Davy — New Method for the Determination of Nitrites. 563 



LXXYII. — 0^ A l^Ew k'SD Expeditious Method eoe the DEXEmMTNA- 



TlOi^ OP THE l^ITKITES, UNDER DUTEEENT CIRCUMSTANCES, By EdMUND 



"W. Datt, a.m., M.D., M.R.I.A., Professor of Forensic Medicine, 

 Eoyal College of Surgeons, Ireland, &c. 



[Eead, April 24, 1882.] 



As the existence of Nitrites and I^itrates in different natural waters 

 lias been regarded (under certain circumstances) as affording eyidence 

 of previous sewage contamination, the determination of the presence 

 or absence of such compounds, and their quantitative estimation when 

 present, in the waters employed for domestic purposes, may be a mattter 

 of much importance in a hygienic point of view ; and though we have 

 some delicate tests for the detection in such of the presence both of 

 nitrites and nitrates, as well as different methods for their conjoint 

 quantitative determination, there is no very simple or expeditious 

 method for the separate estimation of nitrites in waters, which may 

 sometimes be required, if we except that not long since proposed by 

 P. Grriess, which method I shall presently describe, and compare with 

 the one I have myself devised, and which I shall now lay before the 

 Academy. 



In making lately some experiments on certain nitrites, I observed 

 a reaction, which, as far as I am aware, has not hitherto been de- 

 scribed ; and this being one of extreme delicacy, I have founded on it 

 a new method not only for the detection of the presence, but likewise 

 for the quantitative determination of the nitrites under different cir- 

 cumstances, but especially in the case of natural waters, for Avhich it 

 is peculiarly suitable. The reaction referred to is that of nitrous 

 acid, or of a soluble nitrite, on the well-known substance, gallic acid ; 

 thus when an aqueous solution of that latter acid is brought in contact 

 with a soluble nitrite, the mixture, unless the amount of the latter 

 present be very small, will soon acquire a yellow or yellowish-brown 

 tint, which will increase in depth up to a certain point, after which 

 the colour remains permanent, whilst, at the same time, minute 

 globules of gas make their appearance in the mixture. If, however, 

 the quantity of nitrate present be exceedingly small, it will require 

 several hours, or even some days, to complete the reaction at the ordi- 

 nary temperature. By the application, however, of heat,^ and bringing 

 the mixture to the boiKng point, even in the case of the most dilute 

 solutions, the reaction will be completed in a few moments. This de- 

 velopment of colour under the circumstances stated is evidently due 

 to the oxidation of the gallic acid, at the expense of the nitrous acid, 



- The continued application of heat has the effect of slightly diminishing the 

 depth of colour developed in this reaction. 



E. I. A. PROC, SEE. II. VOL. III. — SCIENCE. 3B 



