WATER ANALYSIS 31 



100,000, probably derived irom ground air, increasing with 

 the depth and decreasing with the porosity of the soil. 



2. Carbonate and Bicarbonate. The carbonate is first 

 determined by titration oi the sample with the standard 

 solution of oxalic acid, in presence of phenolphthalein, 

 which gives a pink colour with carbonates and alkalies, and 

 is colourless with bicarbonates and acids. 

 H 2 C 2 4 2,H 2 0+2Na 2 C0 3 =*= 2NaHC0 8 +Na 2 C 2 4 +2H 2 



90 + 36 2 x 106 



From the equation we see that 126 grim of crystallized 

 oxalic acid react with 212 grm. of sodium carbonate, 

 containing 88 grm. of C0 2 , and change it to sodium 

 bicarbonate. Thus 126 grm. of oxalic acid crystals 

 measure the change of 88 grm. of G0 2 from the state of 

 carbonate to that of bicarbonate, or 126/88 = 1-43 grm. 

 measure the change in 1 grm. of C0 2 . Hence a standard 

 solution of oxalic acid crystals, 1-43 grm. per litre, is of 

 such a strength that 1 litre == 1 grm. CO 2 , or 1 c.c. = imgr. 

 C0 2 . When all the carbonate is changed to bicarbonate, the 

 solution becomes colourless, and the number of c.c. used 

 measures the carbonate present. Boil the fluid in the flask 

 briskly for ten minutes, when all the bicarbonate, whether 

 originally present or derived from carbonate, is decom- 

 posed with formation of carbonate (signalized by the return 

 of the pink colour) and evolution of C0 2 , as shown thus : 



2NaHC0 3 - Na 2 C0 3 + H 2 + CO 2 

 Cool and repeat the titration with the standard oxalic 

 solution ; the number of c.c. required measures the 

 amount of carbonate now in the sample. But this figure 

 represents only half the C0 2 present before boiling, halt 

 of the CO 2 having gone into the atmosphere. Hence the 

 figure has to be doubled, and then measures the total 

 amount of C0 2 present originally in the sample. The 

 amount present as carbonate is known from the first 

 titration, and so the amount present as bicarbonate is 

 measured by the difference between the number of c.c. 

 used in the first titration, and double the number used in 

 the second titration. The rationale may be shown thus : 



