COIiTRIBUTIONS TO T H E CHEMISTRY O F BERYLLIUM Beryllium 111. Electrolysis of Solutions of Beryllium Compounds in Liquid Ammonia BY HAROLD SIMMOSS BOOTH AND GILBERTA
G.
TORREY
Introduction In preceding articles' the electrolysis of solutions of beryllium salts in various non-aqueous solvents has been discussed. The only solvents from which beryllium salts gave a deposit of metallic beryllium were several organic derivatives of ammonia. The beryllium salts were very soluble in these solvents but the solutions possessed a high internal resistance so that extremely long periods of electrolysis were required to produce any quantity of beryllium metal. Also these solvents continually formed colloidal, brown, organic products which often adhered to the cathode and tended to prevent the deposition of pure metal. However, this slight success with the organic derivatives of ammonia encouraged the use of anhydrous liquid ammonia itself as a solvent. While this solvent presents greater manipulative difficulties, still it could not form colloidal organic decomposition products and should permit the deposition of pure metallic beryllium. History of Liquid Ammonia as Solvent Liquid ammonia has long been known as an excellent solvent for inorganic salts and various elementary substances, particularly the alkali metals. Gore, and Franklin and Kraus,* have determined the solubilities of many salts in liquid ammonia and state that the most soluble salts are the nitrates, chlorides, bromides, and iodides while the sulfates, carbonates, fluorides, and oxides are generally insoluble. Several investigators have determined the conductivities3 of some of these solutions, chiefly those of the alkali metals, but they have rarely been interested in the possible electrode products. Since the solubility in liquid ammonia of only a few beryllium salts has ever been determined and apparently the solutions have never been electrolyzed, it seemed possible that this might be a means of preparation of metallic beryllium that had been overlooked by previous workers. The chief objection to the use of liquid ammonia as a solvent is the difficulty of handling the solutions which must be maintained in an air-tight apparatus at a low temperature. A new solution must be prepared for each experiment and prolonged treatment of one solution is impossible. A paste J. Phys. Chem., 35, 246.5, 2492 (1931).
* Weyl:
Pogg. Ann., 121, 601 (1864); 123, 50 (1864); Gore: Proc. Roy. SOC., 21, 140 (1872.3); Seely: Chem. News, 23, I69 (1871); h a n k l i n and Kraus: Am. Chem. J., 20, 820 ( 1898). Franklin and Kraus: Am. Chem. J., 23, 277 (1900): Kraus and Bray: J. Am. Chem. SOC.,35, 13x5 (1913); Cady: J. Phys. Chem., 1, 707 (1897).
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of solid carbon dioxide in ether or acetone is the best available cooling agent that will maintain the pressure of the solutions below atmospheric during electrolysis, but requires constant replenishing during the experiment. On the other hand, ammonia possesses the striking iQnizing properties characteristic of water and forms solutions of low resistance. Also ions travel much faster' in ammonia than in water thus improving the conductivities of salt solutions even if the concentrations are less than in water solutions. Its most important advantage in working with beryllium salts is the absence of hydroxyl groups which react with the beryllium salts to form hydrolyzed, slightly ionized compounds from which no metal can be deposited. The usual by-products of electrolysis of solutions in liquid ammonia are gases, Nz and Hp, so that there should be no troublesome residual products to contaminate the deposited metal. Thus ammonia offered a solvent of many advantages and it seemed possible that the difficulties formerly met in attempts to separate metallic beryllium by electrolysis of its salts in solution, might be overcome in this medium. Apparatus A special apparatus had to be devised for the electrolysis of solutions in liquid ammonia. In the first experiments the apparatus described in a previous article (Beryllium I, loc. cit.) was used. This, however, required a cooling bath around the tube containing the solution. The only cooling bath which would keep the ammonia condensed was a paste of solid carbon dioxide in ether which gives temperatures in the neighborhood of - 78OC. This cooled the solution far below the boiling point, of liouid ammonia and consequently lowered the possible conductivity of the solution. Since beryllium is a very light metal and requires considerable time for any weight of the metal to be deposited under even the most favorable conditions, it seemed advisable to maintain the solutions at the highest possible temperature. This was accomplished by allowing the ammonia to boil off from the solution and to be recondensed by a cooling agent kept above the solution. Cue to the bubbles of escaping ammonia gas this method also affords continual stirring. The apparatus consisted of a large tube (T) containing the electrolyte, a Dewar Flask (D) sealed above the electrolyzing tube, and suitable connections to source of ammonia, suction, manometer and current. -4pint vacuum flask was sealed to a long tube of 5 mm. internal diameter and connected by an inner seal to a tube 24 nim. in diameter. This larger t,ube carried two sidearms and a large conical joint ( J ) by which the electrolyzing tube could be attached. Platinum wires were sealed in at the two sidearms and were insulated by a glass capillary down to the point where the elect,rodeswere welded on. A mercury manometer (M) served to indicate the pressures prevailing in the apparatus. The whole apparatus could be maintained evacuated for any desired length of time. The tube (T) was always oven dried and the whole apparatus repeatedly evacuated and rinsed with dry air before use. 'Franklin and Cady: J. Am. Chem. Soc., 26, 499 (1904)
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I n the early work the lower end of the electrolyzing tube was merely rounded off like a test-tube but later it was deemed necessary to wash the deposited metal with liquid ammonia. This was accomplished by connecting the electrolyzing tube to a side flask (F), by means of glass tubing extending from the bottom of the electrolyzing tube to the upper part of F. Since this connecting tubing was not cooled during the electrolysis the ammonia gas which volatilized from the surface of the liquid in the connecting tube into the side flask gradually expanded and caused violent bumping as it was forced back through the solution by its own increasing pressure. The low temperature of the liquid ammonia solution made it dangerous to transfer
FIG.I the residual liquid through a stopcock after the completion of a run so a mercury seal was designed to close off the connecting tubing and side flask during electrolysis and yet leave an easily opened exit for the residual solution and the subsequent liquid ammonia washings. The mercury seal consisted of two small ground joints, (G), into which ground conical floats fitted snugly when pushed into position by the rise of mercury. These valves were joined by a U-tube of 7 mm. glass and connected through a T to a stopcock (C) which regulated the flow of mercury supplied by an auxiliary leveling bulb. The conical floats were loaded with mercury but were often violently driven into place by the sudden boiling of the ammonia solution as it was transferred. TKey were later filled with powdered metallic iron and held in position over tiny glass-point supports at the bottom
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HAROLD SIMMONS BOOTH AND GILBERTA G. TORREY
of the tube by means of an external magnet. The electrolyzing flask, mercury seal and side flask were sealed together as a unit which could be placed in the oven for drying. This unit was attached to the rest of the apparatus through the conical joint and a flat joint (not shown in the cut) on the residue flask. Platinum electrodes were used throughout the work. The anode was a small piece of platinum sheet rolled into a small cylinder and insulated from the large platinum cathode which surrounded it by a glass spiral. Although the electrodes were placed as close as two or three millimeters, the glass spiral prevented short circuiting between the electrodes and yet offered very little hindrance to the passage of current.
Solvent Purification. Solutes Dry ammonia from a tank (A) of the pure gas was passed through a four foot tube of quick lime (L) and condensed directly over the sample to be electrolyzed. In their determinations of solubilities of inorganic salts in liquid ammonia, Franklin and Kraus (loc. cit,.) reported the sulfate, basic carbonate, chloride and oxide t o be insoluble. Fluorides, sulfites, phosphates are in general insoluble so the selection of a suitable salt for solution in liquid ammonia was quite a problem. The iodide and bromide' are rather hard to prepare. hnhydrous beryllium chloride, nitrate and acetylacetonate were prepared as described in Beryllium I. To prepare the double fluoride of beryllium and ammonium,* beryllium basic carbonate was dissolved in pure hydrofluoric acid, the calculated amount of ammonium fluoride added, and the mixture evaporated in a platinum crucible in a stream of dry carbon dioxide. The resulting double fluoride was crystalline, and was preserved in a desiccator over phosphoric anhydride until needed. Practically all the beryllium salts are extremely hygroscopic and must be handled in a dry atmosphere to prevent contamination with the products of hydrolysis. Other investigators have reported several ammoniates3 of beryllium chloride which Franklin and Kraus, under the conditions of their method of determining solubilities, could not have detected. However, no one had noticed any definite solubility of the chloride in ammonia. Since this was the most easily available anhydrous salt further trial of its solubility in liquid ammonia was undertaken. Even if the salt dissolved in very small quantity it was thought that it might furnish a sufficient number of ions to produce metallic beryllium on electrolysis. The solubility of the double fluoride had never been determined but it was hoped that this salt also might be a suitable salt for electrolysis. The acetylacetonate is one of the few salts of beryllium that is not readily hydrolyzed and so is most Satisfactory from the standpoint of ease of handling. Beryllium nitrate dehydrated by heating in a bath of fused ammonium nitrate was found to be a satisfactory solute. Leheau: Compt. rend., 126, 1272 (1898); Ann. Chim. Phys., ( 7 ) 16, 497 (1898). Lebeau: Ann. Chim. Phys., (7) 16 489 (1898). Mieleitner and Steinmetz: Z. anorg. Chem., 80, 71 (1913); Ephraim: Ber., 45, 1322 (1912); Lebeau: loc. cit.
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Behavior of the Various Salts in Liquid Ammonia
For the first tests of the solutions in liquid ammonia the simple apparatus described in the preceding articles was used. Ammonia was condensed over samples of the salts and the conductivity, electrode products, and general behavior of the solution determined before trying the experiment in the larger apparatus. I. Solutions of Beryllium Acetylacetonute. This salt dissolved slowly in liquid ammonia. When electrolysis was first started the solution carried 0.2 A/sq. dm. at IO volts potential but this value slowly increased during three hours to a value of 2 . 0 amperes. After three hours further electrolysis the conductivity dropped to the initial value. .At first a small amount of gas was evolved at the anode, later copious evolution of gas was observed at both electrodes and gelatinous, brown precipitate was deposited at the cathode. There was no indication of deposition of metal on the cathode. 2. Solutions of Beryllium-ammonium Fluoride. Solutions of this salt in liquid ammonia carried only 0.015 amperes/sq. dm. with I O volt potential. With 11; volts potential there was much gassing at both electrodes apparently due to boiling solvent. The conductivity rapidly increased from 0.2 amperes/sq. dm. to 4.70 amperes in the course of five minutes. There was a slight indication of deposition of metal which formed a gray fog on the walls of the tube but the electrolysis had to be discontinued because of the excessive gassing at this high voltage. 3 . Solulions of Beryllium Chloride. Ammonia was condensed over pure, anhydrous beryllium chloride' which absorbed a considerable amount of ammonia before any liquid phase appeared. The volume of the salt increased markedly due to formation of the ammoniates. With I O volts potential the solution carried 0 . 2 amperes/sq. dm. but this value rapidly increased to 2.5 amperes, which was the constant value maintained during the rest of the run. Much gas was evolved at both electrodes and a Mack material deposited on the cathode to which it adhered loosely. The ammonia was evaporated off and the tube filled with absolute alcohol. This dissolved the residual beryllium chloride leaving the black deposit contaminated with some white transparent crystals not completely removed by repeated washings with alcohol. The black material readily dissolved in sulfuric and hydrochloric acids and in sodium hydroxide, with evolution of gas as long as it was in contact with the platinum electrode. The metal is brittle, adheres very slightly to the cathode, and under the microscope has a distinct metallic lustre and crystalline structure. The presence of ammonium chloride in the alcoholic solution was detected microsccpically on evaporation of the wash alcohol. The white crystalline salt, partly soluble in alcohol, is probably a product of the reaction of beryllium chloride with ammonia but its constitution was not determined. When E. C. Franklin was Visiting Professor in the Morley Chemical Laboratory, the senior author called our observations t o his attention and supplied him metallic beryllium. H e initiated studies on the halides and similar salts of beryllium in liquid ammonia. This work was continued on his return to Stanford University and published by F. I$'. Bergstrom: J. Am. Chem. Soc., 50, 652, 657 (1928).
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Since the solutions of beryllium chloride in liquid ammonia seemed the most suitable they were tested on a larger scale in the specially designed apparatus. A sample of beryllium chloride was introduced into the well-dried apparatus, the electrodes adjusted, and the system evacuated for several minutes to remove all permanent gases. The refrigerant was placed in the inner vessel of the Dewar flask (D) and dry ammonia passed slowly into the apparatus from the tank. The gaseous ammonia rose through the innersealed tube into the evacuated space between the two walls of the Dewar flask, was condensed, and dripped down on to the sample of chloride contained in the electrolyzing tube. Ammonia evaporated from this until the tube and chloride were cooled to the temperature of boiling ammonia and then it collected around the chloride. The chloride slowly swelled due to absorption of ammonia. The solid beryllium chloride slowly dissolved but not with sufficient speed to indicate ready solubility. During electrolysis ammonia constantly boiled off from the solution, passed into the Dewar flask and was recondensed into the solution. The drops of liquid ammonia ran down the spiral which was used to insulate the electrodes, and thus prevented splashing of drops into the solution and loss of salt on the walls of the tube. The copious gassing around the electrodes was due to boiling ammonia as there was very little non-condensable gas formed during the electrolysis. From 388 cc. of gas obtained during an experiment only three cubic centimeters of gas were left after absorption in sulfuric acid; from 461 cc. about 4.5 cc. were left unabsorbed. The small amount of gas made impractical a determination of its nature. This electrolysis had been run a t high current densities so it is probable that some hydrogen had been give off at the cathode. On completion of electrolysis the mercury seal was lowered until the mercury stood just below the level of the connecting U-tube in the seal. A11 s t o p cocks were closed, the ether-solid carbon dioxide refrigerant was brought up slowly around the side flask (F) and the liquid ammonia solution siphoned over as a result of the condensation of the ammonia gas. Violent agitation of the conical floats was obviated by insulating the connecting tubing and by holding the cones securely in place by means of a magnet. After the solution had been completely drawn over, the stopcocks to the back line of the apparatus were opened and the ammonia allowed to vaporize and pass back into the main apparatus where it was condensed by the upper Dewar flask and by a refrigerant around the electrolyzing flask. The electrodes were then soaked in the recondensed ammonia for fifteen minutes to permit solution of residual salt and the ammonia siphoned over into the side flask as before. This process was repeated until constant conductivity indicated the metal was clean. The ammonia was evaporated from the side flask, the apparatus was evacuated and dry air admitted slowly. The electrodes were then removed for examination and should have been free from any beryllium compounds that might have been formed by hydrolysis during exposure to air. Apparently ammonium chloride was formed during the electrolysis but it was readily soluble in liquid ammonia.
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Since the met'al tended to scale off from the electrode and to be carried away by the rush of the ammonia solution as it was transferred to the side flask, it was difficult to collect. Effect of Current Density on Deposit As it was difficult to collect completely the metal which was deposited on the cathode due to its lack of adherence several experiments were tried in an attempt to produce an adherent deposit of beryllium. It was thought that the current density might affect the nature of the deposit so one run was made at low current densities. Ammonia was condensed over beryllium chloride until the lower 2.j cm. of the cathode were covered with solution. The total exposed surface of the cathode was about 30 square centimeters. With a line volta'ge of z volts only one milliampere could be forced through the solution. When the voltage was maintained at 3.6 volts which is just about the decomposition voltage of beryllium chloride, the current density was 0.02 amperes/sq. dm. The run was continued for twenty-five hours. The mercury seal was then lowered, the solution of ammonia and beryllium chloride siphoned over, and the electrodes washed three times. After the ammonia had been recondensed and the liquid allowed to stand over the electrodes for several minutes the ammeter readings for the second and third washings were consistently two milliamperes so the metal was thought to be clean. On opening the apparatus the cathode was found only slightly coated with metallic beryllium. Apparently either the solution of ammonium chloride that is formed as the electrolysis continues is sufficiently acid to dissolve off the metal which is readily soluble when in contact with platinum, or the voltage used was so close to the decomposition voltage that very little metal was deposited. The lower part of the cathode carried metallic beryllium that, was perfectly clean. During another run the current density was maintained at 0.6 amperes,' sq. dm. for three hours. Successive washings of the metal gave ammeter readings equivalent to 0.1ampere, 0 . 0 2 and 0.006 amperes per square decimeter. This metal was contaminated with fragments of white material where the solution had splashed up from the boiling liquid. With high current densities the metal is deposited in small nodules that flake off the cathode easily. Apparently the current density is not the only factor that influences the nature of the deposited beryllium. Attempt to prepare Pure Metallic Beryllium Since the attempt to prepare adherent beryllium did not produce metal that would entirely remain on the cathodes, some other method of obtaining pure beryllium in quantity had to be devised. Since the metal tended to suspend in the ammonia solution as it is siphoned away after an experiment it was necessary to arrange some means of collecting the loose metal. A dried cellulose thimble was placed around the electrodes to catch the particles of metal that might drop from the cathode. The sample of beryllium chloride was placed outside the thimble with the ides that no material that had been
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HAROLD SIMMONS BOOTH ASD GILBERTA G. TORREY
hydrolyzed during the exposure to air could get in.to the thimble and contaminate the deposited metal. Ammonia was condensed over the whole surface of the cathode and the thimble. The total electrode surface at the cathode was about twenty square centimeters and carried I .3 amperes//sq. dm. at 8 volts potential. The electrolysis was continued for five hours. After electrolysis the thimble and electrodes were completely covercd with liquid ammonia and n-ashed four times with freshly condensed ammonia. The resulting metal was quite pure. Under the microscope only occasional very fine particles of white residue could be observed in the crevices of the metal. Physical and Chemical Nature of the Metallic Beryllium The metallic beryllium is deposited unevenly over the surface of the cathode as a dark gray or black coating. t-nder the microscope the metal appears in small, warty nodules. These are usually grouped in small clusters of varying size, and resemble heavy warty nickel deposited from aqueous solutions. The nodular fragments are very readily crushed and separate into tiny microscopic particles consisting of small crystals of undetermined form. Some authors have stated that the metal is readily malleable but the pure metal obtained by us is frangible. I t is possible that the metal, like chromium, occludes large quantities of hydrogen during its deposition and this prevents its cohering. Under the microscope the metal appears in small, highly refracting crystals grouped irregularly in small rounded lumps which are hollow and seem to have been produced around a nucleus by successive deposition layers. From its chemical behavior the metal is probably very pure. Previous authors have stated that beryllium is soluble in hydrochloric acid and in strong alkali hydroxides. The metal obtained by this method is insoluble in these reagents and only dissolves in aqua regia slowly on heating with repeated quantities of the acids. The only simple way to dissolve the metal is to place it in contact with metallic platinum and use concentrated hydrochloric acid in which the metal is then readily soluble. If a trace of impurity is present the metal dissolves readily in sulfuric acid, in hydrochloric and in sodium hydroxide with evolution of gas. In this matter of solubility the metallic beryllium which we have prepared resembles pure metallic zinc which likewise is very difficult to dissolve except in contact with a metal lower in the electromotive series. On this basis alone it seems likely that the metal is extremely pure. Obviously it cannot be contaminated with platinum and no carbon is present in the electrolyte. As far as we have found it does not react with pure water, or if so, extremely slowly. Determination of the Decomposition Voltages of Solutions of Beryllium Chloride in Liquid Ammonia An attempt was made t o determine the decomposition voltage of the saturated solution while the pressure of ammonia was continually varying but the temperature of the solution seemed to have a very noticeable effect
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on the decomposition voltage. Therefore it was necessary to maintain the temperature constant in order to obtain values that furnished a smooth plot for the decomposition voltage. The pressure of the solution was taken as a criterion of the prevailing temperature of the solution and was kept at a very definite value during two determinations of the decomposition voltages. l h e pressure of the solution was controlled by careful addition of small amounts of solid carbon dioxide to the refrigerant in the Dewar flask. An empty Dewar flask was kept around the electrolyzing flask to protect it from air currents but the solution was not cooled by any external means. point on the mercury manometer was marked and the readings taken only when the pressure of the solution was exactly at that point. One operator controlled the temperature while the other made the readings of the voltage and amperage. Readings were made for every tenth of a volt change. Determinations were made at a pressure of 3 j o mm. and jj o mm. The curve plotted from the values of the voltage and amperage at 3 j o mm. pressure showed no sharp break while that at j50 mm. gave a decomposition voltage break at 3.5 volts. As the voltage in another experiment was maintained at 3.6 volts and small amounts of metal were deposited this value is apparently very near the actual decomposition voltage of this solution. The slight break on the 3 j o mm. curve was in practically the same position. The temperatures of these solutions may be roughly calculated by comparing the pressures with those determined for liquid ammonia by Brill.' This gives a temperature for the 5 5 0 mm. solution of -4oOC. and for the 3 j o mm. solution of -48.8'C. These values are only approximate as the presence of the solute would somewhat alter the temperature for a given vapor pressure. Current Efficiency The current efficiency of this method of preparing metallic beryllium has not yet been obtained. The fact that some of the metal failed to adhere to the cathode made the ordinary method of determining current efficiency useless. The particles of metal were so small in some cases that they were readily carried away by the washings of liquid ammonia and lost. Solutions of Anhydrous Beryllium Nitrate and Ammonium Nitrate in Liquid Ammonia The meager solubility of beryllium chloride in liquid ammonia encouraged the trial of beryllium nitrate as solute. Beryllium nitrate, dehydrated in fused ?;H4S03as described in Beryllium I, slowly formed a heavy, viscous solution when S H 3 gas was condensed over it. With 8 volts the solution carried 60 A/dm?., but for extended runs the current density was reduced to 8 A/dm2. Smooth, adherent metal was deposited on the cathode. After repeated washings with alcohol minute traces of white material adhering to the metal remained and could not be removed. The metal while in contact with platinum dissolved slowly in hydrochloric acid and was tested microchemically, the 4.
Ann. Physik, 21, 170 (1906);Chem. Abs., I , 268 (1907).
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HAROLD SIMMONS BOOTH AND GILBERTA G. TORREY
test proving the absence of any possible transferred platinum from the anode and giving clear evidence that the deposited metal was pure beryllium. Excess ammonia gas may be allowed to evaporate from the solution until a solution stable a t room temperature is obtained. This solution is clear to slightly opaque and stable for extended periods if protected from moisture. More NHa may be condensed over it for further electrolysis or the heavy viscous solution may be electrolyzed at room temperature. This solution affords an easily controlled method of obtaining metallic beryllium.
Summary Metallic beryllium may be obtained by a new method in a high degree of purity by electrolysis of solutions of its compounds in liquid ammonia. The metal itself is so pure t'hat it can only be dissolved with difficulty in ordinary reagents. h special apparatus has been devieed for electrolysis at the boiling point of ammonia solutions and has been modified to permit the washing of the deposited metal by liquid ammonia. The decomposition voltage of the BeCL-NHa solutions has been measured and the effect on the nature of the deposited metal of different current densities investigated. Electrolysis of liquid ammonia solutions of beryllium nitrate dehydrated in fused ammonium nitrate and containing some of the latter, gave coherent, adherent deposits of metallic beryllium. Even when the solution became concentrated by evaporation to a viscous mass at room temperature, beryllium could still be deposited from the bath. Morley Chemical Laboratory, Western Reserve I-niversity, Cleveland, Ohio.
