AN IN17ESTIGhTIOK OF T H E L A N T H A N J I AJIALGhhI ELECTRODE FOR PRECISE ELECTROlIOTIT'E FORCE MEASUREMENTS'
W.GEORGE
PARKS
AXD
RICHARD W. K I S G L R L E T
Department of C h e m t s t i y , Rhode Island State College, Kingston, Rhode Island Received J a n u a r u 15, 1958 IXTRODUCTIOZ
The purpose of this investigation was t'o determine whether a suitable galvanic cell involving a lanthanum amalgam electrode could be constructed for precise E . M . F . measurements. *% need for such measurements arises from the fact that, x i t h the exception of the work of Hakomori ( 7 ) on indium chloride, there has bceri very little data obtained on electrochemical cells involving salts of trivalent metal?. The more common trivalent metal salts, such as those of iron, chromium, aluminum, and indium, are highly hydrolyzed in aqueous solution and do not lend themselves to precise E.M.F. nieasurements. The addition of an acid to prevent hydrolysis introdwes a difficult correction factor when the data are treated theoretically. Attention is turned to lanthanum which, according to Ley (18), is not hydrolyzed t o any appreciable degree. If this metal can be drveloped into a reproducible electrode, it will give free energy measurements that may be eniployed to test further the interionic attraction theory of Debye and Huckel as extended by LaMer, Gronwall, and Greiff (14) for electrolytes of the uiisymmetrical valence type. Furthermore, if the temperature coefficients of the cell are obtained, certain thermodynamic quantitirs of the cell process can be calculated by means of the Gibbs-Helmholtz equation. The free energy change for the cell process, the entropy, and t'he heat capacity may also be investigated. Muller (21) carried out an investigation of certain electrochemical cells with lanthanum. He was interested primarily in the effect of varying the lanthanum content of the amalgam rather than the concentration of the salt. Furthermore, his cells contained a salt bridge, which introduces factors that are difficult t o treat theoretically. His data were not reproducible enough t o give significant results when subjected to a rigorous mathematical treatment. This article is based upon a thesis submitted by Richard W. Kingerley to the Faculty of Rhode Island State College in partial fulfillment of the requirements for the degree of Master of Science in Chemistry, June, 1937. 483
484
W. GEORGE PARKS A N D RICHARD W. KINGERLEY PREPARSTION OF MATERIALS
Lanthanum salts Several salts of lanthanum were investigated in order to obtain a very pure sample of lanthanum oxide. The most likely impurities in a sample of any lanthanum compound would be the rare earths, principally cerium and praseodymium. Cerium was tested for by the hypochlorite method of Baxter, Tani, and Chapin (4) and found to be absent in all samples. In testing for the presence of the rare earths, absorption spectra of several aqueous salt solutionswere taken. Lanthanum is the onlyone of itsgroup that does not give an absorption in the visible range (25). Any visible absorption was checked against the iron arc for wave length and then -4F.P. sample of lanthanum c2loride showed absorption bands identifie!. a t 4445 A., 4654-4667 A., and 4775-4789 A. in a 1 N solution through 13.5 cm. of solution on an Eastman 40 plate at 2 minutes exposure. This absorption corresponds definitely to that of praseodymium as described by Yntema (25), and indicates approximately 0.5 per cent. -4sample of lanthanum ammonium nitrate obtained from the Maywood Chemical Co. showed no absorption and therefore was selected as the starting material. Lanthanum oxide Approximately 200 g. of lanthanum ammonium nitrate was dissolved in half a liter of water, and the solution filtered free from any insoluble matter. To this solution was added slowly with stirring a saturated solution of oxalic acid until precipitation was complete. It was then stirred for an hour and filtered on a Biichner funnel with abundant washing to remove soluble salts. This precipitated lanthanum oxalate was then ignited in a platinum dish for 4 hours at a temperature of 6OO0C. According to Kolthoff and Elmquist (13), the oxalate is completely decomposed a t this temperature. The lanthanum oxide was preserved in a glassstoppered bottle and was used for the preparation of all lanthanum salts. Lanthanum chloride Forty grams of the prepared lanthanum oxide was suspended in water and treated with slightly less than the required amount of hydrochloric acid. The lanthanum chloride was then recovered from the filtered solution. Since the salt is exceedingly soluble in water and extremely deliquescent, the ordinary methods of recrystallization were not applicable. It was necessary to pump off the water in a vacuum desiccator and then to remove the water of crystallization by continued pumping for 5 days a t 90OC. (22). Analysis showed this salt to have one molecule of water of crystallization instead of seven, as when it crystallizes from water.
LANTHANUM AMALGAM ELECTRODE
485
Repeated attempts to remove this last molecule of water resulted in decomposition of the salt into a basic chloride (8). This decomposition took place either on prolonged heating at 90°C. or upon shorter heating at any temperature above 100°C. Therefore the LaCl8.HzO was used. It was preserved in a \glass-stoppered bottle sealed with paraffin.
Mercury Redistilled mercury was stirred with dilute nitric acid overnight and then twice distilled by the Hulett (9) method in a slow current of air.
A
STORAGE VESSEL
FIG.1. Apparatus for preparation of amalgams
Lanthanum amalgam The lanthanum amalgam was prepared by electrolysis of LaCh .HzO in absolute ethyl alcohol using a mercury cathode (2, 11). The solution was made by dissolving 40 g. of the salt in 100 cc. of alcohol. Any slightly cloudy appearance in the solution was removed by centrifuging it for a short time. The electrolytic cell construction is shown in figure 1. It was made by sealing a right-angled bend with a glass stopcock, as shown, into the bottom of a 250-cc. Erlenmeyer flask. A platinum wire was sealed into the glass bend for electrical connection to the pool of mercury. In this cell w&s placed the alcoholic solution, together with 600 g. of mercury. The flask was stoppered with a rubber stopper that admitted a platinum anode and an air-tight mercury-sealed stirrer for agitation of the mercury. The whole system was cooled continuously by running
486
K . GEORGE PARKS A S D RICHARD W. KINGERLEY
water. A potential of 50 volts wab applied, which allowed a current of 0 3 ampere per square centimeter of mercury surface t o flow. The water successfully kept the solution cool over a considerable period of time. The current was passed through the solution for 20 hours. Then the cell was tipped and the amalgam drawn through the stopcock into a bulb also shown in figure 1. The bulb was alternately eracuated and filled with nitrogen to remove 1a.t traces of oxygen, and then the amalgam set asidr until used. AJitrogen Tank nitrogen 1% as purified by passing it through potassium pyrogallate, sodium hydroxide, and sulfuric acid. The gas was then passed over heated copper gauze in a quartz tube to remove last traces of oxygen Finally, before entering the cell vessel it was passed over soda lime and calcium chloride, and then through a saturating column containing a solution of the same concentration as that in the cell. EXPERIMENTAL METHODS
Cell and electrodes
An attempt \vas made to construct a cell that would successfully measure the electrode process between lanthanum a i d one of its salt.. Lanthanum chloride appears to be the most suitable salt, as it can be prepared readily. The silrer -silver chloride reference electrode is standard and very reprodurible ( 5 ) Therefore it appeared logical to try first t o measure the cell La-Hg (2-phase) -LaClJ( ,n)--igCl -Ag
(1)
in TI hich the cell reaction is represented by La
+ 3hgCl $ LaCl3 -+ 3.4g
(11)
Lanthanum metal is very reactive and readily combines with oxygen t o form the oxide or with water to form the hydroxide. The amalgam is even more active. I t is therefore necessary to use some type of cell that will allon the lanthanum amalgam to remain in contact with the solution for only a brief period of time. The dropping electrode used by Lewis and Kraus (16). MacInnes and Parker (19), and Richards and Conant (24) wab adapted with some modifications. If the apparatus is successful it will aIlow measurements t o be made before any appreciable reaction takes place betn-een the amalgam and the solution. This dropping electrode is shown by h in figure 2. It consists of a tube of 30-cc. capacity t o which are sealed four stopcocks; three of them are sealed in short arms on the top of the electrode and one is sealed in the bottom. The lower stopcock is part of a long capillary tube which ib bent in the form of a hook at the bottom Into this capillary tube and just bplow the lower stopcock is
LANTHANUM AMALGAM ELECTRODE
487
sealed a platinum wire with a connecting tube for electrical contact. When this electrode is in use, the bulb can be filled with amalgam and placed in the cell vessel. By turning stopcock 1 a drop of amalgam can be balanced upon the upturned end of the capillary tube, and, as electrical connection is provided for through the amalgam column, the potential developed between this drop and the standard electrode can be measured. The electrode vessel, shown by B in figure 2 and also by C (top view), is composed of a central compartment from which extend three side arms.
FIG.2. Electrode vessel
Each of these arms, e, e', e", contains a reference electrode and a small glass tube, b, bent at 60' for introducing nitrogen into the cell. The tube, d, in the top of the cell is the exit for nitrogen gas. Having three reference electrodes serves as a convenient means of checking the constancy of each half-cell. The stopcock 5 at the bottom of the vessel is for removing the amalgam after measurements have been made. This prevents any reaction between the amalgam and the solution due to continued contact. The technique for filling the cell was as follows: B 0.03 molal stock soluT E E JOURNAL OF PRYeICAL CREYIBTRY, VOL.
42, NO, 4
488
W. GEORGE P.4RKS AND RICHARD W. KINGERLEY
tion of the lanthanum chloride was made up and diluted by volume to the required concentration for each measurement. After carefully cleaning and drying the electrode vessel, 100 cc. of this lanthanum chloride solution was introduced, and the three silver-silver chloride electrodes and nitrogen tubes put in place. A large stopper was placed in the top of the central compartment, and the cell placed in the constant-temperature oil bath while the dropping electrode was assembled. After being carefully cleaned and dried, the electrode was alternately filled with nitrogen and evacuated to remove oxygen. The amalgam was introduced into the tube by placing the amalgam reservoir in boiling water until it became one phase and then connecting it to the vertical arm of the electrode. Any oxide which formed on the surface of the amalgam was removed by filling the electrode through the fine capillary on the end of the reservoir. slight pressure of nitrogen was introduced above the amalgam to facilitate its flow through the capillary. The dropping electrode was then put into place in the cell vessel and the whole unit clamped firmly in the oil bath. Nitrogen was now bubbled slowly through each leg of the vessel for 20 minutes. At the end of this time the tubes were sealed by means of a short piece of rubber and a glass plug. The cell was allowed to remain in the bath for 2 hours before any measurements were attempted. In making measurements stopcock 1 was opened and the amalgam allowed to run down the capillary and form a single drop on the end. The potential was measured as soon as possible and repeated every minute for a period of about twenty minutes. At the end of this time another drop was formed and its potential measured against one of the other reference electrodes. This process was continued for a period of 3 hours and then the cell was discarded. Measurements were taken at eight concentrations and three temperatures. In many cases duplicate determinations were made to check the results.
Preparation and analysis of lanthanum chloride solutions
For all concentrations of lanthanum chloride less than 0.03 molal the solutions were made from the stock solution by dilution. The stock solution was analyzed by the method suggested by Jukkola, Audrieth, and Hopkins (11). The lanthanum was precipitated in a neutral solution as the oxalate and ignited to the oxide. It was possible to obtain triplicate determinations to check within 0.1 per cent. Several cases gave results to agree within 0.03 per cent. Other methods of analysis were tried, including the precipitation of the chloride as silver chloride and the titration of the precipitated lanthanum oxalate with potassium permanganate. These methods were abandoned, as they were not as reproducible as the oxide procedure.
LANTHANUM AMALGAM ELECTRODE
489
Analysis of the amalgam In the preparation of the lanthanum amalgam the electrolysis was continued for a sufficient time to produce two phases, The time was calculated from the solubility of lanthanum in mercury as determined by Parks and Campanella (22). As long as a two-phase amalgsm existed it was not necessary to know the exact concentration of lanthanum. However, each new supply of amalgam prepared was analyzed. This was accomplished by allowing a sample of amalgam to remain in contact with the air for a period of 10 days. The lanthanum separated from the mercuryas a white solid consisting of lanthanum hydroxide and carbonate. Ac cording to several investigators (2, 20) this separation is complete. To the precipitate 0.1 N hydrochloric acid was added and the liquid was brought to a boil. The solution was back titrated with standard sodium hydroxide and the per cent of lanthanum was calculated.
The apparatus For maintaining the cell at constant temperature a large oil thermostat with automatic heating and cooling coils was used. The desired temperature was maintained to f 0.05"C. The Beckmann thermometer was checked against a laboratory standard certified by the Bureau of Standards. The potentiometer was a Leeds and Northrup type K with a highresistance galvanometer, external critical damping resistance 2300 ohms, and a period of 3.0 seconds. The potentiometer was calibrated and checked against a certified Weston standard cell. EXPERIMENTAL DATA
Constant potentiometer readings were not obtained when the drop was formed on the electrode. A constant shift in the E.M.F. was observed, which increased with increasing temperature. After considerable investigation it was found that the maximum E.M.F. developed could be reproduced within 1 millivolt. I n table 1 is given an example of the measurements taken. A maximum E.M.F. was reached after approximately seven minutes at this temperature. However, in some cases the maxima were not reproducible and some showed several inflection points. All readings except the best were discarded. The cause of the drifting E.M.F. is undoubtedly due to the reaction of the amalgam with the solution. I n order to determine whether this maximum was of any value it was measured at eight different concentrations of lanthanum chloride at 0", 25", and 50°C. Then, using the equation
E: = E O
RT + Nu -ln n F
m
(111)
496
W. GEORGE P.IRKS AND RICH.iRD W. KINGERLET
F'ortalzon
os the
E ?I.?
TABLE 1 of the cell La-Hg(B-phase)-LaCls(m)-rigC1-Ag Temperature. 25°C : 0.0010 $1 LaC13
T I M E IN MINUTES
DROP K O .
2
D R O P NO. 4
1 5139 15311 1 5311 I 5368 1 5454 1 5572' 1 5560 1 5535 1 5540 1 5500 1 5524 1 5516 1 5483 1 5462 1 5439
3
1
8 9 10 11 12 13 14 15
wzih time
1 5133 1 5185 1 519'1 1 5251 1 5390 1 5548 1 55% 1 55591 1 5529 1 5489 1 5452 1 5439 1 5422 1 5110 1 5100
I I
* 3Iaxirnum
0".
t..
Lanthanum ama!gam.
1
1
25T.
I
0.055 per cent
0.018 per
cent
50°C.
,
___--_____ Eobl 1 E: 1 ____ ~
\IO1 4 L I T l
-
0 0 0 0 0
--
02162 01082 006187
004922 003510 0 001970 0 001032 0 005280
E:
Eobs
I 154 1 201 1 220 1 235 1 271 1 313 1 357 1 454
,
___ 1 034 1 059
-
1 060
'
1 068
1 091 1 118 1 151 1 217
0 02217 0 01125 0 006428
0 005115 003753 002048 001075 0005495 _ _
1
1 191
' 1 I
1 335 1 407 1 433 1 461 1 j21 1 558 1 586
~
I
' I
1 060 1 180 1 233 1 251 1 268 1 308 1 324 1 327
~-
~~
___ ___
Cabs
- --
E,
-
I
I
1
, 1
1
-
,
1
0 0 0 0
0.026 per cent
1 290
1 149
1 383 1 161 1 502 1 536 1 628 1 678 1 716 ____
1 216 1 371
,
1 3M 1 329
1 397 1 423 1 437
-
\allies for EL were calculated and plotted against the square root of the concentration. The results are shoan in table 2 and figure 3 The curves i h ~ ~ini figure ~ i 3 should he parallel, fince the difference at any given con-
491
LAXTHANUM AMALGAM ELECTRODE
centration should be governed entirely by the temperature coefficient of the E.M.F. However, the curves are not only not parallel, but at 0°C. the curve approaches the potential axis at a very high value, if not infinity. It follows from this that the cell process (11) m w t be complicated by a side reaction. After careful investigation it was found that in addition to the true cell potential, the hydrogen overvoltage was being measured. This conclusion was reached from the fact that a drop of amalgam on standing a short time in contact n-ith some of the lanthanum chloride solution would change the pH of the solution, especially in the immediate
n
U
im
FIG.4 FIG. 3 FIG.3. Plot of values for E: against the square root of the concentration FIG. 4 . Electrode vessel
area of the amalgam, from 5 . i to 8.0 or slightly above. This was followed over a longer period of time by a slight evolution of hydrogen. STeither one of these processes could be brought about without hydrolysis, and in the cell would definitely cause interference with the cell process. It was necessary t o abandon the use of aqueous solutions and endeavor to find a suitable non-aqueous solvent for lanthanum chloride. ORGANIC SOLVENTS
The solubility of lanthanum chloride was determined in various organic solvents. To 25 cc. of the solvent 3 g. of lanthanum chloride tvai adclrd
492
W. GEORGE PARKS AI\'D RICHARD W. KINGERLEY
and the solution refluxed for 15 minutes. The solution was then filtered and the filtrate evaporated in a Pyrex dish. The residue, if any, was weighed. The results obtained are tabulated in table 3. Upon examination of these data it is seen that the solvents for lanthanum chloride fall into two definite groups: first, the aliphatic alcohols,and second, the aromatic tertiary amines. Acetone appears to be an exception to this rule. However, the solvent action of acetone is attributed to the difficulty of TABLE 3 Solubilitg of lanthanum chloride in carious organic solvents I
______
SOLYENT
'-
,
.I
1,4-Dioxane . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tributylamine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sitrobenzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acetone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Benzene.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dibenzylamine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I I Aniline. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chloroform . . . . . . . . . . . . . . . . ........... Carbon tetrachloride.. . . . . . . . . . . . . . . . . . . . . Quinoline. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Piperidine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pyridine . . . . . . . . . . . . . . . . . . . . . . . . Chlorobenzene . . . . . . . . . . . . . . . . . . . Pyrrolidine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Furfuran.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ether, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ethyl alcohol, ....................... . n-Propyl alcohol+,. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Ethylene glycol*, . . . . . . . . . . . Glycerol' . . . . . . . . . . . . . . . . . . . . . . . . . . . I.
I
i
,
' '
" "
'
::1 :' :
i :1 ~
SOLUBILITY
g r a m per liter
Insoluble Insoluble Insoluble Slightly soluble Insoluble Insoluble Insoluble Insoluble Insoluble Slightly soluble Insoluble Very soluble Insoluble Soluble Insoluble Too low boiling point Very soluble 300ZOO 42540" 166'20O 4&)40° 5030" 150'0°
-
* Private communication from Professor B. S. Hopkins. removing the last traces of water. Different samples of acetone gave varying results. Apparatus In the work with organic solvents the apparatus was reconstructed as shown in figure 4. Since purification of large volumes of organic solvents is cumbersome and sometimes difficult, the new cell had a capacity of only 30 cc. The cell vessel, B, had only two side arms, a and a', fitted with interchangeable ground-glass joints which were drawn down to small diameter to form a close fit with the reference electrodes. The top of each side arm contained a tube, b and b', which was used for passing nitro-
LANTHANUM AMALGAM ELECTRODE
493
gcn into the apparatus. The dropping electrode had a total capacity of 15 cc. It was held in place by a small stopper that fitted into an i n t w changeable glass joint. The glass joints made the assembling of the cell much easier and more rapid; this decreased evaporation and also the accumulation of impurities. The operating technique of the cell was similar to that of the original.
Alcohol cell As one of the classes of solvents for lanthanum chloride was aliphatic alcohols, this type was first investigated by using ethyl alcohol. Since water interferes with the cell reaction, anhydrous conditions were necessary throughout. Both anhydrous lanthanum ch!oride and absolute alcohol were prepared. The preparation of anhydrous lanthanum chloride by the dehydration of Lacla.7H20 was impossible, owing to the formation of the insoluble basic chloride (18). Therefore, it was necessary to seek some other method. Several were tried; the most satisfactory is that proposed by Reed, Hopkins, and Audrieth (23). Ten grams of lanthanum oxide was heated with 15 g. of ammonium chloride in a glass h u h to 25OOC. This temperature was maintained and the excess ammonium salt removed by pumping in an all-glass apparatus. However, the anhydrous chloride prepared in this way, contrary to the statement of Reed, Hopkins, and Audrieth (23), does not give a clear solution in water. The salt was dissolved in absolute alcohol, and the insoluble material removed by centrifuging. This solution was diluted to the correct concentration and introduced into the cell. The absolute alcohol was prepared by treating C.P. absolute alcohol with sodium metal and distilling slowly from an efficient column 120 cm. long. The first and last 150-cc. portions of the alcohol were discarded. E.M.F. measurements were made on the cell, La-Hg (2-phase)-LaC13(C2H60H)-AgCl-Ag The same shift in the E.M.F. was encountered here as in the aqueous cell, However, the maximum was reached in much less time, usually only 15 to 20 seconds being required. This made reading the cell very difficult and the results not very reproducible. TJpon investigation it was found that hydrolysis was taking place, even though extreme care had been taken to obtain anhydrous conditions. The pH of the solution would change slowly over a period of three t o four hours when in contact with lanthanum amalgam. Probably the following reactions took place :
+ 2La + 2(C~H60)3La+ 3Hz (CtHa0)3La + 3Hz0 e La(0H)s + 3CzHbOH 6CzHbOH
494
TV. GEORGE P I R K S .1ND RICHARD W. KINQERLEY
This type of cell with the aliphatic alcohols as solvent wab discarded on thi? evidence. If ultimately absolute condition\ were obtained, the pel1 c-ould still offer side reaction? in that the lanthanum would replace hydrogen from the alcohol.
Aromatic tertiary amine cell The vcoiid class of solvents for lanthanum chloride was that of aromatic tertiary amines The one most easily purified and most readily obtaiiied was pyridine. It also dissolves the lanthanum chloride to the greatest degree, thereby being useful over a wider concentration range. Double d t s of the rare-earth salts and tertiary amineq are reported (3, 12). However, double-salt formation is not particularly undesirable The formation of complexes of the Werner type is not likely to occur, since lanthanum salts do not form complex compounds with ammonium hydroxide. The pyridine was purified by distillation and then treated n i t h 100 g. of activated aluminum oxide for every 500 cc. of pyridine. After standing for 24 hours the mixture was distilled twice, and the fraction boiling at 115 3"-115.5"C. wa5 collected (1). When the cell was set up it was found that pyridine dissoll ed the 4lver chloride from the reference electrode. It was necessary either to find a silver halide that would not dissolre in pyridine or to saturate the pyridine with the silver halide and find the correction to be applied. If n-e draw an analogy between the solubility of the halide.: of silver in ammonium hydroxide and what we chould expect to find in pyridine, we should look for a lower solubility of the iodide, the solubility increasing to the chloride. The only reference in the literature mas that of Laqzczynski (lz), ~ ~ h o rpport. the solubility of silver iodide as 0 1 g. per 100 grams of pyridine at 10°C and 8.01 g. at 121°C. The solubility of silver chloride r a e found to be 1.9 g . per 100 grams of pyridine at 25OC., a value that appeared t o be greater than the solubility of the iodide at the same temperaturr The silver-silver bromide electrode was prepared by the method of Leais and Storch (17), and the iodide electrode by a procedure outlined by Jones and Hartman (10). The iodide cell could not be in\-wtigated, because it way found impossible to prepare anhydrous lanthanum iodide Since the chloride cell offeredthe smallest correction factor we attrmpted to \aturate some pyridine with silver chloride and then dissolve lanthanum chloride in thiq colutioii to the desired concentration. Unfortunately, the mercury of the amalgam replaced the silver chloride dissolrrd in the pyridiiie and precipitated metallic silver. From this evidence it n a' necessary to discard any combination where pyridine and the halide reference electrode\ were used together. The same objection held in the case of pyrrolidine and quinoline.
LAXTH.%NUM AMALGAM ELECTRODE
495
The possibility of a lead-lead sulfate or mercury-mercurous sulfate reference electrode led to the preparation of lanthanum sulfate and an investigation of its solubility in organic solvents. The lanthanum sulfate \vas prepared by dissolving lanthanum oxide in an excess of sulfuric acid, from which it crystallizes in the form of La2(S04)3.9H20 when the solution is cooled. It may be dehydrated by heating to 150°C. for 4 hours. There was no organic QoIvent found that would dissolve this anhydrous lanthanum sulfate. The results of this investigation definitely prove that an E.M.F. method involving a lanthanum amalgam electrode is not applicable to the study of lanthanum salts without considerable difficulty. Some other method such as vapor-pressure or freezing-point measurements should be used even though they are not very satisfactory in dilute solutions. SUMMARY
1. A dropping electrode apparatus was constructed for use with the very active lanthanum amalgam in electrochemical cells. 2. The electromotive force of the cell La(satd. amalgam)-LaClr (H20)AgC1-Ag was investigated a t O", 25", and 50°C. for concentrations of lanthanum chloride of 0.0005, 0.001, 0.002, 0.003, 0.005, 0 011 and 0.022 molal. 3. The solubility of lanthanum chloride was investigated in various organic solvents. 4. The following electrochemical cells were also investigated: La(satd. amalgam)-LaCl3 (ethyl alcohol)-AgC1-,4g La(satd. amalgam)-LaCl:, (pyridine)-AgCl-Ag La(satd. anialgarn)-La~(S04)~ (H20)-Hg2S04-Hg 5 . In all cells where water served as the solvent the E.M.F. was disturbed by the hydrogen overvoltage on lanthanum. h similar reaction occurred when aliphatic alcohols were employed. In cells involving tertiary aromatic amines there was a large solvent action on the reference electrode which could not be corrected for in the customary manner. 6. From this investigation it is definitely concluded that lanthanum amalgams are unsatisfactory for precise E.M.F. measurements.
We wish to express our thanks to the National Research Council for a Grant-in-aid which in part made possible Ihe construction of the apparatus used in this investigation. REFERENCES (1) AVDRIETHAND BIRR: J. Am. Chem. SOC.66,668 (1933).
(2) AUDRIETEI, JURKOLA, MEIXTS,AND HOPKIXS: J. Am. Chem. SOC 63, 1805
(1931). (3) BAREIERIAFD CALZOLARI: Atti. accad. Lincei 20, 164 (1911); 14,119 (1904).
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W. GEORGE P.IRKS AXD RICHARD W. IiIIiGERLEY
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