Determination of Ca/rbon Dioxide. 73 



deficient in carbon dioxide. The observed correction of 

 •0014 grm. for each gram of the carbonate is applied in the last 

 column of the table. 



The low results of the larger amounts of carbon dioxide, 

 in contrast to the higher results of the smaller amounts, point 

 to some action of the iodine upon the precipitated carbonate. 

 It would be natural to suppose that such action would be greater 

 upon the carbonate precipitated from a cold solution, and this 

 proved to be the case. For when barium carbonate, precipi- 

 tated under the conditions of the analysis described, was treated 

 with 10 cm.^ of iodine and boiled again, a loss of 'OOM grm. of 

 iodine (corresponding to '0008 grm. of dioxide) was found ; but 

 when barium carbonate was prepared by passing carbon dioxide 

 through a cold solution of barium hydroxide until the presence 

 of the acid carbonate was proved in solution by filtering a por- 

 tion and boiling — thus insuring the complete destruction of 

 the hydroxide — the precipitated carbonate, after filtering and 

 washing, was acted upon by 10 cm.^ of iodine solution to such 

 an extent that '0253 grm. of iodine (corresponding to '0044: 

 grm. of carbon dioxide) disappeared. The obvious inference 

 is, therefore, that the carbonate should be boiled before the 

 addition of iodine in the process. 



The experiments of Table II were made like those of Table 

 I, excepting in the following points : first, an ordinary flask 

 of about 300 cm.^ capacity, which fitted with a rubber stop- 

 per, was substituted for the more expensive ground stop- 

 pered absorption flask ; secondly, the precipitated carbonate 

 was boiled before adding any iodine ; and thirdly, after iodine 

 was added to a yellow color and boiled, a second amount of 

 iodine was run in to a red color, but the mixture was not boiled 

 again. By this treatment the iodine is kept from acting on 

 the precipitated carbonate, at least to such an extent that the 

 action is not appreciable, and from attacking the rubber stop- 

 per used ; incidentally, it is kept from entering the trap, but 

 one is used, nevertheless, to prevent contamination from the 

 outside air. A separating funnel, reaching below the surface 

 of the liquid, was found a convenient means of introducing the 

 iodine without loss into the hot solution. A higher vacuum 

 is required when the smaller flask is used, especially when large 

 amounts of carbon dioxide are to be determined. A pressure 

 of 200 mm. of mercury was found to be sufficiently low, and 

 in no case did a flask of ordinary thickness and of 300 cm.^ capac- 

 ity collapse at that degree of exhaustion. 



