On the action of nitric and nitrous acids on the sulphocyanides / by Edmund W. Davy.

  • Davy, Edmund William, 1826-
Date:
1865
Catalogue details

Licence: Public Domain Mark

Credit: On the action of nitric and nitrous acids on the sulphocyanides / by Edmund W. Davy. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    of this peculiar red compound may be mistaken for the presence of iron, as, for example, in testing sulphuric, hydrochloric, or nitric acid for iron, as well as in searching for the same metal in the salts of the alkalies, &c., wherein the sulphocyanide of po- tassium (one of our most delicate tests for iron) is used, either alone or in conjunction with hydrochloric, sulphuric, or nitric acid, as the means of detecting that metal; for the apparent indi- cations of iron so obtained may be only caused by the action of the nitric or nitrous acid used, or present in the other acids em- ployed, or even by the presence of some nitrates or nitrites in the compounds under examination, the nitrogen acids of which, being set free by the sulphuric or hydrochloric acid employed, reacting on the sulphocyanide, may be the sole cause of the effect which might be so naturally mistaken for iron. I have ascer- tained, however, that there are several points of distinction be- tween the red compound produced by the action of the nitrogen acids on the sulphocyanides, and the sulphocyanide of iron, resembling it so closely in appearance, which may be easily taken advantage of in distinguishing one from the other. Amongst these the following may be mentioned :— 1st. The former is very unstable, and completely disappears on standing a short time, or immediately on heating it; whereas the latter possesses considerable permanence at the ordinary temperature, and even only very slowly disappears by the con- tinued application of heat. 2nd. The smallest portion of a solution of the ferrocyanide of of potassium added to a mixture coloured red by the sulphocy- anide of iron instantly changes its colour to a deep blue; whereas, in the other case, that reagent appears at first to have very little effect on its colour, till it has been added in consider- able quantity, when its dark red coloration gradually passes into a bright yellow, owing apparently to the conversion of the ferro- cyanide into ferridcyanide of potassium; whilst each drop of the reagent added is attended with the copious evolution of small globules of gas. 3rd. The alkalies (potash, soda, and ammonia) added in excess, decolorize both red compounds; but this in the case of the metal iron is attended with the precipitation of its oxide, whereas there is no precipitation whatever produced with the other compound. On the addition, however, of an excess of either hydrochloric or sulphuric acid the red coloration is restored in both cases. The above differences will easily enable us to distinguish between these two compounds; but, in addition to them, I may mention another interesting peculiarity I have observed in the case of the compound formed by the nitrogen acids and their salts that it is very quickly taken up and dissolved by chloroform, ether,