38 PUBLIC HEALTH CHEMISTRY 



of mgr. of calcium carbonate in the amount of sample 

 taken, because i c.c. N/SO H 2 SO 4 = i c.c. N/5O CaCO 3 . 

 But the molecular weight of CaCO 3 is 100 and N/50 = 

 i grm. per litre, hence i c.c. = i mgr. CaCO 3 . 



Permanent. To a fresh lot of sample add sufficient 

 N 750 Na 2 CO 3 to precipitate as carbonate all the Ca and 

 Mg present, noting carefully amount used. 



Evaporate mixture to dryness on water-bath. 



Dissolve soluble part of residue in 10 to 20 c.c. of aq. 

 dest. and filter through small filter-paper. Wash out dish 

 and filter-paper with a little more distilled water to ensure 

 complete removal of all the Na 2 CO 3 . 



Titrate filtrate with N 750 H 2 SO 4 , using methyl-orange 

 as indicator, until a permanent pink colour is obtained. 



The difference between the number of c.c. of N/5O 

 Na 2 CO 3 used and the number of c.c. of N/5o H 2 SO 4 

 required to neutralize, is the number of c.c. of N/5O sodium 

 carbonate used up in precipitating the Ca and Mg. Every 

 c.c. of same equals i c.c. of N/5O CaCO 3 = i mgr. 

 CaCO 3 . Thus we arrive at the amount of permanent 

 hardness in the quantity of sample taken. 



Total. The sum of the temporary and permanent 

 hardnesses, as determined above, gives the total hardness. 



ORGANIC MATTER IN WATER. 



This is derived from vegetable and animal pollution, 

 and is estimated in a variety of ways. 



Frankland's Method. The water is evaporated to a 

 residue, which is ignited in a hard combustion tube with 

 cupric oxide ; the evolved gases are collected and measured, 

 and the amount of carbon and nitrogen found in these 

 returned as Organic C and Organic N. In a good water 

 suitable for domestic use, the Organic C should not exceed 

 0-2 part per 100,000, and the Organic N should not exceed 

 0*02 part per 100,000. The ratio of Organic C to 

 Organic N furnishes a valuable indication of the nature 

 of the organic matter present in unoxidized waters. Thus, 

 unoxidized peaty waters give a high ratio of from 8 to 12 

 to 20 or even more, the average being about 12, and such 

 a ratio is held to indicate organic matter of vegetable 

 rather than of animal origin. In unpolluted upland surface 



