lowered so as to keep the discharging aperture on a level with 

 the descending water in the cylinder. The operation is terminated when, all the acid 

 in the test-tube having been completely upset and all effervescence being entirely at 

 an end, the level of the water in the cylinder A remains stationary; the number of 

 divisions of the scale corresponding to that level are then read off; they indicate the 

 per-centage strength of the sample. 



The bicarbonate of soda of commerce frequently contains some neutral carbonate of 

 soda, which should be removed before using it for that and for the following process 

 this is easily done by washing it with a moderate quantity of cold water, which 

 dissolves the neutral carbonate, but leaves the greater portion of the bicarbonate in an 

 undissolved state ; it should then be dried spontaneously by spreading it in the air 

 and then kept in stoppered bottles ; for though bicarbonate of soda does not undergo 

 decomposition by exposure to dry air, a moist atmosphere converts a portion of it into 

 a neutral carbonate, with 5 equivalents of water (NaO, C0 2 .5HO). 



10 



Acidimetrical operations may likewise be performed by determining the weight 

 instead of the volumes of the carbonic acid expelled from bicarbonate of potash, or of 

 soda, by a given quantity of acid. For this purpose either of the apparatus contrived 

 by Dr. Ure, and represented above, may be usecL The details of their construction 

 are given in ALKALIMETRY, to which the reader is referred. 



Since 1 equivalent of any acid will disengage 2 equivalents (=44) of carbonic acid 

 from 1 equivalent ( = 75) of bicarbonate of soda, it is evident that by determining what 

 quantity of any pure acid is capable of disengaging or expelling 10 grains of carbonic 

 acid gas, then taking that quantity of the acid to bo examined, and causing it to react 

 upon a mass of bicarbonate of soda more than sufficient to saturate or neutralise it (in 

 order to make sure that the acid has produced all its effect), the loss sustained after 

 the operation from the carbonic gas expelled, multiplied by ten, will at once indicate 

 the exact per-centage of real acid contained in the sample examined. Of course the 

 weight of acid capable of disengaging exactly 10 grains of carbonic acid gas varies 

 with each kind of acid ; and that weight is found by dividing 10 times tile atomic 

 weight of the acid, whatever it may be, by 44 ; that is to say, by the atomic weight 

 of the two equivalents of carbonic acid gas contained in the bicarbonate of soda. 



