Formation of “Hollow Whiskers” from Metals by Reaction with Ferricyanide and Ferrocyanide

Abstract

WHILE devising a microscopic test for the detection of small particles of metallic iron it was discovered that blue whisker-like structures (Fig. 1) were formed from iron particles or iron alloys in a 10 per cent (w/w) solution of acidified (0.06 N hydrochloric acid) potassium ferricyanide. Further microscopic examination of these structures revealed them to be hollow tubes. Considerable branching of these tubules was also evident, giving a general appearance not unlike that of fungal mycelium. Growth of these tubules proceeds at the tip, and the rate of growth, the diameter of the tubules, the amount of branching and shape of the tubules appear to be directly related to the concentration of ferricyanide and the type of iron alloy. With powdered iron, for example, in a 10 per cent concentration of acidic ferricyanide, the tubules vary from about 4 to 10µ in diameter. They may grow outward from the iron particles to a distance of from 0. to 0.5 mm or longer within a second. In less than 30 sec most of the tubules have formed, many of them bent or looped, or both. At a ferricyanide concentration of 5 per cent (w/w) the tubules are more variable, larger in diameter, and grow much longer. At concentrations of 2–2.5 per cent the tubules grow more slowly, some of them forming a blue precipitate at the end which slowly drifts away giving the appearance of smoke coming out from a lengthening smokestack. At concentrations of 20 per cent (w/w) and higher, a few tubules may be seen but mostly blue hemispherical and spherical structures are formed on the surface of the iron. If the solution is then diluted, tubules may be seen to arise from these structures. The tubules formed from acidic ferrocyanide and iron appear to be lighter blue or green in colour.