Sept., 1961
ELECTROLYSIS OF SODIUM AMALGAMS
results (as suggested in other studies)15 from a photosensitive couple of oxygen and the A1208 crystal lattice inhomogeneities such as aluminum vacancies and dislocations. The shift of the lesser peak to a higher energy could, perhaps, be traced to the direct involvement of trace impurities, after the crystal is exposed to solar energy, and latticr?strains. All these samples used above were subsequently treated differently. For a more detailed analysis, data will be obtained from a solarized, non-oxygenated sample; also, data with respect to the decay with time of the color-centers induced by solar energy will be of great importance to the analysis. A microscope examination of the sphere specimens which were used in the above experiment showed
1549
a small amount of cavitation appearing in the crystals exposed to oxygenation and solarization. Cavitation also was produced in straight-rod samples which were merely flame polished. A better controlled study should show to what extent each treatment is responsible for the cavltation and it should help in the understanding of the role that cavitation has in influencing the formation of color centers. Acknowledgment.-We thank the National Science Foundation for supporting this work, Mrs. B. Staker for assistance in calculations and Miss Nola McKee for assistance with the calculations, preparation of drawings and the manuscript. (15) (a) F. P. Claike, P h d . d l a g . , 2, 607 (1957): (b) R. Chang, Report NAA-SR-3339, Atomic8 International, a Division of North American Aviation, Inc., Nov. 13, 1958.
TIIE ELECTROLYSIS OF SODIUX AJfL4LGA?dS’ BY JOHNC. ANGUS*AND EDWARD E. HUCKE Departtr~entof Chemical and Matallurgwal Engineermg, Unaversily of XLchrgan, Ann Arbor, MLchigun Receaued March 16, 1061
The electrolysis of 0.097 and 0.485 weight yosodium amalgams was studied a t temperatuies up to 344” Below approumately 290’ the sodium is transported to the anode; above 290” thc direction of transport reverbes and the sodium niigratrs to the cathode. This effect, which has heretofore never been observed, is postulated as caused by the tiiermal deconiposition of “compounds” or associations which persist in the liquid amalgam.
Introduction Kremann3 showed that when Na amalgams were electrolyzed, the direction of transport of the Ka and Hg depended on the concentration of the amalgam. That is, a t 240” for Na concentrations greater than about 2.0 weight 70E a , the Na migrates to the cathode; when the Na concentration the Ea migrates to the is below 2.0 weight anode. This curious effect has been observed in the K-Hg,4 Ba-Hg4 and Ea-ICi6 systems. I n each case the concentration of the reversal occurs at, or very close to, a composition where compounds are present in the solid phase. Moreover, in each case the component in stoichiometric excess of the compound composition migrates t’o the cathode. It has been pointed out6 that in 38 of 40 binary and ternary alloy systems, the component with the smallest atomic mass moves to the cathode upon electrolysis. On the basis of this empirical correlat’ioii the authors postulated that the reversal was caused by “compounds:” or associations, existing in the liquid metal. I n the Na-Hg (1) Taken f i o m a portion of t h e dissertation submit,ted by John C. Angus t o t h e Rackham Graduate School, University of hlichigan. Ann Arbor, Michigan. in partial fulfillment of t h e requirements of t h r Ph.D. degree. (2) Minnesota Mining a n d Manufacturing Co., St. Paul, blinnesota. (3) R. Kremann, A. Vosrin a n d H. Scheibel, Monatsh. Chem.. 57, 323 (1931). (4) For a review of these experiments see K. Schwara, “Elektrolytische Wanderung in flussigen und festen Metallen,” J. A. Barth. Leipaig, 1940. (5) S. I. Drakin a n d A. K. Maltsev. Z h w . Fie. Khim., S1, 2038 (1957). (0) J. C. Anfius, J. D. Verhoeven a n d E. E. Hucke, Paper presented t o t h e International Symposium on t,he Physical Chemistry of Process Metallurgy, A1:ME. Pittsburgh, 1959.
system, for example, if an undissociated species Ka,Hg, exists 111 the liquid state, one mould expect to find predominantly ?ia,Hgm and Na on thc high S a side of the “compound” composition and NanRg,, and Hg 011 the high Hg side. The correlation then suggests that in these cases a reversal should occur in the direction that is, in fact, observed. If this interpretation is correct, one might expect the reversal to vanish at sufficiently high temperatures if the associations are broken down by thermal agitation. In the present work t n o amalgams (0.483 a n d S a ) that show transport of the ?;a 0.097 w i g h t to the anode at 240” were electio!yzed at temperatures approaching the normal boiling point of the amalgams. Experimental Materials.-Mallinckrodt analytical reagent grade KC'^ with a reported purity of 99.9% and Rlerck and Mallinckrodt A.C.S. Reagent grade niercury was used without further purification. Preparation of Amalgams.--Amalgams of known comgosition were prepared in a glove box under dry nitrogen. odium metal was added to previously weighed polyethylene bottles which were then tightly stoppered, taken out of the dry box, and reweighed. The Na and enough FIg to make the desired amalgam composition were then brought back into the dry box where the amalgamation was performed in a Pyrex beaker. At, room temperature the amaigams could be handled in air for short periods of time with negligible scum formation. Equipment and Procedure.-The electrolyses were carried out in thin walled Pyrex tubes approximately 15 cm. long and with an inside diameter of 0.075 cm. The tubes were closed a t one end and open a t the other. A 0.008 inch diameter tungsten electrode wire Tas Realed into the closed end. The electrolysis tube was situated within a cell consisting of a vertical, 60 mm. diameter Pvrex tubr which was scaled at the bottom. The necessary vacuum and elec-
1550
JOHN C. ANGUSA N D EDWARD E. HUCKE
trical connections were made through a brass fitting att,ached to the top of the large tube. A large, approximately 2000 g., reservoir of amalgam was placed in the bottom of the large tube. The apparatus was designed so that the electrolysis tube could be moved vertically without affecting the vacuum within the cell. To make a run the electrolysis tube was raised above the reservoir while the amalgam was stirred to ensure uniformity and thoroughly degassed. The cell was evacuated to a pressure of approximately 5 X mm. and the capillary lowered below the surface of the amalgam. (The amalgams used in this work are completely liquid a t room temperature.) Helium was bled into the cell until the absolute pressure reached from 25 to 45 p.s.i. The helium pressure served two purposes. First, it forced the amalgam into the capillary tube where it made contact with the electrode a t the far end. (The other current electrode was immersed in the amalgam reservoir .) Secondly, positive prrssure reduced the tendency of small bubbles to nucleate in the electrolysis tube. These bubbles would grow in size and n-auld eventually form an open circuit. The temperature was raised to the desired level by means of a resistance furnace and the direct current turned on. After the current was passed through the capillary for t'he desired length of time, it was turned off and the tube immediately raised from the reservoir. Several extra tubes through which no current was passed mere attached to each electrolysis tube. These tubes were for the purpose of collecting blank samples of the reservoir of amalgam. All of the tubes were chemically analyzed to determine the direction and t,he extent of electrolysis. The direct current was supplied by Udylite and Sobatron rectifiers. The drift during the course of the run and the ripple were less than 1%. Approximately 1 ampere of the total furnace heater current (from 3 to 5 amperes) was controlled by an off.on type controller for temperature regulation. The total temperature variation 2hroughout the couree of the runs was normally about 5 3 . To minimize convection the top of t.he liquid metal reservoir was maintained hotter than the bottom. The gradient was approximately 0.8 '/cm. Three capillary shapes were used: (I) straight tubes, (2) hairpin shaped tubes (one 180" bend), and (3) tubee with two 180' bends. All three shapes were used in a vertical position. During the preliminary experiments the apparatus m s modified so that three electrolysis tubes could be run simult'aneously. The tubes were electrically in parallel. It was helpful to run several capillaries a t once, since a t the high temperatures the capillaries often became open circuited because of t'he formation of bubbles mentioned earlier. Analytical Procedure.-The Na was determined flame photometrically at, 580 mp with a Becknian Model DU spectrophotometer. The S a was dissolved from the amalgams with 1% by volume IITO.,. The emission in a hydrogcnoxygen flame yae coniparetl vith the emission of previouPly prepared standard solutions. The analyses of the wwrvoir metal in all cases TverP n-ithin 393 of the weighed in values. The analysts illvariably m r c loivtr than the wcighcd in values. Thic; probably n-afi caused by the sliglit scum that formed on the amalgam during its formation and subsequent handling in air. Thc standard deviation of an individual analysis from a set of hlanli capillarics was normally less t,hari 170 of the li,veritge value. Discussion of the Experimental Procedure.-Properly titsigned experiments can tie iised to determine the magnitude of the electrical motdity of the solute atoms as well HS simply t'hc direction of transport,. Whcn the electric field is applitd to the liquid alloy, the solute atoms reach a steady state drift ve1ocit)y. Solute mill cross the reservoircaapillary houiidary. The flus of solute across this boundary plane per per sec. is given by the definition of the mobility
TTol.65
I the direct current,
p the resistivity, and t time. Sotice that the cross sectional area of the mouth of the capillary does not appear in equation 2. The experimental method used here is similar to the one described by Mangelsdorf.7 His method is not readily applicable a t high t,emperatures, hon-ever . Equation 2 will give correct mobilities a,s long as the concentration a t the mouth of the capillary remains constant and as long as the only solute flux across the boundary plane is caused by the electrolysis. The first condition is easily met by using a sufficiently large reservoir. The second is met by ensuring that the concentration change in the capillary does not proceed to the point where a concentration gradient is formed a t the reeervoir-capillary boundary. When this happens, solute is transported across the reservoir-capillary boundary by ordinary diffusion. This diffusion flux will always oppose the solute flux caused by the electric field, causing the mobilit,y calculated from equation 2 to be too lorn-. The concentration in the capillary first changes a t the closed end of the capillary. There will therefore be an initial period r h e n the average concentration in the capillary will change linearly with time. If the electrolysis is stopped during this period, equation 2 \vi11 give the true mobility. One can test vhether back diffusion takes place by performing experiments with different capillary lengths and for different, periods of time. A4nyexperiment with too short a tube or run for too long a time will give a mobility lower than the others.&
Results Test of the Experimental Technique.-The met'hod was tested by electrolyziiig t,he amalgams at room temperature. Schwarz4 has previously measured the mobility of Na in dilute amalgams at' room t'emperature by two independent methods. cm.?,!sec. volt. In t h t His value was 1.19 x present work the average of four runs on 0.485 weight % Na and two runs on 0.0976 weight T,S a amalgams was 1.23 X cni.2,1sec. volt,. The standard deviation of a single det'erminatioii was 0.20 X loM4. I n view of the rather large standard deviation the excellent agreement \Tit h Schn-arz's yalue is certainly fortuitous. Clearly, however, the method gives values that are essentially corrcct,. The scat'ter in t'he data arises from the fact that iii order to calculate the mobility, the analysis of the capillary tube must' be subtracted froin the analysis of the reservoir. Since the t,otal change in composition vas normally less than 10yG'any analytical uiicertaiiities were considerably magni. fied. The capillary shape had no apparent cff'cct on t h e results, nor was there any not,iocable difffrcii(~1in the results a t the tn-o diff erelit coiireiitratioiis. Sixty cycle alteriiatiiig current lras passcd through three capillaries. S o st,atistically sigiiificaiit changes were obserrcd. Results a t High Temperatures.---The data arc: shown in Table I. The eixt'h and seveiith coluniiis give t.he original and final coniposit,ions of t,hc capillary tube. The origins1 composit~ionis takeii as the average of the blank tube analyses. Thc: last column gives the per ceut'. confidence of thc ohserved change in alloy composition. It n-as takcii as 100 - P where P is the per cent. chalice that, a blank tube from that particular run could have J = UCE (1) given ail analysis as far from the mean of the blank where u is the mobility of tha solute in cm.2//sec.volt, c the analyses as the capillary in question. Student's concentration of the solute, and E the electric field strength. Ry applying the principlc of the conservation of ma,ss to "t" distribution for n - 1 degrees of freedom was used, where n is tjhe number of blank tiihw. l?or bhe eIPctrolysi? tilbo equation 1 may be rewrit,ten (7) 1'. Mangelsdorf, Jr., .I. Chern. Phgs., 3 0 , 1170 (19.3). ! 8 ) F'nr R more complete disoiisaion of the experiinmtal niptlirjil t l i e '211.1). dissertation of J. U. Angus, University of hlictiigan, 1
arailable
fruiii
L'nircrsity i?licrufilms, Ann h r b o r , Michigan.
1551
ELECTEOLYSIS OF Sor)Iunr AMALGAMS
Sept., 1961
TABLE I ELECTROLYSIS DATA Capillary
Temp., O C .
1 2 3 4 5 6 7 8 9 10 11 12 The sign (
+
D.c.,a amp.
$4.0 232 +3.4 245 $2.48 286 $3.75 342 -3.09 344 -3.40 343 -2.53 344 -2.42 338 -2.80 338 t2.44 334 $3.22 334 +2.95 334 or -) indicates the polarity of
Time, hr.
(9) P. Muller, "Die elektrische Leitfahiakeit der lletallegierungen im flussigen Zustande," Doctor's dissertation, Royal Technical High Scliool a t Aachen, 1911. (10) 0. Kuhasohewski and J. A . Catterall, "ThermochPmical Data of Alloys," Pergamon Press. London, 1956, p. 22. (11) K. Bornemann and P. Muller, iMetalluq7ie, 7, 396 (1910). (12) M. Hansen, "Constitution of Binary Alloya," McGran Hill Rook Po., N e w York, N. Y.,1968.
1.5 j
i
g
1.0
-
% Ka
\vt.
Ht
99.0 94.0 56.0 99.9 99.9 99.9 99.9 99.6 99.7 99.5 98.0 98.5
0.506 ,495 ,480 ,439 ,538 ,532 .512 ,514 ,521 ,0897 ,0934 ,0929
0 486 27.0 0.9335 474 59.0 ,9814 488 49.75 ,8338 480 19 .0 .9398 484 52.0 ,8643 484 31 . O ,8803 484 52.0 1,2730 482 48.8 ,8731 482 48.8 ,8783 0968 30.5 ,8269 0968 30.5 ,9398 0968 30.5 1.0332 the electrode at the end of the capillary.
the capillary run a t 286" the confidence level is the per cent, chance that the composition of any one blank tube lies as far from the mean as the capillary through which direct current m~tspassed. From these dat8aone can see that' the predicted reversal of transference direction was indeed observed. The room temperat'ure data and the mobilities calculated from capillaries 1, 2 , 3, 4 and 6 in Table I: are plotted us. temperature in Fig. 1. At 286", where no significant change was observed, the bar indicat,es the estimated sensitivity of the measurements. A positive mobilit'y means the Xa migratm to the anode; negative, to the cathode. In order to calculate the mobilit'y from equation 2 it, is necessary to know the resistivit'y of the reservoir amalgam and the concentration of Na in the reservoir in g . / ~ n i . ~The . mass fractions of S a were convert'ed to concentrations by multiplying by the density of pure Hg a t the temperature in question. The error introduced by not using the densities of the amalgams is negligible compared t'o the scatt'er of the dat'a. The resistivities were taken from nliiller.9 The capillaries not used in Fig. 1 had significant back diffusion which made a quaiitit,atim estima,t'ion of t'he mobility impohsiblc.x Discussion There is wpport from other type:: of trieasuwrneiits tor the idea that "compounds" do exist in liquid alkali metal amalgams. Kubaschemski and Catt#era.ll1Ocome to the conclusion t,hat there are "fairly strong io bonds" in the liquid K-Hg system on the b of the eiit,ropy of mixing of K and Hg. lIullergpll concluded from his resistivity meadurements that' the KaHg2 species may persist in the liquid state. The eompoinzds KaHgq Itlid SaHgzarr iorrncd at, 2.8 and 5.4 weight yo Sa.12 The former melts i~icongrueritlyand the latt'er congrucntlv. Conscqueiitly. oEe might expect NaHg, t,o be the species present in the liquid metal; hon-ever, the re-
Confidence level of change
Final
Origina! 70 h a
Wt. of capillary metal, g.
.
-. - .
T
X
x -1.0
3 .r=
-1.5
1
I
I
50
100
I
I
I
I
I i
150 200 250 300 350 Temp., "C. Fig 1. --Mobility of Na versus temperature: f , bchwwdi value; 0 , this work.
verbal of transport direction with concentration takes place at approximately 2.0 weight % S a . The associations or "compounds" that persiit in the liquid state may not correspond exactly to the (*ompositions of compounds in the solid. One would expect, however, that the composition of any association in the liquid metal to he in thc general region where strong compound forming tendencies are found in the solid. It has been p r e d i ~ t e d l ~that , ' ~ in dilute alloys the solute should concentrate a t the anode if it decreases the conductivity of the alloy compared with the pure metal, and vice versa. At 240" the Na-Hg system obeys this rule. At 340°, ho.wrvrr, the Na goes to the cathode, even though the conductivity decreases with increasing Ka coiiceiitration a t this temperature.3 Acknowledgments.-The authors gratefully arknowledge the financial assistance of the U.S. Atomic Energy Commission under contract AT(11-1)-771 and the invaluable assistance of ILlr. John Verhoeven in the experimental work. 113) P lIsngelsdorf, J r , paper presented t o t h e International Symposium on t h e Physical Chemistrg of Process Metallurgy, A I h I P Pittsburgh 1959 (14) F. Rkaupq, Z p h y s t k C h ~ m Sa, , ,560 (1907). Foi a review o f all of Skaiipl Is riorks RPC ref 4
