QUANTITATIVE METHODS OF ANALYSIS 151 



8. Now add about 6 drops of phenolphthalein to the 

 solution and titrate with N/2 sulphuric acid until the 

 pink color begins to change. Care should be taken not to 

 overtitrate. In order to secure the best results titrate with 

 the tip of the buret in the solution. 



9. At this point, when the carbon exists as acid carbonate, 

 add 5 drops of methyl-orange and titrate with N/io sul- 

 phuric acid until the color changes. 



10. Prepare color standards for each indicator by adding 

 to carbon-dioxid-free water the same amount of the in- 

 dicator as used in titrating. 



11. The number of cubic centimeters of N/io sulphuric 

 acid used during the titration with methyl-orange as an 

 indicator, minus the blank, multiplied by 1.2, represents 

 the weight in milligrams of carbon produced as carbon 

 dioxid from i kilogram of soil. 



Equation for the double titration of carbon dioxid: 



2KOH + CO 2 = K 2 CO 3 + H 2 O 



K 2 CO 3 + H 2 SO 4 = KHSO 4 + KHCO 3 



KHCO 3 + H 2 SO 4 = KHSO 4 + H 2 CO 3 = H 2 O + C0 2 



(10) Soil Acidity: 



Lead acetate paper. 



Calcium chlorid solution plus zinc sulphid solution (2 per cent, of 

 ZnS in 20 per cent. CaCl 2 + 2H 2 O). 



1. Place 10 grams of the soil to be analyzed in a 3OO-C.C. 

 Erlenmeyer flask. 



2. Add 50 c.c. of a mixture of 45 c.c. of water and 5 c.c. 

 of a well-shaken suspension of zinc sulphid in calcium 

 chlorid solution. 



3. In order to remove the zinc sulphid adhering to the 

 vessel, refill with 50 c.c. of water and add to the flask. 



