44 SEWAGE AND ITS PURIFICATION 



can be done has enabled me to determine nitrates in situ, at 

 intervals of a few minutes, on river banks or in runnings from 

 filters, so as to ascertain the fluctuations. If samples were 

 simply collected, transferred to the laboratory, and determined 

 by a longer process, they would have undergone such changes 

 as to make the results erroneous. 



Nitrites in comparatively large quantities do not interfere 

 with this test ; 25 c.c. of a solution containing 5 parts per 

 100,000 of nitrous nitrogen, an amount not often met with, 

 require only o'2 c.c. of standard indigo solution to produce 

 a blue colour, whereas the same quantity of a liquid containing 

 I part per 100,000 of nitric nitrogen would require over 5 c.c. 

 R. Warington^ and others have stated that the indigo method 

 measured both nitric and nitrous acids, but conducted in the 

 way I adopt, it only measures the former. The mistake 

 has probably arisen from the sodium nitrite, reputed pure, of 

 commerce containing a considerable proportion of nitrate or 

 chloride ; when prepared fresh from pure silver nitrite by re- 

 action with pure sodium chloride I obtained the above negative 

 results with indigo. 



The amounts usually found to be suitable for a determination 

 are — For ordinary drinking-waters, 25 c.c. ; for highly nitrated 

 waters or effluents, 10 or 5 c.c. made up to 25 c.c. ; for sewages 

 or low nitrated effluents, 50 or even 100 c.c, with 100 or 

 200 c.c. of H2SO4, may be used. With practice the slight tint 

 of blue produced by o*i c.c. excess of indigo is distinctly 

 perceptible, even when the liquid acquires a brown tint after 

 the acid is added. Occasionally the flask cracks when the 

 mixture is made — as it should be — suddenly, so this contingency 

 must be provided for. The titration should be done as quickly 

 as possible, but a blue that is permanent for two minutes 

 remains as a rule for half an hour. 



Nitrites. — The solutions required are — 



1. A half per cent, solution of metaphenylene-diamine in 

 dilute sulphuric acid ; if much coloured it can be bleached by 

 filtration through purified animal charcoal. 



2. Diluted sulphuric acid — i part to 2 parts of water. 



3. Standard NaNO.2 solution. Fifty c.c. of the sample are 

 placed in a Nessler glass, i c.c. of the metaphenylene-diamine 

 and I c.c. of acid added ; on standing, a yellow brown colour 

 slowly develops even with traces of nitrite. The colour is 



^ " Detection of Nitrous and Nitric Acids," Chemical News, li., 39. 



