30 PUBLIC HEALTH CHEMISTRY 



QUANTITATIVE ESTIMATION OF LIME, MAGNESIA, 

 PHOSPHATES, AND SULPHATES. 



These are all estimated for gravimetrically by precipita- 

 ting as above, collecting the precipitate, which settles 

 after twelve hours, on a filter-paper of known ash ; drying, 

 and igniting ; and weighing ash. The ash of lime is weighed 

 as calcium carbonate, of magnesia as magnesium pyro- 

 phosphate (Mg 2 P 2 O 7 ), of sulphates as barium sulphate. 

 The phosphates are precipitated as triple phosphates of 

 magnesium and weighed similarly. In the case of magne- 

 sium, the lime salts must be removed before precipitating, 

 hence the filtrate from the lime estimation is suitable for 

 the purpose. 



GASES IN WATER. 



Carbon Dioxide in Water. Exists as free CO 2 ; bicar- 

 bonate and carbonate ; and free CO 2 and bicarbonate. 



i. Free CO 2 . Determined by titration with N/20 

 sodium carbonate solution, using phenolphthalein as indi- 

 cator. On adding the N/20 solution from a burette to 

 the sample, a red colour appears which gradually fades as 

 the carbonate absorbs the free carbon dioxide and changes 

 to bicarbonate, which is neutral to phenolphthalein. 



Na 2 CO 3 + CO 2 -f H 2 O = 2NaHC0 3 

 1 06 44 



N/i sod. carb. = 53 grm. per litre and absorbs 22 

 grm. carbon dioxide per litre ; hence N/20 sod. carb. 

 = 2-65 grm. per litre and absorbs i-i grm. carbon 

 dioxide per litre ; hence I c.c. N/20 will absorb o-oon 

 grm. free CO 2 . 



PROCESS. Take 100 c.c. of sample in an Erlenmeyer 

 flask on a white slab, and add one drop of i per cent 

 alcoholic solution of phenolphthalein. 



Fill burette with N /20 solution of sodium carbonate and 

 add to sample, c.c. by c.c., until a permanent red tint 

 remains after waiting a minute or two. 



Calculate out result. 



Free carbon dioxide is almost constantly present in 

 ground waters, and in inverse ratio to the amount of 

 dissolved oxygen. It may be as high as 13 parts per 



