52-54] Estimation of Carbon Dioxide 33 



is first of all weighed, care being taken that no moisture 

 adheres to its exterior. About 1 gram of finely ground Iceland 

 spar is then introduced into the apparatus, and it is weighed 

 again, the difference giving the weight of spar added. 



If the first apparatus be used, care must be taken that none 

 of the added substance fall into the test tube of acid. To 

 avoid this it is desirable to remove the tube whilst pouring the 

 substance into the flask. If Schroeder's apparatus be used, the 

 carbonate may be poured through the opening d. 



The next operation is to drive off the carbon dioxide. 



52. If the apparatus in fig. 20 be used, f is unstoppered, 

 and the flask is tilted on one side so as to allow the acid in e 

 to run over on to the substance below. As soon as effervescence 

 ceases a little more acid is run over. This operation is con- 

 tinued until the whole of the substance is dissolved. The 

 flask is then placed on top of the water oven for about ten 

 minutes to expel any C0 2 which may be dissolved in the 

 liquid. Finally, the stopper at a is removed, and air slowly 

 aspirated through the apparatus (by attaching f to an air-pump 

 or by sucking a tube attached to f) until all C0 2 is expelled 

 from the flask. The stoppers at a and f are then replaced, 

 and the apparatus is weighed. 



53. Calculation- 

 Weight I. = Apparatus. 



Weight II. = Apparatus + Substance. 

 Weight III. = Apparatus + Substance — • C0 2 . 

 .*. Weight II. — Weight I. as Iceland spar, 

 and Weight II. - Weight III. = C0 2 . 



From this the percentage of C0 2 in the Iceland spar may 

 be readily calculated. 



54. If Schroeder's apparatus be used, the acid is let in 

 gradually as before by the tap at the bottom of the cistern b, 



D 



