96 CHEMICAL ANAL YSIS OF SOIL 



with a little water, the apparatus closed and again 

 tested as to tightness. The acid is now gradually 

 run in, the evolution of the carbon dioxide being 

 maintained at such a rate that about two bubbles per 

 second pass into the potash bulbs. When evolution of 

 the gas has ceased, the remainder of the acid is run 

 in and the flask gently heated. An aspirator is attached 

 at hy the tap of the dropping funnel opened, and a gentle 

 stream of air is drawn through the apparatus in order 

 to displace any COg which remains. Carbonic acid 

 from the atmosphere is excluded by attaching a small 

 soda-lime tube to the dropping funnel. 



When all the carbonic acid has been absorbed the 

 potash bulbs and U-tube are disconnected and weighed. 

 The increase in weight is the amount of COg present 

 in the carbonate. 



76. Amos' Method. 



This method, which is suitable for the estimation of 

 smallquantitiesof carbonate (under i per cent), depends 

 on the well-known double titration principle. The 

 carbon dioxide is liberated from the soil with acid 

 and absorbed by caustic-soda solution. The alkali 

 is then titrated, using successively phenolphthalein and 

 methyl orange as indicators. 



Apparatus. — The construction of the apparatus may 

 be gathered from Fig. 17. It consists of: — 



A. A Reiset absorption apparatus containing 100 c.c 



of a 4 per cent, sodium hydroxide solution, for 

 washing the air free from carbon dioxide. 



B. A Jena flask, in which is placed the soil to be 



treated. The flask is provided with a side-tube. 



C. A second Reiset apparatus containing 100 c.c. of 



4 per cent, sodium hydroxide solution for 

 absorbing the COg liberated in B. 



D. A small reflux condenser. 



