NITRATES AND NITRITES 225 



nitrates and nitrites estimated in the Lunge nitrometer 

 as nitric oxide (see p. 1 10). 



{b) Estimation by Reduction to Ammonia. — A few 

 pieces of zinc are immersed in a dilute solution of copper 

 sulphate until they are just covered with a firm deposit 

 of precipitated copper. The " copper-zinc couple " thus 

 obtained is washed with distilled water; it has the 

 power of reducing both nitrates and nitrites with the 

 formation of ammonia. 



About 5 grams of the freshly prepared copper-zinc 

 couple are placed in a stoppered bottle together with 

 100 c.c. of the water and about i gram of pure oxalic 

 acid to precipitate lime. After about twelve hours an 

 aliquot portion of the clear liquid, more or less according 

 to the concentration of the nitrates and nitrites in the 

 water, is transferred to a Nessler glass, diluted if 

 necessary to 50 c.c. and the contained ammonia 

 estimated by Nessler reagent in the ordinary way. If 

 any free ammonia has been found in the water this 

 must be deducted, previous to calculation of the nitrates 

 and nitrites. 



The amounts of nitrites and nitrates present in the 

 water are calculated as parts of nitrogen per 100,000 of 

 water. 



Nearly all waters contain nitrates, generally as the product of the 

 oxidation of nitrogenous organic matter. Nitrites are also 

 occasionally met with, either as an intermediate stage in the 

 oxidation, or by deoxidation of nitrates. The presence of nitrates 

 in itself does not affect the quality of a water, but it generally 

 points to past pollution if any quantity is present. In most 

 cases nitrites obviously imply more recent sewage contamination. 

 A general average content cannot very well be stated, since the 

 conditions vary considerably in different districts and for different 

 classes of water. In the London water supply the nitrogen as 

 nitrate and nitrite generally averages about 0-25 part. Anything 

 above 0-5 part is suspicious, although well-waters frequently give 

 much higher values. 



P 



