Magnetic Mercury tathode E. J. CENTER, R. C. OVERBECK, AND D. L. CHASE Battelle Memorial Institute, Columbus, Ohio This study was undertaken to develop a more practical mercury cathode for use in analytical separations. A new mercury cathode has been designed which employs a magnetic circuit. The cathode removes metals rapidly and completely, is rugged and convenient to operate, and contains no mechanical moving parts. The novel magnetic circuit provides rapid countercurrent stirring at the mercury electrolyte interface, continuously cleans mercury surface (ferromagnetic element), and requires a minimum amount of mercury per analysis. Electrolysis with the mercury cathode is often used as a means of separation of certain elements in analytical techniques. It is particularly useful where large amounts of interfering metals must be separated from small quantities of other elements to be assayed. The new cathode provides the analytical chemist with a practical, rapid tool for quantitative analytical separations.
T
H E mercury-cathode electrolysis is often one of the most reliable, if not the only practical, means for the separation of certain elements. Because of the large number of elements deposited and the inconvenience of weighing the mercury, the cathode is usually employed for separation rather than direct determination. The technique is particularly useful where large amounts of interfering metals must be separated from small quantities of other elements which are not deposited. I n dilute sulfuric acid solution, elements such as aluminum, titanium, zirconium, vanadium, and uranium can be quantitatively separated from iron, nickel, cobalt, copper, and other elements of the mercury-cathode group ( 7 ) . In general, the electrolysis has been carried out in a glass cell with a mercury-pool cathode and a platinum anode. Xumerous cathode cells employing a variety of stirring mechanisms, electrode designs, current densities, heat exchangers, etc., have been described in the technical literature ( 1 , 2, 4, 6, 8-11). However, the mercury cathode has never been fully utilized as an analytical tool, because of low rates of deposition; lack of quantitative separation in certain systems due to instrument and cell design; difficulties in the handling of the electrolyte, mercury, and amalgam during and after electrolysis: or high initial and operating cost of equipment. The following work was undertaken in an attempt to develop a more practical mercury cathode.
pulled below the mercury surface as rapidly as it was produced; thus, re-solution of the iron was completely prevented. The strong magnetic field caused the electrolyte to rotate when current flowed through the cell. The electrolyte behaved like a simple conductor in a magnetic field and acted as the rotor in a simple direct current motor. The speed of the rotation depended on the amount of current flowing and the strength of the magnetic field. Frary has described the use of a solenoid for stirring solutions during electroanalysis ( 5 ) . A number of electrolyses were performed with iron, nickel, cobalt, copper, chromium, and zinc in sulfuric acid electrolyte under identical conditions with the magnetized and the unmagnetized cells. Currents of 5 to 10 amperes were used. In both cells an increase in temperature resulted in a somewhat higher rate of metal deposition. Optimum geometry of the anode and the cathode was essentially as shown in Figure 1 for normal cathode operations; the area of the anode was critical up to 28 sq. cm., but a larger anode showed no increase in efficiency. The volume of the electrolyte should be kept as small as practical (the removal of 1.0 gram of iron from 50 ml. was accomplished in
PLATINUM ANODE
PRELIMIVARY STUDIES
OUTLET
To investigate the factors affecting the removal of metals from solution, two identical cells, 13 cm. deep and 6.5 cm. in diameter, were made of borosilicate glass. A cooling jacket, 1 cm. thick, surrounded each cell from the top to within 1 cm. of the bottom (Figure 1). In this type of cell, the electrolyte was withdrawn from the top of the mercury, thus avoiding the necessity of attempting to force the mercury amalgam through a stopcock upon completion of electrolysis. The cathode consisted of approximately 50 sq. cm. (35 ml.) of mercury. A platinum wire, B. 8r S. gage No. 17, weighing about 16 grams, was formed into a flat spiral (Figure 1) to serve A S the anode. Because the removal of iron and paramagnetic metals is desired in about 95% of all analyses using the mercury cathode, the magnetic properties of these metals were utilized to assist in electrolysis. Thus, a lar e, horseshoe-type magnet of Alnico \- was placed under one of t t e cells, as shown in Figure 1. The second cell was used for control to compare with the over-all effect of the magnet. To obtain comparable results, the two cells were placed in series with an ammeter and a variable resistance, and connected to a direct current power supply.
( T n ELECTRODE
-STRONG ALNICOPERMANEM MAGNET
By placing the magnet so that one pole was below the center of the cell and the ,other pole a t the outer edge of the cell, two novel and most desirable effects were produced. The iron amalgam was
Figure 1. Experimental Model of hfercury Cathode Cell 1134
V O L U M E 23, NO. 8, A U G U S T 1 9 5 1 HWK-UP WIRE SHOWN S O W E R E D iNm ELECTRODE WNNECTOR
y8RnSS E L E C T R W E WhNECrCn
1135
The heat exchanger consists of a single ooil of borosilicate glass. The prohe assembly can be quickly dismantled and reassembled in event of breakage, adjustment, or, if desired, use of a special electrode. Split watch glasses of plastic cover the cell and probe unit and effectively prevent spray losses. Magnetic Circuit. Horseshoe-type Alnico permanent magnets are placed under each cell of the cathode unit. The geometry of the magnet and the cell is the same BS that shown in the trid cell in Figure 1. General Construction. The completely assembled magnetic mercury cathode is shown in Figure 3. The unit contains no moving mechanical parts; only the electrolyte and the mercury are in motion during electrolysis. The ease is constmeted of heavy-gage, corrosion-resistant stainless steel and east aluminum with black wrinkle finish. The mercury crtthade is 16.5 inches wide, 14 inches deep, and 21.5 inches (over-all) high, with probe assemblies in operatine position. The probe a&emhliei can be raised a maximumbf 5.5 inches. The sloping instrument panel carries a cell-seleotor switch for meration of richt or left cell onlv. or both cells aimnltaneously; an ammeter yeading 0 to 25 amperes showing direct current input to cell or cells; a fuse held in a panel receptacle which is mounted in the direct current circuit and protects the electrical components; a pilot light indicating power to unit (on or off); and a variable autotransformer knob controlline direct current input to cell or cells. ~~
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100 ml.). "Ripple-free'' direct current v i ~ snot essential for efficient operation of the cells. Optimum deposition rates were ohtained with an acid coneentration (sulfuric acid) from 0.1 N t o 0.5 N , although much higher acidities could he maintained in the magnetized cell because of the minimized re-solution effect. The deposition of 1.0 gram of iron on a mercury surface of about 50 sq. cm. produced a crust of amalgam in the unmagnetised cell. In the msgnetiaed cell, t h e cathode was clean and silvery after 10.0 grams of iron had been deposited in 35 ml. of mercury. The magnetized cell showed consistent advantage in speed and efficiency of removal of both paramagnetic and diamagnetic metals. The advantage was most marked in the tests with cohalt, where the use of the speoially placed magnet (Fignure 1) cut the time of deposition in half. MERCURY-CATHODE DESIGN
Cells. The final design of the mercury-cathode cell is shown in Figure 2. The cell is constructed of horosilioate glass and has a capacity of 400 to 450 ml. A special stopcock is welded near the bottom of the cell so that, when 35 to 50 ml. of mercury are added, the mercury level will he flush with the stopcock outlet. This arrangement permits all but a few milliliters of electrolyte to drain from the cell when the stopcock is opened. Rapid quantitative removal of the oell contents is easily aeeomplished with a small volume of wash water. The cell is washed in position and the mercury is not disturbed. Electrodes and Cooling Unit. The anode, cathode contact, and cooling coil are combined t o form a prohe unit (Figure 2). The anode is fabricated from heavy (B. & S. gags No, 17) sandblasted platinum wire in the form of a flat spiral. Platinum is also used as a cathode contact t o the mercury. Graded seals of No. 3320 uranium glass to borosilicate glass are used in the construction of the platinum anode and cathode connectors.
Figure 3i.
Magnetic Meroury Cathode
The vertical panel has a needle valve controlling the volume of water flowing through the heat enchangers in the cells, m d an off-on toggle switch controlling alternating current power into the unit from line Eource (Figure 4). The stainless steel pillar in the center of the mercury cathode carries the probe support, which can he raised 5.5 inches when
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mide of stainless steel and bakelite with a, hlack'wrinkle-finished aluminum cap covering the connections in the head of the prohe support. The two cathode cells are held in place with a stainless steel and Bakelite clamp which provides positive centering of the pmbe , ?nit , with the d , I . Thefe is F?mp!ete visibility of the
'The magnetic mercury cathode is a completely self-contained, dual unit operating directly from 115-volt, 50- to 60-cycle alternating curront. It needs a source of cooling water, although
A N A L Y T I C A L CHEMISTRY
1136 this is not required for all operations. The unit is built for continuous laboratory operation with a maximum power requirement of 400 watts. Stepless current control provides 0 t o 20 amperes for either or both cells. By introducing the cathode connector into the center of the mercury (Figure 2), the change in direction of current flow causes the mercury to rotate in the same manner as the electrolyte, but in the opposite direction. Thus, the mercury and the electrolyte become independent rotors of a simple direct-current motor, which results in countercurrent stirring at the interface of the mercury and the electrolyte.
4CooL ELECTROLYTE
VARIABLE AUTOTRANWMER POWER TRANS
POWER
16-IEV -2OA
50/60 CY PILOT
Figure 5 . Flow of Current Through Mercury Cathode Cell
RIGHT 4
c
I
c
BOTH
ELECTROLYTIC CELLS POLARITIES AS FACING CELLS
Figure 4.
Schematic Wiring niagram
The detailed mechanism of countercurrent stirring is shown in Figures 4 and 5. Figure 5 graphically illustrates the flow of current through the mercury-cathode cell. Figure 6 shows magnet, cell, and cathode probe location along with flux, current, and force vectors for various positions in the electrolyte and mercury. hlagnetic lines of force are assumed to be essentially vertical a t B and C and approaching horizontal a t 9 and D (the force a t A will be greater than the force a t D). The counter rotation of the mercury is confined to the area adjacent to the cathode connector probe. The over-all effect is shown in Figure 7 . The countercurrent agitation is independent of the electrolyte or the metals being removed. In addition, the magnetic field immediately removes deposited ferromagnetic metals from the working interface and retains them beneath the surface of the mercury. Thus, electrolysis is complete and re-solution effects are negligibk. Temperature is controlled when necessary hy adjusting the flow of cooling water to the heat exchangers. Probes and cells can be rinsed and ne4 electrolyte inserted without raising the probe support. The prohe support can be raised by loosening the knurled-head setscrew, lifting the prohe support, and resetting the screw. It is not always necessary t o change mercury during an electrolysis. Ten or more grams of iron may be deposited in one 40nil. charge of mercury, although a larger 70-ml. mercury cell may be more convenient for 10-gram samples. On completion of electrolysis, only the electrolyte is drained from the cell through the stopcock. This avoids the troublesome operation (and resulting contamination of the electrolyte) of attempting to force mercury plus amalgam through a stopcock. The vertical distance between the platinum anode and the mer-
cury may be easily adjusted by opening the split rubber stopper a t the top of the prohe assembly. For general work, a separation of 6 to 8 mni. is optimum; however, with large amounts of electrolyte containing high concentrations of cathode-group metals, greater spacing may be used.
To Operate Magnetic Mercury Cathode. Raise probe units and place a cathode cell in each position. Lower probes carefully to see that the cathode lead just clears the bottom of the cell (adjust by removing the housing over the probe units and loosening the probe holder). Pour mercury into the cells until the level is just flush with the lower edge of the cell drain and then add the electrolyte. Place the split plastic cover glasses over the cells. With power control at the low end of its scale, and the cellselector switch in the desired position, turn on the main switch. rldjust power setting to the required amperage (as the resistance of the cell decreases as the electrolysis proceeds, the initial setting should be low enough or a readjustment made later t o prevent exceeding 20 amperes). (.Occasionally during the electrolysis of concentrated iron solution in excess of 5 grams and at very high current densities, trees may form on the surface of the mercury. They increase the effective area of the cathode
CATHODE PROBE\_
:TROLYTE
@ ELECTROLYTE @
MERCURY
@
MERCURY
@
ELECTROLYTE F E FORCE I CURRENT FLUX
-
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Figure 6. Vector Diagrams Illustrating Countercurrent Rotation in Cell
V O L U M E 23, NO.
8, A U G U S T 1 9 5 1
Figure 7.
Stirring Action in Magnetic Mercury C a t h o d e
and thus aid in deposition to some extent. As the trees disappear a t the completion of electrolysis, they are of no importance unless they grow sufficiently to short-circuit the cell. Should this occur, it can be remedied b y increasing the electrode spacing by raising the probe unit slightly, or avoided entirely by using a larger cell.) Turn on and adjust cooling water, if necessary, b y means of the needle valve on the front panel.
e of Magnetic Mercury Cathode Grams 50
50 100 100 100 150 50
loo
50 100 100 50
100
100 (0.5 N
Fe++* Fe+**
Pet++
Fe+++ Fe+++
Fe++ cut+
CU++
Ni*+ Ni++
2n++
CO+* CO+*
Pb++
0.5
1.0
1.0 3.0 5.0 5.0
1.0 1.0 1.0 1.0 1.0 1 0 1.0 1.0
1137
Time of Removal by Mhgnetio Mercury Cathode, Minutes" 10 t o 15 15 t o 20 20 t o 35 Lesa than 65 Less than YO 55 to 65 7 to 10 10 to 15 7 t o 10 10 t o 12 16 t o 17 10 t o 12 13 t o 17 Less than 30
HC100 Based on final eleotrol te Content of less than 70 miorograma of metal per 100 ml. of solution. &ectrolly8iS conducted s t 15 to 20 ampere^ with 40 ml. of mercury (cathode area approximately 45 84. om.).
MtLnganese is inrompletrly separated from the electrolyte, when the mercury cathode is used under normal conditions (7). Some of the manganese is deposited on the anode as the hydrated dioxide and some is deposited in the mercury. Table I1 shows the behavior of manganese using the magnetic mercury cathode. The chemical technique employed is essentially that used by Clopin (S). During the electrolysis (Table 11) 30% hydrogen peroxide was added dropwise to the electrolyte to remove the dioxide from the anode. Manganese removal is approximately 99.6% complete under the conditions given. The iron remaining after a standard sodium bicarbmats separation (determination of residual aluminum in a &gram sample of plain carbon steel) can be removed on the magnetic mercory cathode in 15 minutes.
Table 11. Experiment 1
2
3 4 5
6 7 8 9
At the completion of electrolysis, drain electrolyte from the cell and wash as required. In cmes requiring a minimum of cathode-group metals remaining in the electrolyte-i.e., less than 50 micrograms per 100 m1.-wash with the current on. This is easily accomplished by draining the electrolyte to a level just above the anode, adding a small amount of water (or 0.1 N acid), and again draining t o the same level. This technique is particularly important with metals that are not ferromagnetic. PERFORMANCE
Data on the removal of various elements by electrolysis on the magnetic mercury cathode are given in Table I.
Deposition of Manganese Using Magnetic Mercury Cathode*
0.1 N &SO,,
1:5
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MI.
MI.
95
10 (+?I ml.
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85 80 75 80 80 80 84
5 4 (ooncd.)
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