01154 gram in 30 c.c. of aqueous solution at 20° in a 4-dcm. tube gave: Nayellow. -1.25° 81.2 445 The rotation dispersion ratio for Hggreen/Nayeilow=1-232 and for Hgyellow/ Nayellow = 1.056. The molecular rotatory power of the l-phenylbenzylmethylallylammonium ion is [M] −166°, a value agreeing well, within experimental error, with that obtained for the d-base. 1-Phenylbenzylmethylallylammonium d-a-Bromocamphor-π sulphonate. This compound was also prepared in a similar manner to its isomeride, and had similar properties. It melted at 147-149°. The rotatory constants were determined in aqueous solution; 0.2360 gram made up to 30 c.c. at 17° in a 4-dcm. tube gave: The rotation dispersion ratio for Hggreen/Nayellow=1.328 and for Hgyellow/ Nayellow = 1.094. The molecular rotatory power of this salt ([M] +1110) gives a value which agrees within experimental errors with that assigned to its enantiomorphously related isomeride and also with the value obtained by calculation from d-phenylbenzylmethylallylammonium d-a-bromocamphor--sulphonate. The recovered quaternary iodide had [a]-565°. There is no immediate prospect of continuing this work or the comparative study of the corresponding derivatives of the optically active acids with the cyclic ammonium compounds, owing to the undertaking of other duties. For this reason the work is published in its present form. I desire to acknowledge my indebtedness to Prof. Pope for his suggestion of this work and for his kind interest during the course of the experiments. THE CHEMICAL LABORATORY, THE UNIVERSITY, CAMBRIDGE. [Received, November 16th, 1916.] V.-Lead Subiodide, and an Improved Method for Preparing Lead Suboxide. The Solubility of Lead Iodide. By HENRY GEORGE DENHAM. THE existence in aqueous solutions of subvalent salts of lead has been demonstrated by the joint work of Denham and Allmand (T., 1908, 93, 424), wherein it is shown that in the presence of platinised platinum hydrogen is capable of reducing bivalent salts of lead to a lower state of valency. Further support of this was obtained from the "circulation" experiments, in which it was shown that by maintaining a constant flow of an aqueous solution of lead acetate over a heated column of lead appreciable quantities of the metal could be dissolved and again precipitated on cooling the solution. These two results were summarised by the authors in the equations Pb++ +H Pb++H+ Pb+++ Pb2Pb+. A further possible explanation has since been suggested by Pick and Ahrens (Abegg's "Handbuch," 4te Gruppe, Blei, 637), the mechanism of the second reaction being represented according to their view by the equation Pb+++ PbPb2++. Further evidence as to the existence in aqueous solutions of the ions Pb+ or Pb2++ has recently been furnished by the work of Bell (Trans. Faraday Soc., 1916, 11, 1, 79), who investigated the problem by comparing the weight of lead actually dissolved in various electrolytes during the passage of an electric current with that calculated from the electrochemical equivalent for bivalent lead. The discrepancy observed was found to substantiate the earlier work of Denham and Allmand. The obvious method to obtain sub-salts of metals which give definite suboxides, for example, lead (Tanatar, Zeitsch. anorg. Chem., 1901, 27, 304; Brislee, T., 1908, 93, 154), is to act on this suboxide with the necessary acid; but this method failed utterly owing to the following decomposition : Pb2O → Pb+ PbO. Other methods, such as the action of finely divided lead on the various solutions of lead salts, also gave negative results. It has been shown that the vapour of methyl iodide acting on heated cupric oxide gives cuprous iodide without the liberation of iodine (Denham, Zeitsch. anorg. Chem., 1911, 71, 303). This sug gested that the vapour of methyl iodide acting on lead suboxide might yield a lower iodide of lead, and although many unforeseen difficulties have been met with, this method has ultimately proved successful. A preliminary experiment was carried out by distilling methyl iodide over lead suboxide at a temperature of 250-260°. After a distillation lasting about thirty minutes, the apparatus was cooled and the reaction tube examined. The substance in the tube was found to have changed from its original dark grey colour to a dark yellow with globules of lead scattered throughout the mass, but in the immediate neighbourhood of the glass walls a homogeneous, bright yellow band was clearly to be seen. The inner mass was extracted with hot water, and the filtrate gave, on cooling, a copious crop of lead iodide crystals, whilst the yellow band under similar treatment not only gave no such deposit, but the filtrate gave no trace of a precipitate with potassium chromate. This reaction, several times repeated, pointed to the desired reaction having proceeded with considerable evolution of heatsufficient to raise all but the layer in contact with the relatively cool walls to a temperature at which the subiodide decomposed into a mixture of lead iodide and lead, the melting of the metal proving that the temperature had risen at least 70°. This supposition was at once tested by mixing the suboxide with four times its weight of silica, and it was found that the tendency to decomposition was by this means completely checked. Before describing in detail the apparatus and method used in preparing the subiodide, the details of the method for obtaining pure samples of lead suboxide merit attention. Methods of Preparation of Lead Suboxide. Two methods have been described for the preparation of this suboxide, namely, that of Glaser (Zeitsch, anorg. Chem., 1903, 36, 1) and that of Tanatar (loc. cit.), but the actual working details of their methods have been so sparingly given that it is wellnigh impossible to follow their work without repeating the whole of their experiments. Glaser, for example, records that so long as the temperature does not exceed 235°, lead oxide is quantitatively reduced to the suboxide, and not to lead. The duration of this reduction is, however, not mentioned. On the other hand, the author has found that samples of lead oxide prepared by the decomposition of lead. oxalate in a stream of air are reduced to lead at temperatures that do not exceed 220°. In an experiment at this temperature lasting 100 hours not the slightest sign of a halt in the neighbourhood of a composition approximating to that of lead suboxide was found. Glaser has pointed out in the case of copper oxide that the actual temperature of reduction depends on the previous history of the oxide, and it is highly probable that the samples of oxide used in this research possessed a finer grain than the specimens used by Glaser, hence the difference in the temperature of reduction. Obviously a method of preparation depending for its success on a knowledge of the particular temperature of reduction of each sample used is of little value as a method of preparation, and attention was then turned to that of Tanatar. This author describes how he prepared the suboxide "by heating the lead oxalate in a combustion tube at the lowest possible temperature, carbon dioxide being led through the apparatus during the decomposition." It has required a considerable amount of work to rediscover the conditions that enabled Tanatar to obtain a product showing the properties of lead suboxide; under no conditions has the author been able to obtain a pure product when using a stream of carbon dioxide, for considerable traces of this gas were always tenaciously retained, possibly as subcarbonate. If nitrogen is used, according to Tanatar's suggestion, this objection is removed, but the inordinate length of time necessary for the decomposition-nearly a week at 300°-is a great disadvantage. Consequently, the author has been compelled to introduce various modifications in Tanatar's original method, and to lay down precisely those conditions that enable an independent worker to repeat his experiments. In the first place, the dilution effect obtained by the passage of an inert gas was obtained by removing the products of decomposition (carbon monoxide and carbon dioxide) by an automatic three-fall Sprengel pump. Provided the total pressure does not exceed 5 cm., the carbon monoxide does not reduce the suboxide to lead. The main part of the decomposition was carried out at 270-275°, but when the pressure had fallen nearly to zero the temperature was steadily increased to a maximum of 335°. Under these conditions it is possible to convert the oxalate into suboxide in twenty-six hours. The product is a dark powder exhibiting all the properties ascribed to it by Tanatar. So far as the preparation. of the suboxide itself is concerned, the author has been unable to find any reason why the temperature of decomposition should not rise as high as 375°. Even at this temperature no decomposition into lead and lead oxide occurs, but the product becomes distinctly paler in colour, and in this form is much less reactive, especially towards methyl iodide. Table I gives the composition of the last nine samples of lead suboxide prepared according to the method outlined above. Experiment... TABLE I. 1 2 3 4 5 6 7 8 9 Lead per cent. 96-16 95.93 95-99 96.04 95-74 95.86 96-16 96-19 96-01 (theory, 96-28) Method of Preparation of the Subiodide. Although it is relatively easy to prepare samples of the subiodide approximating in composition to the theoretical, it has been found that the pure substance may be obtained only by the closest attention to the conditions stated below. The apparatus found to be most satisfactory for the preparation of the subiodide is shown in Fig. 1. J was a distillation flask into which methyl iodide could be introduced from K at the completion of the decomposition of the oxalate into suboxide; H was a tube containing phosphoric oxide; Ga spiral of thin glass; F, E bulbs containing the reaction mixture of lead oxalate and silica; D a bulb containing lead oxalate capable of being sealed off at the conclusion of the decomposition of the oxalate into suboxide and separately analysed; C a water condenser; M a receiver into which the tube from the condenser projected to a considerable extent, so that after the first few c.c. of methyl iodide had been condensed the evolution of any gas |
