Art. LXIII.—On the Decomposition of Water by Tannic Acid in conjunction with an Alkali.
[Read before the Wellington Philosophical Society, 16th February, 1898.]
Though tannic acid, when alkalized, rapidly combines with free oxygen, it has not hitherto been supposed capable of combining directly with the oxygen of water—that is, of decomposing water for its oxygen—under any circumstances. This decomposition, however, of water by tannin can be easily brought about, and in the following manner: A sheet of metal—say, platina—is partly immersed in an alkalized solution of tannin, over which is a layer of oil to shut off the atmosphere, and thus prevent interference with the results.
This sheet is clamped to another sheet of the metal standing in another vessel that contains water feebly acidulated—say, with hydrochloric acid. The two vessels are then connected by a bent glass tube full of gelatine in a firm but gelatinous condition, when a lively evolution of gas proceeds from the platina in the hydrochloric acid, while the tannin solution acquires a deep-brown tint in the immediate vicinity of the metal. The gas is hydrogen. If the acid solution is changed for a solution of auric chloride or cupric sulphate, metal is rapidly deposited upon the platina there. Evidently water* has been decomposed, its oxygen combining with tannin, and its hydrogen depositing the metal in the other vessel: we have, in fact, a galvanic cell formed in this way and brought into action.
In this experiment I take it that the metals do not contribute to the results by any so-named catalytic action† or condensation of oxygen, but only by affording the means by which the chemical combinations in the liquids may produce, not heat, but electricity, and electricity in the current or dynamic form—that is, speaking more in accord with the electrical theories of the day, a series of electrical polarizations. Of course, it is only the tannin that is in absolute contact with the metal that acts on the water—that is, in fact, oxidized.
In accordance with the custom of designating new forms of voltaic cells after some distinguishing or novel feature in them, I propose to name the apparatus here described the “tannin cell”; a group of these, the “tannin battery.”
[Footnote] * I take here the most liberal view of the electric liberation of hydrogen from acidulated water. The case may really, so far as we know, be one where decomposition of the acid is effected—one that science has yet to settle.
[Footnote] † May not many of our cases of catalytic action be, in reality, electrical action as in this case.