Lead and potassium were determined by dissolving the substance in dilute nitric acid, evaporating with sulphuric acid, separating the lead sulphate by filtration, weighing it, and determining potassium in the filtrate by weighing it as sulphate. Iodine was determined by treating the substance with a solution of sodium arsenite, acidifying with nitric acid and digesting with an excess of silver nitrate, and finally weighing silver iodide. Carbon and hydrogen were determined by combustion with lead chromate, where the front part of the tube contained a layer of metallic silver which stopped the passage of any iodine. The variations in the results of the analyses are considerable, and it is probable that the salt, being always deposited in a concentrated mother-liquor, was never quite pure, but there is no evidence that the variations in composition have been regularly influenced by the variations in the conditions of preparation. The analyses are given in the order in which they were made. The last three probably represent better material than the others. Oxygen Lead. Potassium. Iodine. Carbon. Hydrogen. (difference.) It must be admitted that the results do not agree very satisfactorily with the calculated quantities, and that the formula is somewhat uncertain. It seems probable, however, that the compound is a combination of lead acetate with potassium triiodide with the formula 5Pb(CH,CO), . 3KI,. It is not certain that the extra iodine is combined with the potassium rather than with the lead, but since KI, is a well-known compound, and since the acetic acid radical is present in the proper proportion to form lead acetate, this view seems to be the most plausible one. Johnson's analysis differs chiefly from the new ones in its higher iodine and consequently lower oxygen as determined by difference. His oxygen is considerably too low for the amount required to give CH,CO, with the carbon and hydrogen, and this was evidently the main cause of his inability to arrive at a rational formula. It seems probable that there was an error in his determination of iodine. 3 Gröger's salt.-A compound has been described by Max Gröger as corresponding to the remarkable formula, PbO. PbI. I. As he prepared it, it was an amorphous precipitate which had been washed with water, and exposed to the air for a long time in order to allow iodine with which it was mixed to evaporate, and, consequently, there seemed to be room for doubt as to its freedom from decomposition after it had undergone these operations, even if it could be supposed to have been a pure substance when it was precipitated. I have undertaken a reinvestigation of this salt, and have succeeded in preparing it in a beautifully crystalline condition in which there was no doubt about its purity, and have found that Gröger really analyzed a pure compound, but that he overlooked some water that it contained. With the addition of one molecule of water his formula becomes correct, but this formula, Pb,IO.H2O, or as it may be written Pb,I(OH),, is no less remarkable than the one which Gröger advanced. This substance, in a crystallized condition, had been observed in this laboratory a short time before Gröger's work was known here. At my suggestion, Mr. J. H. Pratt had made some experiments with the dark colored precipitate produced by mixing strong aqueous solutions of lead acetate and potassium triiodide. Such precipitates were collected upon filters, treated while still moist with boiling alcohol and the resulting liquid, after filtration, was evaporated over sulphuric acid, with the result that small, brilliant, black crystals were sometimes obtained. Several partial analyses of this substance showed that it contained lead and iodine in the ratio 2:5, and were as follows: The yield of this product was very small, and it was difficult to obtain it in a pure condition since it was often mixed with the well-known compound PbIOH and with other substances which were not identified. The presence of water in the salt was established, but the circumstances were such that the investigation was interrupted at a point where the pure *Monatshefte für Chemie, xiii, 510, 1892. material at hand had been exhausted and no accurate determination of water had been made. My thanks are due to Mr. Pratt for his valuable assistance in the investigation of the compound up to this point. When I subsequently obtained Gröger's salt in a crystallized condition it proved to have the same form and composition as the product mentioned above, so that a further study of the latter was deemed unnecessary. In order to obtain Gröger's compound in a well crystallized condition, it is necessary to modify his method of preparation by using a small amount of acetic acid. It is also advantageous to use boiling water instead of cold water for the precipitation, and to use a somewhat larger volume of this than is recommended by him. I have obtained the best results by the following method: Dissolve 10 g. iodine in 100ce absolute alcohol, then 50 g. crystallized lead acetate in 150cc water, 3c glacial acetic acid and 300ce absolute alcohol. Mix the two solutions, let stand 14 to 16 hours at the temperature of the room, filter to remove the small precipitate, then dilute with 1500° of boiling water. Let the whole stand until cold, when the compound sought will have crystallized out mixed with iodine. Pour off the liquid and wash the crystals with cold alcohol in small quantities until the iodine is removed. Dry the product upon filter-paper, and then in the air at ordinary temperature. The product consists of very brilliant black crystals, usually 0.5mm or less in diameter. They form octahedrons, apparently of the tetragonal system, with faces that are much curved and otherwise distorted. The powder of the crystals is similar in color to Gröger's precipitate, and it agrees with it in being practically stable in the air and scarcely acted upon by cold water or alcohol. Two separate crops of apparently perfect purity were analyzed. Lead and iodine were determined by the methods. described above under Johnson's compound. Water was collected and weighed in a calcium chloride tube, the substance being ignited in a tube behind a layer of granulated sodium carbonate which held back the iodine completely. Free iodine was determined volumetrically by the use of sodium thiosulphate solution. The results were as follows: I have also prepared the compound, exactly according to Gröger's directions, as a reddish-brown precipitate, and after the product was apparently free from intermixed iodine and air-dry, it was dried for three days, spread out in a very thin layer under a bell-jar well charged with solid potassium hydroxide. This product gave the following results on analysis: Water.. Found. 1.80 Calculated for 1.66 This result indicates that Gröger overlooked water in his compound, and that his precipitate is identical with the crystallized product. I have observed the formation of this salt under various conditions when alcoholic solutions containing lead acetate and iodine, and in some cases potassium iodide also, were diluted, but the purest crops have been obtained only when the ingredients were used nearly in the proportion which Gröger recommends, and also when the alcoholic mixture has been allowed to stand for the proper period. The compound cannot be recrystallized from water, alcohol or mixtures of the two liquids, and it seems probable, as Gröger suggests, that it is formed by the decomposition of some other compound by water. This view does not conflict with the fact that it was prepared, as described above, by the evaporation of certain alcoholic solutions, because these always contained water which increased in proportion to the alcohol as the evaporation went on. The presence of an acetate seems to be indispensable to its production, for I have made a number of experiments using lead nitrate instead of the acetate with no indication of its formation. It seems probable that a soluble compound closely related to Johnson's salt is formed at first and that this yields Gröger's compound by the action of water. I have made unsuccessful attempts to prepare a bromide corresponding to Gröger's salt, and my attempts to replace a part of the iodine in it by bromine have also failed. Conclusion. The two compounds which have been re-investigated, 5Pb(CH,CO,),. 3KI. 61 and PbI,. PbO.31. H,O show no evident relation to each other nor to the compound 2PbI,. 3KI. I. 4H2O, which I have previously described, except that all of them are of complicated composition and they all contain extra iodine without showing evidence of the existence of lead tetraiodide. Classen and Zahorski's quinoline salt, previously referred to, seems to furnish the only evidence of the existence of this higher iodide in combination. Sheffield Scientific School, New Haven, Conn., March, 1895. ART. III.-The Estimation of the Halogens in Mixed Silver Salts; by F. A. GOOCH and CHARLOTTE FAIRBANKS. [Contributions from the Kent Chemical Laboratory of Yale College-XXXIX.] KNOWN methods for the estimation of chlorine, bromine, and iodine in mixed silver salts depend either upon the reduction of the salts to metallic silver or their conversion to a single definite silver salt. The old but by no means ideal methods for the determination of chlorine and bromine in mixed silver chloride and bromide by reduction of the salts to silver in hydrogen at high temperatures or conversion to silver chloride in an atmosphere of chlorine are typical. Perhaps the best of all are the electrolytic method of Kinnicutt* for the reduction of the fused chloride and bromide, the battery process of Whitfield+ which involves the electrolysis of the solution of the silver salts in potassium cyanide and the method of MaxwellLyte according to which the silver in the cyanide solution of the silver salts is thrown down by potassium iodide and sulphuric acid. Even in these processes there are points against which objection may be raised with reason. Thus, in the processes of Whitfield and Maxwell-Lyte it is next to impossible to secure complete and speedy solution of the dried silver salts in potassium cyanide without recourse to intermediate washing and treatment with nitric acid; and in Kinnicutt's method, which has been applied only to the analysis of the mixed chloride and bromide, difficulty is found in the speedy removal of all sulphuric acid from the spongy mass of silver formed in the reduction. We have tried many experiments with a view to simplifying the analysis of the mixed silver salts. Ignition with mercuric cyanide according to Schmidt's method for sulphides; treatment with cuprous chloride dissolved either in ammonia or in hydrochloric acid; the action of ferrous oxalate dissolved in potassium oxalate, Eder's reagent; treatment with chromous chloride or chromous acetate; contact with powdered magnesium under dilute acid; and many other plans of action with powerful reducers have failed to yield analytical results comparable with those of the known methods. Hydrogen sulphide, dry or moist, and ammonium sulphide attack the halogen salts of silver with varying intensity, the chloride very *Am. Chem. Jour., iv, 22. Chem. News, xlix, 3. Am. Chem. Jour., viii, 421. |