A simple and expeditious process for the estimation of nitric acid in water analysis, with some remarks on the estimation of nitrous acid

M. Whitley Williams
1881 The Analyst  
IT is well known that when zinc is immersed in copper sulphate solution it becomes covered with a spongy deposit of precipitated copper. If the solution of copper sulphate be sufficiently dilute, this deposit of copper is black in colour and firmly adherent to the zinc. It is, however, not so generally known that the zinc upon which copper hae thus been deposited possesses the power of decomposing pure distilled water at the ordinary temperature, and that it is capable of effecting many other
more » ... ecting many other decompositions which zinc alone cannot. Among these is the decomposition of nitrates, and the transformation of the nitric acid into ammonia. Messrs. Gladstone and Tribe have shown that the action of the I ' copper-zinc couple " (as they call the conjoined metals) upon a nitre solution consists in the electrolysis of the nitre, resulting in the liberation of hydrogen and the formation of oxide of zinc. This hydrogen is liberated upon and occluded by the spongy copper, and when thus occluded it is capable of reducing the nitre solution in its vicinity. The nitrate is first reduced to nitrite of potassium, and the nitrous acid is subsequently transformed into ammonia by the further action of the hydrogen. I n a paper lately read before the Chemical Society I have shown that even in very dilute solutions of nitre the nitric acid can be completely converted into ammonia in this manner with considerable rapidity; and I have further shown that the reaction may be greatly hastened by taking advantage of the influence of temperature, acids, and certain neutral lsalts which increase the electrolytic action of the couple, I there showed that carbonic acid-feeble acid as it is-suffices to treble the Epeed of the reaction, and that traces of sodium chloride (091 per cent.) accelerated it nearly as much as carbonic acid. A rise of a few degrees in temperature was also found to haden the reactien in a very marked degree. The presence of alkalies, alkaline earths, and salts having an alkaline reaction, was found to retard the speed of the reduction. Upon those experiments I founded a simple and expeditious process for estimating the nitric and nitrous acid in water analysis, which, when used with skill, may be applied to by far the greater number of waters with which the analyst is usually called upon to deal. Before describing this process I will first say a few words upon the nature and use of the copper-zinc couple, ahout which considerable misapprehension appears to exist even in the best iriformed quarters. This appears very plainly from the description of the copper-zinc couple process given in the Handbook of Water Analysis, recently published by Frankland, in which mistaken directions are given for making the couple and applying it to the estimation of nitric acid in water analysis. There are two kinds of copper-zinc, one (the " dry couple") intended for anhydroue reactions, the other (the wet couple") intended for hydmgenizations in watery or alcoholic solutions. These two are quite different reagents, differently prepared and different in their application, It is the " wet couple" alone which is adapted to bydrogenizations, such as the transformation of nitric acid into ammonia ; but in the handbook
doi:10.1039/an8810600036 fatcat:awj2tamevfb55jef3c3ka3bocm