PREPARATION OF ALKALI METAL HYDROXIDES BY ELECTROLYSIS
967
T H E LABORATORY PREPARATIOK OF .4LKALI METAL HYDROXIDES BY ELECTROLYSIS A . F. WISSLOW, H. A. LIEBHAFSKY,
AND
H. hf. SMITH
Research I,nborntoi,!l, General Electric Company, Schenectady, New York
Received January 16, 1947
From time to time chlorides of cesium and rubidium which have long been in this Laboratory must be converted into ot'her salts, such as azides, chromates, or iodides. In the past, the carbonates served as intermediates for such conversions, and the carbonates were prepared either via the sulfate (chloride to sulfate to hydroxide to carbonate) or via t,he oxalate (chloride to nitrate to oxalate to carbonate). Both methods are cumbersome and time consuming. The former, moreover, is liable to give a carbonate contaminated with chloride unless a large excess of sulfuric acid is used-probably the formation of acid sulfates interferes with the elimination of hydrogen chloride. The preparation of the hydroxides by the electrolysis of alkali metal chloride solution in an oscillating mercury cell seemed to olinr an att*ractive way out of these difficulties. In view of the industrial importance of this process, we were surprised to find i,elatively little detailed information about cells of the Castner-Kellner type. Fetzer (2) electrolyzed sodium hydroxide in such a cell to eliminate the carbonate. .I cell resembling his, escept t,hat the joints were fused, was accordingly constructed, studied, and finslly operated to prepare the hydroxides of cesium and rubidium from their chlorides. Since electrolysis of the chlorides involves special problems, preliminary ivork \vas done on sodium chloride so that the apparatus could be modified where necessary. In its final form, the cell yielded hydrosides of acceptable purity, but two electrolyses were necessary. S o attempt IYW made to exclude carbon dioxide. EXPERIMESTAL DETAILS
The various parts of the apparatus are shown in figure 1 (more exact specifications are given on G. E. Drawing M-5917537, dated October 12, 1939). The dimensions are not critical. The troughs to seal the compartments are about 8 mm. Tvide and 4 mm. deep. The apparatus must meet these requirements: (1) Throughout the oscillations, there must be effective sealing by the mercury to prevent the passing of olution from one chamber to another. ( 2 ) In order to reduce the oxidation of mercury in the center compartment, there must be provision for shunting the current past the nickel-mesh cathode (see figure 2). (3) Cooling must be adequate (temperature always below 40°C.) to minimjze chlorate formation.
Electrolysis of the chloride The clean glass cell is firmly clamped to the table, which can tilt through 15", asbestos pads and tape being used a t all points of contact to reduce strain.
968
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I n order to minimize creeping of the cnhloride solutions, a Iieavy layer of stopcock greasc is applied to the cell inside and out over bands estending 2 mi. clowv~i from the top. The cam is i w d v e c l to give masimum tilt; &an mercury is then poured into the lowest compartrncnt rintil the liquid extends 2.5 ern. beyond
FIG.1. The apparatus
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Mercury (see note) (see notej 1 Pyres 5OA3IC 1 Fibre 3 I'yr~x 50A3E 1 Nickel 1 Nickel (18 mesh) 2 Graphite AllABA 1 Pyrex50A3E 1 Steel (see note)
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>I>..TERI ALS
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I'*YTI
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1 3 husili,rry clectrodt!
12 Wiring diagram 11 Insulaticrti tube
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10 Hrackct 9 Cooling w i l 1
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8 Cathode by-paau 7 Cathode screen 6 Anode
5Tank 4 I3ase-i" x 7" s 21" channel 3 Cam 2 Drive 1 Assenibly
-
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the upper seal. Finally, the cell is leveled; the nickel-mesh cathode and the nickel shunt-rod are positioned ; water and current connections are made. The charge of solid chloride is divided, half being placed in each end compertment. With the cell a t rnasimuin tilt, distilled water is poured into the upper-
PREPARATION OF ALKALI METAL HYDROXIDES BY mwmtoI,YsIs
969
most compartment until one-half of the lower surfitc-e of the graphite anode is immersed; the other end compartment is then elevated and treated likewise. (With the anode thus exposed, the escape of chlorine during electrolysis is facilitated; this reduces chlorate formation.) When the center compartment has been filled to aborit two-thirds its depth with distilled water, the cell is ready t o operate. (In a typical run the charge was 2400 g. of cesium chloride; of this, enough remained undissolved t o give layers 2 cm. deep on thc mercury.) The motor is nom started. The clectrolysis circuit is closed with the mesh cathode disconnected and the estcrnal resistances adjusted t,o give 22-25 amp. through the shiint-rod. After aborit 20 min.. the amalgam will be sufficiently concentrated to warrant connecting the mesh cathode so that the electrolytic oxidation of the dissolved alkali metal can begin. In the initial stages of this oxidation the cell is usually quite unstable, owing to the low conductivity of the solution in the center compartmmt. This instability may be compensated by increasing the proportion of shnntrd current diuing this short initial phase, but
FIG.2 Provision for shunting the current past the nickel-mesh cathode. For connections. see assemhly.
it is better to eliminate it by charging the center compartment with dilute hydroside solution. After. the initial period, the current is adjusted for stable operation, about onefifth of the 22-25 amp. being shunted through the nickel rod. Beyond replacing part of the hydroxide solution with distilled water every 8 hr. or so in order t o reduce attack of the glass. the apparatus should require little attention until qualitat,ive tests show that virtually no chloride remains in the solutions in the end compartments. 21t this point the total current is lowered to 10 amp., of which half is shunted. When the appearance of an oxide film on the mercury in the center Compartment shows that little dissolved alkali metal remains, 0.7 of this total current is shunted for 10 min.; then all of it for 10 min.; and so on until all the alkali metal has bcen oxidized, this point being established by shaking samples of mercury from an end compartment with distilled water and phenolplithalcin. The cell is then disconnected. At the end of the run, the solution from the end compartments is usually black with suspended graphite. It cmtains most of the impurities from the
970
A . F. WINSLOW, H. A. LIEBHAFSKY, AND H. &I. SMITH
charge and the chlorate ion (about 4 g. from 2400 g. cesium chloride in a typical case) that has been formed. The center compartment usually seems filled with mercury oxides. The solution and these oxides are sucked out, and the compartment is washed repeatedly with distilled water, the washings being combined with all the hydroxide solutions previously removed.
Electrolysis o j the hydroxide The remaining chloride (in one case, about 1/25 of the charge) is removed by a second electrolysis. After all the mercury oxides have settled out, the crude hydroxide solution is evaporated to a volume that the end compartments can accommodate. For this evaporation, platinum is preferable; nickel (or certain other metals) might do; glass or porcelain introduces risk of contamination. The chief reason for concentrating the solution is to avoid the necessity of withdrawing dilute solutions (from which cesium or rubidium vould have to be recovered) during the run. When hydroxide is being electrolyzed, troublesome “crusts”, rich in alkali metal, form on the mercury surface in the end compartments. The tendency to form these crusts, which probably varies inversely with the solubility of the metal in mercury, was much more pronounced with cesium than with sodium. The crusts will not pass into the center compartment, with the result that the mercury there, being soon stripped of dissolved metal, becomes liable to oxidation. Crust formation was reduced by ( 1 ) using more mercury to increase turbulence during tilting, (2) stirring the mercury in the end compartments, and (3) decreasing total current and increasing shunted current to reduce oxidation of mercury. The second electrolysis yields a hydroxide solution, usually near 1 N , that is practically free of chloride but contains some carbonate. (Carbonate formation, as Fetzer has shown, can be prevented by covering the cell to protect it from the atmosphere.) As before, suspended mercury oxides must be removed by settling, centrifuging, or filtering. Since the alkaline solution (cesium hydroxide especially) attacks glass, it should be stored in a container of an inert metal or-if permissible-onverted to the solid carbonate for storage.
Purity Considerable purification of the charge ought to result from a double electrolysis. Non-metals should not pass into the mercury. The low concentration of cation impurities militates against the electrodeposition of the corresponding metals even though the standard electrode potentials favor it; moreover, some of these cations tend to be removed as insoluble hydroxides along with the mercury oxides. On the other hand, an increase in sodium content owing t o the attack of glass is to be expected. The results bear out these expectations. Once, with 2400 g. of cesium chloride, the solution from the end compartments contained about 4 g. of calcium ion after the first electrolysis, and a negligible amount after the second. In another case, cesium hydroxide was re-converted to the chloride, which was compared
PREP.iRAT1OK OF AI,KAI,I MhT.LL HYDROXIDES HY ELECTROLYSIS
971
spectrographically with the starting material. The results follow, those for the starting material being given first: aluminum, present, trace; calcium, low, trace; magnesium, very low, trace; sodium, very low, present; silicon, trace, trace: copper, not found, trace. (“Present” indicates 8 greater amount than “low”.) In every case, the cesium and rubidium salts made from the hydroxides prepared in the cell were pure enough for the applications to which they were put. Had the presence of the sodium derived from glass been objectionable, we should have built the cell of: or lined it with, an alkali-resistant materialperhaps a plastic.
Electrical qficienc?j S o attempt was made to attain maximum electrical efficiency, \vhich ivould have entailed operating a t loiv current densities with a minimum of shunted current, lowered the output, and increased the attention required to prevent oxidation. The results in table 1 were obtained in a preliminary investigation. h current of 20 amp. was selected as a reasonable compromise current; additional experiments showed that the shunting of one-fifth of this current during steady operation was satisfactory from the viewpoint of attention required. TABLE 1 Elsctrical efficiency in electrolyses of sodium chloride
so. . , ... . .... . . . .... Total current (amp.) . ...... Time of run (hr.). . . ... ,
1 10
1
2
111 5.7
1
3 20 1.8
1
4 20 5.0
1
5 1 6 30 : 3 0 0.5 1.5
theoretical; the chromate had 99.8 per cent of the theoretical oxidizing power, which was determined iodimetrically by the method of Bray and Rliller (1). Rubidium chromate was prepared from the hydroside and chromic oside with an over-all yield of 81 per cent,. srlIJl.\ ItT
To facilitate the preparation of cesium and rubidiuin salts from their chlorides, a laboratory-scale oscillating mercury cell has been constructed and operated to make available the pure hydroxides for these preparations. Operating instructions and data are given. Two electrolyses proved necessary to produce a hydroxide virtually free of chloride, other impurities also being removed to a considerable degree, Toward the end of an electrolysis, the cell requires dose attention if efficient operation is desired. Care is necessary throughout in order to minimize losses whenever expensive materials are being processed. The chief drawback encountered was a contamination-not serious in the present case-of the hydroxide by sodium ion derived from glass. This contamination could be prevented by building cell and containers of more alkaliresistant materials. In this work the hydroxides, carbonates, and chromates of cesium and rubidium were prepared, as mere also cesium azide and dichromate. REFERENCES (1) BRAYAND MILLER: J. Am. Chern. SOC. 46, 2204 (1924). (2) FETZER: J. Phys. Chern. 82, 1787 (1928).
HEAT GUARL) FOR THE McBAIS-BAKR SORPTION BALANCE R E N E D . ZENTNERI- 3 Department of Chemistry, Stanford University, California Received March 11, 1947
Of the numerous techniques presently utilized in the measurement of the weight changes of various materials during sorption, probably the simplest, most reliable, and most accessible is that known as the McBain-Bakr sorption balance (2). By the proper manipulation of this method, isobars, isotherms, and isosteres of various two-component systems may be conveniently, accurately, and reproducibly studied. In general the sorption characteristics of such relatively stable materials as soap (3), textiles, and inorganic materials (1 ) have been investigated, with respect to both water and liquid hydrocarbons. Lever Bros. Company Research Assistant to Professor J. W. McBain. Present address: Patent Division, Plastics Department, E. I. du Pont de Nemours and Company, Arlington, New Jersey. 1 2
