PREPARATION OF CHROMY L DICHTAORIDE. 191 Chant y 1 Dichlo ride. By HERBERT DRAKE T,AW and FREDERICK MOLLWO PERKIN. THE method usudly given for the preparation of chromyl dichloride is t o act on a mixture of potassium clichromate and sodium chloride with concentrated sulphuric aci(1. g t a r d (Ann. China. Phys , 1881, [ v 3, 23, 218) used fuiriing snlphuric acid and noticed that clilorine was illways given off during the reaction, but he succeeded in obtaining a yield of 70 per cent. of chromyl dichloride. As n matter of fact, a certain amount of reduction of tho chromyl chloride with the excess of hydro- chloric acid, liberated in the reaction, always takes place, even when ordinary strong snlphiiric acid is used The objection t o the above method of prep,wation is the extreme frothing and fuming which occurs.I n order to lessen the frothing, it is generally recom- mended to fuse sodium chloride and potassium dichromate together in equivalent proportions, and then to break up the fused product into small pieces. Even when this is done the operation requires constant attention, and it is very difficult t o distil the chromyl chloride owing t o the frothing. Moissan (Conzpf. ?*end., 1884, 98, 582) prepared it by passing dry hydrogen chloride over chromic anhydride, but even by this method a certain amount of reduction takes place, a green substance being left behind a t the end of the reaction. We tried several methods for preparing the product, for example, that of passing dry hydrogen chloride into chromic anhydride sus- pended in concentrated sulphuric acid, the mixture being cooled by running water.This method gave practically theoretical results, but the process was tedious owing to a tendency of the chromic anhydride to cake in the sulphuric acid, when i t is only slowly acted on by the hydrochloric acid. W e found the most satisfactory method mas to dissolve chromic arihydride in concentrated hydrochloric acid and then to add an excess of strong snlphuric acid. Chromic anhydride clissolves with the greatest ease in concentrated hydrochloric acid, forming a brownish-red so1ut)ion. On adding con- centrated sulphnric acid to this solution and cooling, chromyl dichloricle separates, and, being denser than the hydrochlol.ic-sulphuric acid solu- tion, sinks to the bottom, and is readily isolated by means of a t'ap funnel.We find t h a t if 1:trge quantities of chromic anhydride are acted on at one time the reaction does not proceed as snioothlyas when smaller quantities are used, owing to the difficulty of cooling the mix- ture and thus keeping the reaction under control. When the reaction becomes too vigorous, and consequently heating results, tbe yield is192 PREPAHATION OF CHROMYL DICHLORIDE. much smaller, owing to reduction of the chromyl dichloride. The best results were obtained by proceeding as follows. Fifty grams of chromic anhydride are dissolved in rather more than the calculated quantity of concentrated hydrochloric acid (1 70 c.c.) in a 1 s litre flask and then 100 C.C. of concentrated sulphuric acid added in quantities of about 20 C.C.a t one time, the mixture being cooled between each addition. The sulpharic acid should be poured into the middle of the solution and not clown the sides of the flask, as i t then mixes bettey and very little fuming takes place. The whole of the sulphuric acid inny be added in the course of about two minutes. I n order to prepare larger quantities, about six flasks are taken, and into each flask 50 grams of chromic anhydride are placed and then the requisite quantity of hydrochloric acid. The chromic anhydride immediately dissolves. Sulphuric acid, 100 c.c., is then added to each flask, cooling between the additions of the acid, and when the reaction in the six flasks is complete, the contents are poured into a large separating funnel and allowed to stand for twenty minutes. At the end of this time the whole of the chromyl di- chloride mill have separated as an under layer, and is then drawn off from the specifically lighter layer of sulphuric and hydrochloric acids.Proceeding in this manner, there is no difficulty in preparing a kilo- gram or more in one hour. Dry air is now aspirated through the chromyl dichloride for about an hour in order to remove any dissolved hydrochloric acid. It may then be distilled, when i t will be found to boil constantly a t 115-11Ci0. For most purposes, however, it is sufficiently pure without this operation. Our own experience is that, provided i t is freed from hydrochloric acid by aspirating air through i t and is then distilled to remove any sulphuric acid, it may be preserved for a long time. It is, however, essential to have the bottles well stoppered to exclude moisture, as, owing to absorption of the latter, the stoppers often become fixed. I n using a hurette or separating funnel the taps should never be lubricated with vaseline or oil, because of the vigorous action of the chramyl di- chloride on hydrocarbons. It is often stated that chromyl dichloride does not keep well. CHEMICAL LABORATORY, BO~LOUGII POLYTECHNIC INSTITUTE, I,ONDON, S. E.