Note O — Chapter VI

It has been noted by several chemists, and especially by Kathreiner, and later by Prof. von Schroeder, that the quantity of permanganate reduced by a given amount of tannin varies within rather wide limits, according to the rate at which the permanganate is added; and the "1 c.c. method" was suggested by Prof. von Schroeder, to secure uniformity in this particular. It has, however, been found by the writer, in the course of experiments not yet completed, that the quantity of permanganate required, was a function not simply of time, but of the rapidity of diffusion through so large a bulk of liquid; and by the alternate use of a simple glass rod, and of a specially constructed perforated stirrer, he was able, while adhering strictly to Prof. von Schroeder's directions, to obtain results even more divergent by the "1 c.c. method" than could be obtained by the drop method previously recommended, when properly carried out. Employed in conjunction with the use of tannin for standardising, as recommended by the Commission, either method gives perfectly dependable results.

The explanation of the variation is a simple one. The oxidation in the Löwenthal process should be limited to indigo, and bodies more oxidisable than indigo, but there exist both ready formed in liquor, and among these oxidation products many substances which in the absence of indigo will readily reduce permanganate. When the latter is added rapidly, and with insufficient stirring, it destroys the indigo and tannin in contact with it, and proceeds also to oxidise the other matters present, although in other parts of the beaker indigo and tannin still exist. Thus more permanganate is reduced than corresponds to the indigo and tannin, and this is especially so towards the end of the process, when very little of either remains. The more slowly the permanganate is added, and the more vigorously it is stirred, the more closely it will approximate to the theoretical quantity required merely to oxidise the indigo and tannin. It seems to the writer more scientific to approach this as nearly as possible, than to attempt to establish a purely arbitrary standard such as the "1 c.c. method;" but he would rather refrain from committing himself to a definite opinion till his experiments are complete.