Exchange of Radiochlorine between Hydrogen Chloride and Carbon Tetrachloride. ``Red Form'' of Solid Hydrogen Chloride

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Abstract
Radiochlorine‐36 has been used to investigate the HCl*–CCl4 exchange in systems of high purity at 22–35°C in the dark and in sunlight for exchange times up to 200 and 117 days, respectively, with solutions 0.14–0.18 f in HCl and 10 f in CCl4. The exchange rate law is not known, but results have been presented in terms of an assumed bimolecular rate law, R=k(HCl)(CCl4). For the dark exchange, k≤2×10—11 liter mole—1 sec—1, an upper limit which is about 4×107 times smaller than the upper limit set by earlier workers. There is no evidence that the exchange is accelerated in sunlight. The so‐called ``red form'' of solid HCl was observed during the work, as well as solid HCl which was yellow. An apparent red‐inducing impurity was found separable from most of the HCl by slow vacuum distillation of the latter from a —140°C bath to a trap at —196°C. The solid HCl distillate was colorless and failed to develop a red color even in six flash freezings at —196°C, whereas the pale yellow HCl residue from the distillation gave a rose‐red color on being flash frozen at —196°C. Volatility characteristics of the latter apparent impurity suggest that, if it be an oxide of nitrogen as proposed by Clusius and Haimerl, it is probably not NO but rather N2O or possibly N2O3. If the red color arises from imperfection color centers in the crystal lattice of HCl, as suggested by Johnston and Martin, generation of the color seems to require both reasonably rapid freezing at or near —196°C and the presence of an impurity separable from the HCl by a proper distillation. Consequently the flash‐freeze test for traces of HCl in other substances must be interpreted with caution.

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