752 DR CHARLES HUNTER STEWART ON THE ESTIMATION OF 



The Estimation of Carbon and Nitrogen in Potable Water. 



(The Authors Method.) 



500 c.c. of the water are evaporated down over a flame in a bohemian glass flask 

 (similar to that used in the determination of carbon and nitrogen in organic substances) 

 to about 100 c.c. This gets rid of the "Free Ammonia." Should the water be very- 

 impure a few drops of caustic alkali must be added previous to evaporation. Meanwhile, 

 the presence of nitrates has been tested for in another sample of the water, and, if present, 

 their amount quantitatively determined. If no nitrates be present, or their amount less 

 than 1 grain nitrate of potassium per gallon of the water, then 3 c.c. of diluted sulphuric 

 acid (or enough to make the water distinctly acid) is added to the residue in the flask 

 and the evaporation continued to about 15 c.c. This can be done quite easily without 

 any risk of decomposing the organic matter. This treatment frees the water residue 

 from carbonic acid derived from carbonates. The flask is now connected up with the 

 apparatus which has already been described, and the process conducted exactly in the 

 same way as for the determination of carbon and nitrogen already described, except that 

 no permanganate of potassium requires to be added. The author found, from numerous 

 experiments on the same water, with and without addition of permanganate, that the 

 same results were got. In the examination of sewage the addition of permanganate 

 is advisable. Though the amount of carbonic acid evolved in water analysis is small, 

 two carbonic acid absorption tubes are necessary. In all the experiments some carbonic 

 acid has been found in the second tube. The rate at which the air is aspirated through 

 the apparatus is about 5 litres per hour. 



Estimation of the Nitrogen. 



Into a distillation flask an amount of 10 per cent, caustic soda solution — rather more 

 than is necessary to completely neutralise the contents of the combustion flask — is put, 

 and rendered ammonia free by distillation and allowed to cool. To this is added the 

 diluted and cooled contents of the combustion flask and the whole distilled. 200 c.c. 

 collected in a marked flask is taken over and the ammonia estimated by Nessler's 

 reagent, the ordinary standard solution of chloride of ammonium, 1 c.c. contains 0*01 

 milligram of ammonia, being used. In ordinary cases, 50 c.c. of the distillate is taken 

 for this purpose, but if the water be very impure, 25 c.c. may have to be taken, and 

 in extreme cases even less. In these latter cases it is better to operate on 250 c.c. of 

 the water rather than 500 c.c. The titration for carbonic acid has been already described. 

 There is always a trace of ammonia present in pure sulphuric acid, and this has to be 

 deducted from the total ammonia. As a rule, the pure sulphuric acid obtained in 

 Edinburgh contains in 10 c.c. about 0'03 milligram of ammonia. The exact amount 

 must of course be determined in each sample of the acid, and the bottle containing 



