T H E PREPARATION O F GALLIUM TRIBROMIDE AND GALLIUM TRIIODIDE* BY WARREN C. JOHNSON AND JAMES B . PARSONS
Lecoq de Boisbaudran and E. Jungfleisch’ announced many years ago that metallic gallium reacts with chlorine, bromine and iodine to form the corresponding halides. They described the relative intensities of the reactions as being in the usual order, and the boiling points and melting points of the compounds as increasing with increasing atomic weight of the halogen. Shortly following this announcement, Lecoq de Boisbaudran2 made a very brief statement regarding the preparation of several chlorides, bromides and iodides of gallium. He later showed that two chlorides of gallium, namely, gallium dichloride3 and gallium tri~hloride,~ are readily formed by the direct combination of the elements. The existence of these two chlorides was substantiated by quantitative data. Although the reactions of bromine and iodine with metallic gallium are described by this investigator, there is no evidence to show that these halides were ever analyzed or studied, nor is any mention made of the technique employed in their preparation. According to the literature no attempts have ever been made to prepare a fluoride of gallium. I n the present investigation gallium tribromide and gallium triiodide are prepared and some of the physical properties of the compounds are determined in order to further our knowledge of the chemistry of this relatively unknown element. Qualitative experiments are carried out to show that a fluoride of gallium exists. Source of Material The gallium used in the investigation was obtained from the mineral germanite6 whose gallium content varies from 0.5-0.75 percent.6 After the germanium’ had been removed from the mineral, the gallium was extracted with hydrochloric acid according to the procedure of Kraus, Johnson and Foster.* In order to obtain a pure product, the gallium hydroxide was reprecipitated several times by means of sodium acid sulfite. The hydroxide
* Contribution from
the Kent Chemical Laboratory of the University of Chicago. Lecoq de Boisbaudran and E. Jungfleisch: Compt. rend., 86, 577 (1878). Lecoq de Boisbaudran: Compt. rend., 86, 756 (1878). Lecoq de Boisbaudran: Compt. rend., 93, 294 ( 1 8 8 1 ) . Lecoq de Boisbaudran: Compt. rend., 93, 329 (1881). Pufahl: hletall. u. Erz., 19, 324 (1922); Kriesel: 20, 257 (1923). bKriesel: Chem. Ztg., 48, 961 (1924); Lunt: 8. hfrican J. Sei., 20, 157 (1923); Kiel: 2. anorg. Chem., 152, I O I (1926). ’ A pyrogenic process for the removal of germanium from germanite ore was described by Kraus and Johnson at the Swampscott Meeting of the American Chemical Society, September, 1928. a A report of this extraction process was given by L. S. Foster a t the Swampscott Meeting of the American Chemical Society, September, 1928. The results of the methods for both germanium and gallium will constitute the subjects of forthcoming publications.
GALLIUM TRIBROMIDE AND GALLIUM TRIIODIDE
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was dissolved in potassium hydroxide and the solution was electrolyzed. Metallic gallium collected at the cathode as a small metallic-appearing ball which dropped into a cup below the electrode. Although the electrolysis was carried out a t room temperature, sufficient heat was generated a t the cathode to keep the metal in a molten condition. (The melting point of gallium is 29.75'). The gallium was purified by fractional crystallization on a platinum wire from the supercooled liquid metal.'O
Gallium Tribromide Vacuum Preparation. Gallium tribromide was prepared by the direct combination of the elements in vacuo. Metallic gallium (about I g.) was introduced into a Pyrex tube as one solid piece. As soon as the L tube was sealed off, the entire system was evacuated by means of a mercury nr: vapor pump supported by a Hy-S'ac oil pump, Liquid bromine, which had previously been twice distilled from phosphorus pentoxide, was then distilled in vacuo into the tube containing the gallium which was cooled to -33.j0 by means of a Dewar flask of liquid ammonia. The gallium reacted with the bromine D very rapidly a t the temperature of the boiling point of liquid ammonia. At room temperature and even a t oo, the metal reacted on the surface of the liquid bromine in much the same manner as does an alkali metal on the surface of water. A considerable amount of energy R was liberated both in the form of heat and FIG.I . light. When the initial and more vigorous Apparatus for the Sublimation of reaction was over, the mixture was allowGallium Tribromide and Gallium ed to stand for two days to insure comTriiodide. plete reaction, the excess bromine was removed by condensation into a small side tube, which was then sealed off from the main reaction tube, and finally the entire system was evacuated to a pressure of 0.001 mm. Under this low pressure the salt began to sublime a t 90' into the neck of the reaction tube. This process was greatly hastened by increasing the temperature to about 125'. When the major part of the salt had
\ u -
N o definite procedure was followed in the electrolysis of this alkaline solution. The current density was varied to obtain the greatest yield of metal, h u t the results Rere found to be quite inconsistent even when the same conditions were thought to have been employed in different experiments. "Richards and Boyer: J. Am. Chem. SOC., 43, 274 (1921).
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W A R R E S C. JOHNSON AND J A M E S R. PARSOKS
sublimed into a limited space in the tube, it was sealed off from the system. This tube, containing the gallium bromide, was scratched with a file, cracked slightly with a fine hot glass rod, and then introduced into a larger tube, A, as shown in Fig. I . After sealing off the system at B, the entire apparatus was exhausted to a low pressure. The tube containing the salt was then broken by applying a slight jar in the vicinity of A. A Crisco bath was placed about the apparatus and heated to 140' a t which temperature sublimation of the salt into D took place rapidly. Then A was removed by sealing off at C. The smaller tube D was of such a length as to allow for several successive sublimations. The tube was finally sealed off a t D1 and the salt was then sublimed into the various branches above. F1and Fz represent melting point tubes while GI and GP are small tubes to collect samples of the salt for analysis. During the process of sublimation some of the salt appeared in E. It was thus possible to obtain several small samples of the bromide of high purity for carrying out analyses and determining some of its physical properties. Snalysis. A sample of the salt which had been sublimed several times was sealed off in a tube (GI, Fig. I ) . The tube was scratched, washed with alcohol and ether and then weighed. I t was broken and then dropped immediately into water where the salt dissolved quickly. The parts of the tube were .removed, washed, dried and weighed in order to obtain the weight of the dissolved salt. Two aliquot parts of the solution were analyzed for bromine by the usual gravimetric method of precipitation as silver bromide. Anal. Subs., 0.3226, 0.3893: AgBr, 0.5861, 0 . 7 0 7 2 . Calc. for GaBrs: Br, 77.47. Found: 77.31, 77.31. The third aliquot sample was analyzed for gallium by precipitation of the hydroxide with ammonium acid sulfite according to the procedure described by Porter and Browning." Anal. Subs., 0.3642. Gaz03,0.1098. Calc. for GaBra:Ga, 22.53. Found: 22.43.
Melting Point. Several melting point determinations were carried out with three different samples of gallium tribromide and in each case a value of 122.5 was obtained. Density. The density of gallium t,ribromide was determined a t 1 2 0 ' and a t 125'. Ordinary methods could not be used due to the hygroscopic nature of tjhe salt and the small quant,ity available. Nor was it found feasible to determine the density of the solid salt as it showed a marked tendency to contract from the walls of glass upon solidification. It was accordingly found necessary to determine the density of the fused substance. This was accomplished by comparing the weight of the melted salt a t 120' and 125' with the weight of an equal volume of mercury a t the same temperatures. The mercury was purified according to the method of Hulett and Ninchin.12 The gallium bromide was subjected to several sublimations for "Porter and Browning: J. .4m. Chem. Soc., 41, 1491 (1919). Hulett and Minchin: Phys. Rev., 21, 389 (1905).
GALLICM TRIBROMIDE A S D GALLIUM T R I I O D I D E
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purification and then sealed in Pyrex capillary tubes of uniform bore. The inner diameter of the capillary tube was determined by weighing a given length of mercury, the length being measured with a travelling microscope. The menisci were read with a cathetometer. A meniscus correction was applied by assuming a value of one-third the radius of the tube for the height above the bottom of the meniscus. The following values for the density were.obtained : Temperature
Density
0 . 1 ~ 0cm. 5
120°
3 . 123
0.1215
I20
3.138
0.1205
125
0.1215
125
3.095 3 ' 109
Diameter of Tube
Other Properties. Gallium tribromide is a white crystalline substance melting a t 122.5'. It dissolves readily in water undergoing hydrolysis. It is exceedingly hygroscopic. The salt begins to sublime at 90' when subjected to a pressure of 0 . 0 0 2 mm. No decomposition occurs when it is sublimed and melted. When melted and then allowed t o cool, it appears to supercool several degrees. In the initial preparation of gallium tribromide a residue remained in the reaction tube which did not sublime with the bulk of the material. When it was subjected to a much higher temperature, about 2 0 0 ° , a small quantity of a white salt appeared on the walls of the tube above the bath. This subst,ance may be a lower bromide of gallium due to a reaction between the normal bromide and free gallium. Its properties will be described in a later paper.
Gallium Triiodide Preparation. This salt was lilrewise prepared by the direct union of the elementary constituents. Resublimed iodine was dried for several days over phosphorus pentoxide. The metallic gallium was purified by crystallization. Equivalent quantities of these two elements were reacted in a small Pyres tube in the absence of air and moisture. Reaction did not take place at room t e m p e r a t ~ r e ; 'however, ~ on warming gently with a flame, a vigorous reaction followed with the evolution of much heat and light. A crust of the iodide, which appeared t o form on the surface of the small pieces of metallic gallium, was removed by shaking the tube and subsequent heating. By repeating this process several times, it was found possible to complete the reaction. The tube was then cracked and placed in a larger tube, -1,as shown in Fig. I . Several sublimations of the salt were carried out to obtain samples of a pure product. The same general procedure was followed as in the case of gallium tribromide. Bnalysl's. TIVOsamples were analyzed for iodine by the usual method of prrcipitation RS silver iodide. l 3 Reaction n-ould undoubtedly have taken place here providing the constituents were in a finely divided condition.
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WARREN C. JOHNSON AND J A M E S B. P A R S O S S
Anal. Subs., 0.0290, 0.1604: AgI, 0.0453, 0 . 2 5 0 j . Calc. for GaI,: I, 84.53. Found: 84.44, 84.42. The gallium was determined by precipitation of the hydroxide with ammonium acid sulfite and weighed as Ga203according to the previously described procedure. Anal. Subs., 0.3300, 0 . 2 j 2 1 : Ga203, 0.0690, 0.0516. Calc. for GaL: Ga, I 5.47. Found: I j.55, 15.58. Melting point. The melting point of gallium triiodide was found to be at 213.5'. A slight amount of decomposition evidently takes place a t this temperature since the liquid appears darker than the original salt, The color, amber to red, is probably due to the presence of a small amount of free iodine. Color. Gallium triiodide is a lemon yellow crystalline substance a t ordinary temperatures. Lecoq de Roisbaudran undoubtedly obtained a colored salt by reacting iodine with gallium since he niakes the statement that it would, without doubt, be colorless if it were pure. Whether or not he obtained a pure product is not known; he gives no analytical data to support his description of the salt. We have carried out several preparations of gallium triiodide and in each instance a light yellow salt was obtained even after it had been subjected to several sublimations. Some of the small tubes of the iodide were broken under a starch solution and in no case was a test obtained for the presence of free iodine. However, when this solution was allowed to stand for a few hours, free iodine was found, due to the hydrolysis of the salt and subsequent decomposition of hydriodic acid. Other Properties. Gallium triiodide sublimes readily in vacuo a t a temperature as low as 160". It hydrolyzes readily and is hygroscopic. When it is exposed to the air iodine fumes are liberated and the residue appears to be much darker than the original salt. At higher temperatures decomposition takes place. Gallium Fluoride When a small quantity of metallic gallium was treated with hydrofluoric acid ( s o y o solution) in a platinum crucible, a white substance, insoluble in an excess of acid and of much greater volume than the original amount of metal, was formed. The reaction was allowed to continue for several hours and the excess of acid was evaporated over steam. The white residue gave tests for fluorine. The details of the preparation and properties of this substance will be described in a later article.
summary Gallium tribromide and gallium triiodide have been prepared by the direct combination of the elements. Some of the physical properties of these salts have been determined. University of Chicago, Chieago, IlBnoas.
