The limitations of the copper-zinc couple method, etc. 441 
TJie limitations of the copper-zinc couple method in estimating 
nitrates. By J. E. Purvis, M.A., St John’s College, and R. M. 
Courtauld, M.A., M.B., Pembroke College. 
[Bead 9 March 1908.] 
Gladstone and Tribe (Journ. Chem. Soc. 1878, p. 140) proved 
that a copper-zinc couple quickly reduced an aqueous solution of 
potassium nitrate to potassium nitrite and ammonia. The obser- 
vation confirmed a previous one by Thorpe that such a couple 
converted the whole of the nitrogen of nitre into ammonia. Several 
theories were suggested by the authors to explain the changes; and 
they were inclined to the view that, on the whole, the changes 
could be represented best by the following equations : 
(1) Zn | NO* K | N0 3 K | Cu = Zn (N0 3 ) 2 1 K 2 j Cu, 
(2) K 2 + 2H,0 = 2KHO + H 2 , 
(3) KN0 3 + H 2 = H 2 0 + KN0 2 , 
(4) KN0. 2 + H 6 = H 2 0 + NH 3 + KHO. 
The only nitrates investigated by them were potassium nitrate 
and ammonium nitrate. They did not investigate the changes 
when nitrogenous organic substances were also in solution with 
the nitrate. 
M. Whitley Williams {Journ. Chem. Soc. 1881, p. 100) also 
investigated the action of a copper- zinc couple on nitrates ; and he 
suggested a method of estimating nitric acid in water analyses 
based upon his results. He pointed out further the influence of 
temperature and of varying amounts of substances like lime, 
calcium sulphate, sodium chloride, and phosphoric acid when these 
are in solution with potassium nitrate. But the influence of 
dissolved organic substances containing nitrogen was not in- 
vestigated. 
It is probable that the estimation of nitrates by the copper- 
zinc couple is reliable, provided the waters have inorganic 
substances only in solution, and little or no substances of an 
organic nature. But when waters contain the latter compounds, 
as, for example, sewages, sewage effluents, or waters polluted by 
animal or vegetable refuse, it seems probable that the effect of the 
couple might influence the breakdown of organic substances con- 
taining nitrogen, and, particularly if the latter are in an unstable 
condition, one of the results would be the production of ammonia; 
and it would be a fictitious one so far as it affects the estimation 
of nitrates. For the ammonia estimated would be the result, not 
