Unitized Mercurv Cathode Appar .. . . . . ........___._.....___..=~_._._._._il______ for rapid electrolytic removal of heavy metals fmm solutions of reactive elements, such as alkali metals, alkaline earth metals: and aluminurn, using a mercury cathode of new design. The apparatus consists of a self-contained immersion electrode assembly coupled to a high-capacity Tungar rectifier and a suitable control panel. It allows convenient, efficient use with hieh current densities, permits easy washing of the electrode a s ~ mhly e without re-solu0
dIIF
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tion of ___.._ -..., ..bakes possibleasy changing of mercury, and pmvides for agitation of the mercury-solution interface by a magnetic stirrer. The apparatus removes approximately 0.5g r a m quantities of copper, iron, nickel, cohalt, ohromium, zinc, and many other elements in one hour or less from an acidic solution. It removes interfering elements prior to polarographic analysis for aluminum, sodium, potassium, and other metals not removed by electrolysis with B mercury cathode. ~
I . , prior to thr determination remova~ orC mrerienng mwais other means. Gfmxally, t,he most useful application of the mercury cathode is the removal of heavy met,als prior to the deterof other elements offers numerous difficulties. If ureciuita. . minadion of such elements as the alkaline eart.h and alkali groups, tion of the undesirable elements is to he employed, it is necessary titanium, vanadium, tungsten, aluminum, and silicon. One of t,o anticipate occlusion and to choose a suitable method or reagent that will not. introduce int,erfering elements. It is often difthe earliest applications of the meroury eat,hode to chemical determinations was the sevaration of aluminum fromiron (5). ficult t,o obtain reagents fiufficiently free from the element to he ,,usins a primitive unit consisting of a beaker containing mercury and B determined; sometimes, subsequent removal of excess reagent platinum't m '' anode. offers additional difficulties. By contrast, electrolysis with the mercury cathode (7) eliminates virtually all these problems when applicable. With suitable apparatus it is generally possible t o D E V E L O P M E N T OF A P P A R A T U S perform the electrolysis in a fraction of the time required for When the mercury cathode is used for removal of metals from B most chemical separations, leaving a dilute acid solution almost large number of samples it is necessary t,o consider the importance completely free of metallic elements. of certain characteristics which may generally he overlooked in According to Lundell and Hoffman (8),the metals which-are aumtitativclv deoosited in the mercuw cat,hodeare: chromium. the electrolysis of occasional samples. In this respect, the effiiron, cobalt, nickel, copper, zinc, galciency and rate of electrolysis are among lium, germanium, molybdenum, rhothe most import,ant characteristics; dium, palladium, silver, cadmium, therefore, it is desirable that the appairidium, tin, rhenium, indium, phti--Polarized Jack ratus he capable otf operating wit,h a cnrJpner. N0.P-302-CCT num, gold, mercury, thallium, bisrent of severd a mveres without overmuth, and polonium. Other elements heating. Tho rernaval of the solution which are completely removed from without eontamiination with mctallio -Bakelite solution but not quantitatively demercurv is rmer ally aecamplishcd by posited in t.he mercury are osmium, various methods such as decanting and which is partially volatilized as the filtering, hut a much faster and more --Cement t,etroxide a t the anode, selenium and dependable method is desirable. A IPbOt Glycerinel t,ellurium, which are reduced t o the simple, sturdy clectrode (preferably solfelemental state but remain unamalgacontained) is required for use by ana5 CM mat,ed, ]earl nariirllvi ,", nrhirh ma., hn rl.".y ..., lysts not part,icularly adept in the operaSCALE deposited In thc anode, and arsenic, tion of special, delicat,eapparatus. which ma)' he partially volatilized at Scvcral cells for elcctrolysis with a the cathmde as arsine. Manganese, mercury cathodc have been described ruthenium, and antimony arc incom( 1 , 8, 4,9,11,12) , hut none was judged to fill the requirernents for a cell to be - P I - l r (S%,)Wire pletely sepi%rated(8). .. 8 . 8 5 Ga.No.12 Electrolysis with a mgrcury cathode used in service an alyses. I n the search has been found extrcmely useful in for a more suitahle design, several variathe polarographic dcterminstion of tions of cells con taining mercury pools - P I Wire itlkali metals where practically all the were const,ructeda,nd tested. 0 . 8 5 . Go.Na.19 metals must be removed prior t o the One style consilsted of 8. tall beaker analysis; in general, it finds wide ap- P I Gauze having a dmin coo plication in the preparation of soluN0.l-45 Mesh above the level of t,ions for determination of any eleutilizing an anode -Uranium Glass ment not removed by electrolysis. tsched to a elas; ".>,,,ution. This cell was It is sometimes possible to, weigh slow to drain hecause of the low hydrothe mercury hefare and after elecstatic head and thus permitted re-solutrolysis and thus determine the weight. tion of the more active metals. Moreof metals deposited, hut it is ususlly over, it was impossible to change the merFigure 1. Unitized Mercury cury cathode without draining and Cathode Electrode preferable t.odetermine theseelements by
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washing the cell, the mode was fragile, and some danger existed of igniting the evolved hydrogen if the two electrodes tlccidentally touched. Another type tested consisted of a tall cell with a sealed-in horizontal platinum gauze anode, similar to the cells described by Melaven (8) and Smith ( 1 1 ) . A two-way stopcock a t the bottom of the cell permitted removal and replenishment of the mercury cathode and withdrawal of the sample solution. This cell had a more sturdy anode and drained more readily, but it was difficult or impossible to withdraw the viscous amalgamssometimes formed. The development of a special cell with built-in eleotrodes was abandoned in favor of a unit utiliaing B self-contained immersion electrode assembly to be used with 8 removable tall-form electrolytic beaker. Special advantages were found in the use of this convenientarrangement: (1) The electrodeiseasilywashed witha stream of water, quicklyenough so that elements in the amalgam have little opportunity to redissolve; (2) electrolysis with fresh mercury is accomplishedeither by changing that in the electrode cup or by simply moving the beaker.to a second fresh electrode; (3) very little preparation is required between successiveapplications; (4) difficulty from loss of mercury is minimized. Besides serving as a container for the mercury used as cathode, the cup on the end of the electrode protects the anode, so that there is little chance.ofdamage during cleaning h*nalin* Figure 2.
APPARPLTUS
Dual Unitized Mercury Cathode Apparatus
The anode-cathode assembly is illustrated in Figure 1 by the heaG-~&tinum external wire which ionneck it to the polarized jack. The Bakelite extension which covers the electrical leads a t the top of the glass tube serves as a convenient rugged fitting for the supportclamp, ~h~ simple oompresion clamps used t? support .. theelectrode.assemhly_,and t_he &djyt?hle table supports for the beamrs me shown m rigure 3,w n m also snows the location of the electrim1 controls. The uriring arrangement, shorn in Figure 3, p r o ~ d e ssome control over the ourrent hy means of resistances and individual switches so that either unit may he operated independently. The General Electric Tungar rectifier (Model 6RB-19Y2 or 6RB33B2) is a convenient source of direct current.
Table I. Effeotiveness of Mercury Cathode in Removing Metals from Solution Element* % 5
Fe
CU
CT
Eleotrolysis Intervals
NO. of Meraury Chanees
30,20.10 30,30 20.15 30,ZO.10
Elemeqt
Remaining in Solution
2
42 2'1
1 2
100
1
6
Ni 30.15 1 30 Ni 30.30 1 2 =Equivalent of 500 mg. of metal oxide i n solution st beginning of eketr01ysis.
PROCEDURE
Concentrate to 25 or 30 ml. the neutral or slixhtly acid solution
tom of the beaker. Turn an the cukrent and carefully add concentrated sulfuric acid until a current of 5 amperes is sttaine'd. Cover the beaker with a split, notched watch glass, electrolyze for 30 minutes, and add water 8s necessary to maintain constant volume. With the cukrent an, quickly lower the beaker and rinse the electrode with a stream of distilled water. Replace the amslgam with clean mercury, electrolyze for 20 minutes, again rinse, replace the amalgam with clean mercury, and electrolyze for 10 minutes. OPERATING CHARACTERISTICS
I n order to establish optimum cundilions for removal of metals, solutions of several elements were electrolyzed under different
were geno&ly desirable and that, re. changes f, short periods of elecof the hulk of the element, use of trolysis with an intervening change of mercury was more effective than a longer second period. .Electrolysis . far successive periods of 30,20, and 10 minutes was judged most generally satisfsctory. Typical results, shown in Tsble I, indicate that the mercury cathode is capable of reducing concentrations of many metals to extremely low levels. For most praotioal purposes the removal of the metals listed above may he considered complete under the specified conditions. I n special cases requiring extremely thorough removal, it may he necessary to use longer periods of electrolysis (afber the first period), and may possibly be desirable to change the mercury for a third time. The effectiveness of the mercury esthade for removing metals prior to the polarographic dctermination of sodium was tested. Chromium, copper, cadmium, lead, and iron were added to known amounts of sodium and the mixtures electrolyzed for 45 minutes. The excess acid was then removed from the solutions by evaporation, and polarograms were prepared from solutions of the residues. The results in Table I1 show that the interfering elements were effectively removed by the treatment, and the sodium was quantitatively rotained during thc process. Either hydrochloric or sulfuric acid solutions may be electrolysed successfully, although nso of the latter is generally preferable, When hydrochloric acid is used the acid vapors become a problem, and the curront through the cellmust be watched more carefully. Other acids, such as perchloric and phosphoric, have also been used (10) but do not appear to have advantages over sulfuric acid for most applications. The removal of chromium from solution requires special treatment, since chromium amalgam is somewhat unstable, and a finely divided precipitate of mctallic chromium often forms in the cell and must he removed from the f i n d solution by filtration. Chirnside (4)st.udicd this problem in detail, but did not find a completely satisfactory procedure. While conducting experiment,s along the same lines, it was discovered that when rhromium was in the trivalent condition, amalgamation proceeded in a normal manner, hut hexavalent chromium almost invariably yielded a fine black precipit.ato. Various methods were tried for reducing chromium to the trivslcnt state; rcduction with 30% hydrogen peroxide was found to be the most satisfaet,ory, as the ~
JULY
1947
483
excess peroxide is easily decomposed by boiling prior to electrolysis. The amalgam formed by electrolyzing trivalent chromium will eventually decompose and form a black precipitate in the cell if the current is not maintained, although the reaction is not so rapid as to cause r + d u t i o n when the electrode is removed from the solution. Evans (6) has described a rather elaborate apparatus designed to avoid the lowered rate of amalgamation caused by local high concentration of amalgam a t the surface of the cathode. To test the effectiveness of simple agitation of the mercury pool, a magnetic stirrer was placed under the cell, and a small (1.25 em., 0.5 inch, long) iron-in-glass agitator placed on the surface of the mercury. The results of several tests, shown in Table 111, indicate that the stirrer speeds the amalgamation process and is useful whm it is dc5ired to remove the bulk of the metals quickly. However, when it is desired to make the electrolysis as complete as possible it is about as satisfactory to change the mercury twice as to use agitation. Serious contamination of the electrolyzed solutions by mercury was not expected, iince mercury is one of the metals removed from solution by means of the mercury cathode. To test this point, a sulfuric acid solution containing the equivalent of 0.5
Table 11. Polarographic Determination of Sodium after Removal of Metals by RIercury Cathode Element Removed
Sodium Added
Sodium Found
5
70
0.100,0. 100 0 100,o. 100 0 . 1 0 0 , 0 . 100
0 108,O 106 0.099,O 105 0 099,o 099
500 nig. of C r as (SHd>KrOd. Equivalent of 170 mg. each of CuO, P b O , CdO. Sulfuric acid added t o solution a n d precipitated PbSOr filtered off before solution electrolyzed. e 500 mg. of Fel(SOaia. Q
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Table 111. Effect of Agitating Mercury on Rate of Amalgamation Elementa
Magnetic Stirrer Yes No Yes No Yes
No. of Mercury Changes
Electrolysis Intervals Min.
n
N O
Cr Cr Cr Ni Ni Ni Ni sis.
Yes No Yea NO Yes NO Y es NO
Yes No
60 60
Element Remaining in Solution Micrograms F,7
288
30 30 60 60 20,15 20,15 30,15 30,15 30,20,10
182 286
30,20,10 30,15 30,15 30,30 30,30
6 32 30
24 77
22 100 540 804
4 2
Equivalent of 0.5 gram of metal oxide present a t s t a r t of each electroly-
gram of iron was electrolyzed for 1 hour, and then analyzed for mercury: On four such tests, the largest amount of mercury found was 33 micrograms. DISCUSSION
In preliminary designs, considerable difficulty was encountered in sealing platinum wires through the Pyrex. Heavy wires 'CI ere necessary because of the large current and cracks sometimes developed. A tungsten-to-uranium glass seal was successfully applied a t the cathode where the tungsten mas covered with mercury and therefore not in contact with the acid solution. A graded seal at the anode was not possible because of the dimensions involved, and the most successful Pyrex-to-platinum seal was obtained by covering approximately 1.25 em. (0.5inch) of the wire with a thin sheath of Pyrex. Experimental units were also constructed of soda glass, and, while the platinum-to-glass seals were easy to make, the poor resistance to thermal shock and chemical attack made the use of this material undesirable. Eventually, the problem was solved by using a very heavy platinum n ire which is not sealed into the glass at all, and is supported by the cap a t the top of the, electrode. A very heavy nire iq essential, as otherwise its temperature may become high enough to cause the evolved hydrogen-oxygen mixture to detonate. ACKNOW LEDGBI ENT
I
I I
I
The authors wish to express their appreciation to T. D. Parks for his valuable suggestions. LITERATURE CITED
Rectifier Figure 3. A.
h, h.
h.J z .
P.S.
RI,R2. R3, Rd.
Si, S2.
V.
Wiring Diagram
Ammeter, 0 t o 15 amperes Indicator lights, 6 v o l t s Jacks t o electrodes Power switch 2-ohm, 7-ampere maximum 50-ohm resistors Single-pole single-throw switch Voltmeter; 0 to 15 v o l t s
(1) Alders, H., and Stahler, A, Ber., 42, 2685 (1909). (2) Cain, J. R., J . I n d . Eng. Chem.,3, 476 (1911). (3) Chirnside, R. C., Dauncey, L. A , , and Proffitt, P. M .C., Analyst. 65,446-52 (1940). (4) I b i d . , 68, 175-80 (1943). (5) Drown, T. M.,and JIcKenna. A. G., J . Anal. Chem., 5, 627 (1891) * (6) Evans, B. S.,Analyst, 60, 389 (1935). (7) Gibbs, W., Chem.
