The Inhibitor Agent in Subramanian's Dislocation Etchant for KCl

A useful solution (10 mgr of Fe CI3 in 25 cc. of isopropanol plus two drops of water) for observing dislocations in KCl by the etching method has recently been reported by Subramanian [1]. The ability of solutions of this kind to selectively etch is attributed to an inhibitor effect [2, 3, 4], but little is known about the nature of the inhibitor agent. It should be kept in mind that transitionmetal salts, when dissolved, may give rise to several systems, the nature of which is often difficult to determine [5, 6]. In this paper the Optical Absorption (0. A) spectrum of Subramanian's (S) solution is studied in order to elucidate the nature of the inhibitor agent in the case of KCl. The O. A. spectra were recorded using Beckman Acta CII and Cary 17. D spectrophotometers, both working in double beam mode. We have first verified that pits formed on the KCl faces by means of fresh S-solutions are actually related to dislocations, using matched cleaved faces [3] as well as by observing the variation of the pit density under annealing and plastic deformation processes. In the latter case a relation n — 108-9 e between the plastic deformation, e, and the dislocation density, n, was found, as observed similarly for LiF [4]. We have noticed that the addition of water to the fresh solution inhibits progressively its ability for etching. In a 2.5 10~3 M solution of FeCl3, when the concentration of the added water is about 2 M, the etching power of the solution disappears almost