CHAPTER III 



CHEMICAL ANALYSIS (continued) 



Determination of nitrates — Nitrites — Organic nitrogen — Dissolved 

 oxygen — Carbonic acid — Incubator tests — Weights discharged 

 per day — Proposed standards for effluents— Ratio of chlorine to 

 total nitrogen, and of oxidized to unoxidized nitrogen. 



Nitrates. — This determination is of very great importance. 

 Sewages, as a rule, unless very fresh, contain no nitrate, and 

 only traces of nitrite, while effluents may contain large amounts 

 of both. 



I have adopted a slightly modified indigo process, which 

 determines nitrates only, the w-phenylene-diamine method being 

 used for nitrites. Duplicates with the Crum method and with 

 the copper-zinc couple, giving the total oxidized nitrogen, have 

 agreed closely with the above. 



For the indigo titration, a standard indigo is made by dissolv- 

 ing o'5 gramme of crystallized indigotine in 20 c.c. Nordhausen 

 sulphuric acid, allowing to stand twenty-four hours, diluting 

 very cautiously, filtering if necessary, and making up to i litre. 

 This solution keeps well in the dark ; its strength is determined, 

 and controlled at intervals, by means of KNO3 solutions of 

 different strengths, and a curve is constructed giving directly 

 the relation between the number of the c.c.'s used and the 

 grammes of nitric nitrogen present in the quantity taken, since 

 the ratio between the c.c.'s of indigo and the nitric nitrogen 

 diminishes gradually in a curve as the solutions become 

 stronger. The quantity of water or effluent used should not 

 require more than 10 c.c. of indigo, and is better kept at about 

 5 to 6 c.c, or even less. In titration the strength is uniformly 

 arranged at 25 c.c. liquid to 50 c.c. H2SO4 free from nitrate ; 

 and it is important to make blank estimations, as several 

 samples of acid have been found to be faulty. The quantity 

 of liquid found suitable, and made up to the 25 c.c. with dis- 

 tilled water, is mixed rapidly in a thin flask with the acid (over 

 a sink or dish, in case of fracture), and is immediately titrated, 

 while still hot, with the indigo. The rapidity with which this 



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