CARBONATES BY AMOS' METHOD 97 



Procedure. — A quantity of the soil (not exceeding 

 50 grams) is weighed out, in which the amount of lime- 

 stone present is not more than 0-5 gram. The material 

 is introduced into the flask B, shaken up with 75 c.c. of 

 carbon-dioxide-free water, and the end Y of the rubber 

 tube connected directly with the pump, so that a stream 

 of air is drawn through the apparatus and all the 

 atmospheric COg swept out. The apparatus is now 

 connected up as in the diagram, and a steady stream of 

 air drawn through. Twenty c.c. of strong hydrochloric 

 acid are run into B by means of the dropping funnel, 

 and the contents of the flask gradually brought to the 

 boiling point; the boiling is continued for twenty 

 minutes, to ensure all the carbon dioxide being swept 

 into the Reiset C. 



The titration is carried out in the lower part of the 

 Reiset apparatus, into which the contents of the absorp- 

 tion tube are washed, phenolphthalein is added, and 

 normal hydrochloric acid run in until the pink colour 

 begins to fade, then decinormal hydrochloric acid until 

 the colour is completely discharged. A reading of the 

 acid is now taken, methyl orange is added, and the 

 titration continued until the liquid shows an acid 

 reaction. The amount of acid used up during the 

 methyl - orange titration is that required for the 

 equation : 



NaHC03+HCl = NaCl+H20 + C02. 



From this is calculated the weight of calcium carbonate 

 originally present in the soil. 



The object of employing normal HCl in the first 

 part of the titration is to prevent unnecessary dilution, 

 but the liquid must be kept in motion so that the acid 

 is never in excess at any point, with consequent evolution 

 of carbon dioxide. 



G 



