32 PUBLIC HEALTH CHEMISTRY 



boiled=Na 2 C0 3 -}-H 2 0+CO, 



fintoair) 



(3). 2Na 2 CO 3 +H 2 C ,,O 4 +Phth=2NaHCO 3 +Na 2 C 2 O 4 



EXAMPLE : 100 c.c. of sample, plus phth., required 8 c.c. 

 of std. oxalic sol. to decolorize. Boiled ; cooled ; and 

 titrated again, when 1 1 c.c. were required. 



CO 2 as Carbonate, 8 parts per 100,000. 



CO., as Bicarbonate, 22 8 = 14 ,, 



3. Free Carbon Dioxide and Bicarbonate. These are 

 estimated by the addition of excess of alkali in the shape 

 of a known quantity of baryta solution. The baryta 

 fixes the free carbon dioxide and that halt bound in the 

 bicarbonates, precipitating both as barium carbonate. 

 The excess of alkali which remains unused is measured by 

 titration with standard oxalic acid solution. 



CO 2 + BaH 2 O 2 = BaCO 3 + H 2 O 

 CaCO 3 -CO 2 + BaH 2 O 2 = BaCO 3 + CaCO s + 2H 2 O 

 HCO + BaHO = 



224 22 



From these equations we see that one molecule of oxalic 

 acid and one molecule of CO 2 are each able to neutralize 

 one molecule of baryta in solution. Therefore, 126 grm. 

 of crystallized oxalic acid neutralizes 171 grm. of baryta, 

 which fixes 44 grm. of CO 2 ; or 126/44 = 2 '86 grm. of 

 oxalic neutralizes 171/44 grm. of baryta, which fixes 

 44/44 = i grm. CO 2 . Hence a standard solution of oxalic 

 acid 2*86 grm. per litre is such that i c.c. measures the 

 quantity of baryta which fixes i mgr. of CO 2 . 



PROCESS. To 100 c.c. of sample in a flask or bottle, 

 add a drop of phenolphthalein, and then 5 c.c. of BaCl 2 

 solution, 5 c.c. of AmCl solution, and 40 c.c. of baryta 

 solution (o - 5 per cent). If excess of baryta is present, 

 the liquid should become red, and remain so. Stopper 

 the vessel and set aside for twelve hours. Thereafter 

 titrate the whole sample (or an aliquot part) with 

 standard oxalic acid solution. Titrate 40 c.c. of fresh 

 baryta solution. The difference between the number 

 of c.c. required for the fresh baryta solution and that 

 required for the baryta mixe'd with sample, measured 

 in c.c. of standard oxalic solution the amount -of baryta 

 used up in fixing free CO 2 and changing bicarbonate 



