ANALYTICAL CHEMISTRY
906
LITERATURE CITED
tained from the polarographic determination of palladium after the separation of this metal, using dimethylglyoxime, from platinum, rhodium, and iridium are shown in Table 11.
Ayres, G. H., and Berg, E. W., ANAL.CHEM.,24, 465 (1952).
Ibid.,25, 9 8 0 ( 1 9 5 3 ) . Ayres, G. H., and Tuffly, B. L., Ibid.,24, 9 4 9 (1952). Ayres, G. H., and Wells, W. N., Ihid., 22, 317 ( 1 9 5 0 ) . Gilchrist, R., and Wichers, E., J . Am. Chem. SOC.,57,
DISCUSSION
A study of the polarography of palladium in ammonia-ammonium chloride solutions indicates that palladium can be determined accurately over a moderate concentration range The precision of the method is less than 1%, when the concentration of palladium is greater than 0.35 mM. This method can be used to determine palladium after this metal has been separated, using dimethylglyoxime, from platinum, rhodium, and iridium. A brief study of the polarography of palladium in acetic acidacetate buffer showed that the diffusion currents of these waves were proportional to the concentration of palladium and could be used for analytical purposes. The diffusion current constant ( I ) value ( I = id/Cm*/3t*/6) for palladium in 0.431 ammonium chloride buffer at 25” C. is 3.76.
2565
(1935).
Hillebrand. W. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” p. 278, Tlriley, New York, 1929. J . Am. Chem. Soc., 62, 3172 Kolthoff, I. AI., and Langer, -I,, (1940).
Kolthoff, I. ll.,and Lingane, J. J., “Polarography,” 2nd ed., Vol. 1, p. 3 6 2 , Interscience, S e w York, 1952. MacNevin, W.ll.,and Kriege, 0. H., .$TAL. CHEM.,26, 1763 (1954).
Toropova, V. F., and Yakovleva, G. S.,Zhur. -4naZ.Khim., 1 , 290 (1946).
Willis, J. B., J . Am. Chem. SOC.,67,
547 (1945).
RECEIVED for review July 13, 1954. Accepted January 28, 195.5. vestigation supported by a grant from the Research Corp.
In-
Polarography with Platinum Microelectrodes in Fused Salts EDWARD D. BLACK’ and THOMAS DE VRlES Department o f Chemistry, Purdue University, Lafayette,
Conditions for obtaining polarograms in fused salts with platinum microelectrodes were established. Automatic recording was employed in most experiments. In the eutectic mixture, lithium chloride-potassium chloride between 380” and 450’ C., waves were obtained with a microcathode and a platinum coil anode. The theoretical shape of the cobalt and nickel w-ates was expressible by a linear relation between log ( i d - i) and applied potential. A linear relation between wate height and mole fraction was obtained for dilute solutions of cadmium chloride, cobalt chloride, nickel chloride, lead chloride, zinc chloride, and potassium chromate in the alkali halide melt. Half-wave and decomposition potentials of cobalt chloride and nickel chloride shifted in the expected direction and magnitude. The effect of variation in polarization rate, area of microelectrode, speed of rotation of the electrode, and temperature was examined with respect to polarograms of nickel chloride. The temperature coefficient and energy of activation for the limiting digusion currents of nickel have been determined. In a melt of lithium nitrate, sodium nitrate, and potassium nitratc as solvent, with rotating spherical microcathodes of platinum or silver, a linear relationship between wave height and concentration of copper sulfate was observed.
I
S R E C E 3 T years an increasing interest has been shoan in the study of polarography with fused inorganic salts as solvent media. Sachtrieb and Steinberg, employing the dropping mercury electrode, verified the application of the Ilkovii. equntion to the reduction of nickel(I1) ions in the ternary eutectic of lithium nitrate-ammonium nitrate-ammonium chloride at 125’ C. (6) and to the reduction of cadmium, lead, nickel, and zinc in the eutectic mixture of lithium nitrate-sodium nitratepotassium nitrate at 160” C. ( 7 ) . A dipping platinum electrode has been adapted by Lyalikov and Karmazin ( 6 ) to the polarographic determination of cadmium, copper, and nickel in potassium nitrate fusions at 360” C. In extensions of this work, 1 Present address, Quartermaster Research and Derelopinent Laboratories, Katick, M a s s .
Ind. Lyalikov ( 4 ) has reported on the analytical use of polarography for a number of cations and anions rvith various fused salts as solvents. The latter included the following salts or mixtures: potassium nitrate, potassium hydrogen sulfate, potassium nitrate with potassium hydrogen sulfate, and potassium nitrate with potassium chloride at temperatures in the range 360” to 520” C. il measure of success was attained a t temperatures as high as 1000” C. in obtaining polarographic waves for copper in a mixed silicate melt. Also results were claimed by Lyalikov for the use of molten lithium chloride-potassium chloride as solvent, but details in these instances were not given. The ultimate objectives of the adaptation of polarography to fused salt media concern the analysis and/or study of metallurgical slags, molten rocks, and glasses as well as the kinetics of the decomposition of substances at high temperature. The polarographic technique also offers an approach to the in situ study of corrosion in the presence of molten electrolytes as ne11 as possible means of elucidating the nature of complexes in fused salt media. Since most of the progress to this time has been with relatively simple salt systems, the application of polarography to many of the more complex problems cited above a~ sits further development. E Q U I P V E N T AND PROCEDURES
The mixture lithium chloride-potassium chloride a t a mole ratio of 3 to 2 has a eutectic point of 352” C. Mixtures in this proportion (31.8 grams of lithium chloride, 37.9 grams of potassium chloride) were prepared by weighing the dry salts, stirring rvell, and immediately placing them in the preheated furnace The container and polarographic cell was a 100-ml. borosilicate glass beaker open to the atmosphere. Prolonged contact with the molten salts resulted in considerable etching and weakening of the glass. The cell holder consisted of a steel cup, 3 inches in diameter and 5 inches deep, in which the beaker vas suspended by means of a nickel \Tire triangle. Electrical grounding of this steel beaker provided necessary shielding for the electrolysis cell. The platinum microcathodes viere constructed by sealing the wire into a small diameter bulb a t the end of a length of 6-mm. glass tubing. The wire, which was perpendicular to the axis of the tubing. was cut off flush with the glass surface and buffed smooth. Wire sizes employed were Iioe. 24. 27, and 30 (B. and S. gage). In the studies nith rotating electrodes, the electrical connection was effected with a commutator of platinum foil pressing against a spiral of platinum wire wound on the glass tubing of the electrode. The anode consisted of a platinum wire spiral made from 5 inches of KO.24 wire. The separation of anode and cath-
V O L U M E 2 7 , NO. 6, J U N E 1 9 5 5
907
ode in the melt was 1.5 cm. A lead glass was used in the construction of these electrodes. Platinum to borosilicate glass or platinum to "uranium" glass seals gave rise t o high, erratic residual currents, a difficulty encountered also in other phases of fused salt polarography(4, 7 ) . Because of the rather low chemical resistance of the lead glass in the molten salt, i t was necessary to renew electrodes after several days of intermittent use. The cavity in the electrically heated furnace (Hevi D u t y Electric Co.) was 5 inches in diameter and 13 inches deep. The temperature of the furnace could be held constant to 1' C. or less with a Leeds & Korthrup Model C temperature indicatorcontroller. A Sargent Model XXI polarograph was used for the recording of all the data. When a mixture of lithium chloride-potassium chloride had been molten for about 2 hours a t 430" C., the electrodes were inserted and a current-voltage curve was recorded for the solvent melt alone. This procedure served as a check on the purity of the melt as well as on the proper operation of the electrodes. Solutions were prepared by adding weighed amounts of the substances to be investigated directly to the melt. I n general, three methods for pretreatment of the electrodes could be employed: immersion for a few minutes in nitric acid, followed by a rinse with water; renewing the surface of the microcathode by buffing with No. 0 emery cloth; or shorting the cathode and anode together in the melt, a t the conclusion of a polarogram until the original state was restored (about 5 to 10 minutes). Both the reproducibility and shape of the polarographic waves sometimes depended upon the pretreatment of the electrodes. For example. lead waves of different shapes were obtained depending upon nhether the second or third method of Pretreatment had been employed. Xickel and cobalt waves, on the other hand, were independent of the treatment of the electrodes. Only the second method was effective in attaining fairly reproducible cadmium waves. Nearly all polarograms were obtained a t a polarization rate of 1.24 mv. per second, except for those experiments in which the variation of wave height with the rate of increase of polarizing e.m.f. was studied. Manual polarograms were also obtained. h period of 2 or 3 minutes as generally sufficient for the current to attain a steady state. The platinum anode appeared to behave as a nonpolarized electrode during the recording of the polarographic waves in the molten salt. All potentials in this M ork have been reported with reference to the platinum anode and as read directly from the polarographic instrument. Increasing cathodic polarization was employed in the obtaining of all current-voltage curves. A4tacit assumption, of course, in all polarography in fused salts is that such a solvent is its o n n supporting electrolyte. Also. because of the high concluctivit>-of the solvent, ohmic corrections n ere not nerrssary
value the residual current for the solvent melt alone increased rather rapidly and a t about -2.2 volts began to increase without limit. Within the range 0 to -1.5 volts, reproducibility of the residual current was considerably better than a t more negative potentials. -is a general rule the system v a s considered t o be functioning properly ii the residual current a t -1.0 volt had a value of 0.8 i 0.3 pa., irrespective of such variables as the area of the cathode, speed of rotation, and temperature. During a particular reduction wave the platinum cathode it. of course, plated a i t h a different metal. This fact is probably responsible for the observation that the beginning of the solvent decomposition current occurs a t more positive potentials than that observed n ith t n o platinum electrodes in the electrolysis of the solvent melt alone. Tracings of typical 11 aves for nickel, cobalt, and lead ions are shonn in Figure 1 for both stntionaiv and rotating rlectrodes, the latter at 261 r.p.m., a t 400" to 408" C.
d I
a a
3 V
1
I
-8
-9
I
cr 04=
-2 APPLIED
Figure
I
-7
POTENTIAL,
-4 VOLTS
-10 I
I
-6
-8
(VS. P t
I
-13
ANODE)
2. Automatically recorded polarograms lithium chloridepotassium chloride
in
Stationary cathode (platinum, h'o. 27). 408' C., mole fractions. Yirkel chloride, 3.03 X 10-4; cobalt chloride, 3.53 X 10-4; potassium chrom a t e . 1.24 X 10-4
Similarly shaped waves were obtained for both rotating and stationary microcathodes. The waves for lead chloiide, however, n ere better defined at a rotating than a stationary cathode. Not shown in the figure are cadmium ion viaves which appeared in the vicinity of -1.1 volts and zinc ion waves near -1.3 volts. Examples of multiple waves are given in Figure 2. The polarogram (a tracing) for the cobalt chloride-nickel chloride mixture was obtained with the aid of a fine sintered glass diaphragm interposed between the electrodes. This device seemed to improve the general appearance of the waves in this instance. The multiple wave for potassium chromate in Figure 2 gives an approximate ratio of 3 to 1for the wave heights at -0.25 and - 0.82 volt which suggests that the first wave corresponds to the reduction of chromium(V1) to chromium(II1) and that the second nave represents the reduction of chromiuni(II1) to chromium(I1).
EXPERIMENTAL RESULTS
The useful range for cathodic polarograms under the conditions of these experiments extended to -1.5 volts. Beyond this
THEORETICAL SHAPE O F THE WAVES
I
The theoretical expression for the shape of the polarographic wave at a solid microelectrode is given (3) by
I IPt/
Figure 1. Automatically recorded polarograms in lithium chloride-potassium chloride
where E and i are the applied potential and current at succesive points along the wave, E.; is the standard potential for the deposited metal, R is the gas constant, i d is the limiting diffusion current, fs is the activity coefficient of the metal ions near the surface of the electrode, k , is a proportionality constant, T is the absolute temperature, n is the number of electrons involved
/
-4,
L LJ-;-
0
-8
0
APPLIED POTENTIAL,
Sickel chloride. Cobalt chloride. Lead chloride.
-9
VOLTS
-10
(VS.
I
-I I
I
-I
2
P t ANODE)
iY = 4.05 X 10-4, stationary cathode (platinum No. 27), 400' C. N = 5.35 X 10-4, stationary cathode (platinum No. 27), 407' C.
A'
= 2.32 X 10-4, rotating cathode (platinum No. 30), 264 r.p.m., 408" C .
908
ANALYTICAL CHEMISTRY .
~
Table I.
~~~~
______
~
Typical Data of Waves Obtained w i t h Rotating a n d Stationary Electrodes" CdCln KiClz coc12
ia/*vx
id/iv X 102 2 . 8 1 0.83 1.75 2.11 1.83 3.70 2.02 2.78 2.76 5.33 1.93 2.66 4.03 8.65 2.14 2.65 4.71 10.55 2.24 2.75 5.62 13.50 2.40 2.62 6.82 15.45 2.26 2.62 .kv. (id/.V) 2.16 ?c 0.12 2.70 i 0 06 a Cadmium chloride and nickel chloride. Rotating cathode (platinum, N o . Stationary cathode (platinum, No. 27). Temperature of melt 407' =t1' C. IV X
id,
pa.
lo4
101
IV X
id,
104 0.93 1.38 1.94 2.58 3.46 4.01 5.34
pa. 2.61 3.84 5.16 6.95 9.53 10.5 13.95
X 104 0.59 1.45 3.49 5.58 6.53 10.55 13.85 i v
T a b l e 11. Variation of Wave Height w i t h Speed of Rotation of Microcathode" Mole Fraction of NiClz X I O 4 R.P.M. 1.70 2.92 0 1.25 2.07 264 4.04 6.30 5.34 7.84 516 5 Cathode No. 27 platinum:, polarization rate X 1.24 mv. temperature 407O C.: wave helght in microamperes.
1
4.08 3.05 7.72 8.83 per second;
I'"
I
2.0
A
1.61
5-
1.21
I
-.2
g J
0
.2
-60
I
:68
APPLIED POTENTIAL,
Figure 3.
- 76I VOLTS
I
-. 84
(vs.
I
Pt ANODE)
M a n u a l polarogram of nickel chloride. log ( i d - i ) us. applied potential
Plot of
Stationary cathode (platinum. No. 27), 413' C., mole fraction, 1.50 X 10-4
in the electrode reaction, and F is the Faraday constant. A test of Equation 1 is possible if f S is constant, in which case a plot of log ( i d - i) along the ordinate against E along the abscissa should yield a straight line with a slope nF/2.3RT. A manually obtained polarogram for the reduction of nickel chloride at 413" C. a t a stationary microcathode was in close agreement with the predictions of Equation 1. The polarogram and linear plot are shown in Figure 3. The value of the reciprocal slope is 69 mv., which is in agreement with the calculated value of 68 mv. for a reaction involving the transfer of two electrons a t this temperature. The lack of correspondence between the observed curved inception of the current wave and the theoretical expression for the wave is due to the fact that a certain amount of metal must be deposited on the electrode before the activity of its surface becomes constant. Predictions of Equation 1 were also demonstrated to a fair degree by automatically recorded polarograms a t a stationary microcathode. Using the data for cobalt chloride and nickel chloride which was plotted in Figure 1, the reciprocal slopes of the straight lines were 76 and 60 mv. for cobalt and nickel, respectively, as compared with a calculated value of 67 mv. Contributing to this discrepancy could be the difference in the electrode surface, a delay in the establishment of diffusion equilibrium, and any time lag of the recorder mechanism. Besides affording a means of determining n, these results for manual and
id.
pa. 0.52 1.15 2.64 4.28 5.40 7.89 11.00
id/'Y X 103
(8.82) 8 21 7 57 7.67 7.79 7.48 7.95 7 . 7 8 i 0.20
a ut,om a t i c polarograms also indicate t,hat the rate of d 8 u sion is the controlling factor in these reductions and that irreversible phenomena in the electrode reactions are a t a minimum. FACTORS AFFECTI6G THE WAVE HEIGHT
Mole Fraction. At a particular mole fraction the wave height was reproducible to 2% under identical experimental conditions. If different electrode surfaces were used, the results ranged from 5 to 10%. The Jyave heights were propoitional to the mole fraction with the straight lines passing through the origin. Some typical data obtained with both stationary and rotating electrodes are given in Table I. The listed values of i d / X indicate the precision to be expected in analytical applications. The larger deviation for cadmium chloride is attributable to the fact that a freshly buffered cathode surface was employed for each polarogram of the series. In some instances the data deviated from a straight line plot a t the higher concentratlons. This downward trend was probably due t o a gradual hydrolysis of the reducible species with the formation of insoluble oxide or hydroxide. Linear relationships were also obtained between wave height and mole fraction at 407 C. for lead chloride, zinc chloride, potassium chromate, and a mixture of cobalt chloride and nickel chloride. Rate of Polarization. The effect of varying the rate of increase of applied e.m.f. vias studied only a t a rotating cathode (No. 30 platinum, 264 r.p.m.) a t 391" C. for several solutions of nickel chloride a t different mole fractions. -4s one would predict, there was an increasing trend in the wave height with an increase in the rate of polarization. This increase was the more marked the greater the mole fraction of nickel chloride. Thus for a sixfold increase of polarization rate, from 0.62 to 3.72 mv. per second, there was a 25% increase of diffusion current )Then the mole fraction was 1.57 X 10-4 and a 40y0 increase when it was 2.24 x 10-4 Area of the Microcathode. A41thoughreproducibility for different electrodes was relatively poor, a direct proportionality between wave height and the area of the flush electrode was observed for both rotating and stationary electrodes at 407' C. Three electrodes of different area made from S o . 24, 27, and 30, B. and S. gage, platinum wire were employed in this study. In general, it is difficult to duplicate the behavior of a particular electrode surface. Speed of Rotation of Microcathode. The effect of a change in the speed of rotation of an electrode on the wave height was investigated for a solution of nickel chloride at three different mole fractions (Table 11). The wave height at a stationary cathode was compared with that at a cathode rotating at 264 and 516 r.p.m. An increase in wave height, tending toward a limiting value, was observed. Although the data are insufficient for an exact mathematical formulation, the trend observed \\as proportional to the cube root of the speed of rotation ( 1 ) . Temperature. Five series of experiments in the temperature interval 380" to 450' C. to study the increase of wave height, using both stationary and rotating microelectrodes, resulted in a linear relation between log i d and l / T o K. for nickel chloride. The enthalpy of activation for the process v.-as determined from the slopes of these lines to be -5.6 kcal. with the stationary electrode and -4.3 kcal. with the rotating electrode. These values correspond to an increase in wave height of 0.59 and 0.4570 per degree, respectively, a t 415" C. This result implies that a variation of 2" C. may be allowed for temperature control. It seems reasonable to expect the activation energy for diffusion in
30, 264 r.p.m.).
-
2.4
___
Cobalt chloride.
V O L U M E 27, NO. 6, J U N E 1 9 5 5
909
a concentration gradient (staTable 111. Shift of Half-Wave Potential with Concentration for Cobalt Chloride tionary electrode) to be smaller and Nickel Chloridea than the activation energy for COCll NiClz ( a ) KIC12 (a) self diffusion (rotating elecCathode Xo. 27, Pt No. 30, Pt N o . 30, Pt trode). A comparison can be R.P.1\L0 0 264 0 Temp., C. 407 407 411 made with the change in visA x 104 iog s E , ( ~voit , N x 104 log 5EU?. IOIT s x 104 log s E ~ I ?volt , cosity of the lithium chloride0 93 -4.033 -0,698 1.17 -3.933 -0,713 1.45 -3.839 -0,897 potassium chloride melt with 2.44 -3.613 -0.879 1.88 -3.861 -0.688 2.40 -3.622 -0,695 1 94 -3.713 -0.679 3.41 -3.469 -0,681 3.49 -3.457 -0.867 t e m p e r a t u r e . The data of 2.58 -3.590 -0.671 5.07 -3.296 -0,669 4.60 -3.337 -0.862 6 . 3 4 3 . 2 0 0 -0,662 5 . 5 8 3 . 2 5 3 0 . 8 5 4 3 . 4 6 3 . 4 6 3 0 . 6 6 3 Karpachev ( 8 )for the tempera7.62 -3.120 -0,657 6.93 -3.159 -0.848 4.01 - 3 398 -0.fi38 ture interval 450" to 600" C. 5 31 -3.274 -0.650 8.92 -3.051 -0,652 8.72 -3.059 -0.842 1 0 . 5 3 2 . 9 7 8 0 . 8 3 4 1 0 . 2 5 2 . 9 9 1 0.648 vields a value of +7.0 kcal. for 2 . 3 E T 69 mv. (obs.) 64 mv. 70 mv. the energy of activation, corre67 mi-. 2F 67 niv. (oalcd.) 68 mv. sponding to a decrease in the a Polarization rate, 1.24 mv. per second. viscosity of 0.730j, per degree. Considering - that the diffusion Table IV. Comparison between Half-Wave Potentials and current usually varies approxiStandard Molal Potentials Standard mately according to the square root of the diffusion coefficient, Mola! it is apparent that the change of viscosity is not the sole factor -T x 104 Potential responsible for the variation of diffusion current (wave height) ZnCl2 1.33 -1.40 -0.763 CdCh 1.83 -1.18 -0.402 with temperature. The small temperature coefficient provides PbClz 1.22 -1.01 -0,126 COCl? 1.45 -0.90 -0.277 further evidence that diffusion rather than rate of reaction is the Cr(II1) Cr(I1) 1.24 -0.84 -0.40 controlling factor in these polarographic waves of nickel. NiClz 1,19 -0.70 -0.23 ~
~~~~~
-.
KzCrO4 [Cr(171) + C r ( I I I ) ]
1.24
-0.30
-0.12
HA LF-WAVE POTENTIALS
An equation for the change of half-wave potential with concentration a t a platinum microelectrode can be derived from Equation 1 since i d is proportional to N and i is &/2 a t the halfwave potential. In the resulting equation
E,,* = E&
+2 . 3 ' logf,k' + nF
log N / 2
(2)
k' is a proportionality constant between concentration and mole fraction, and the other symbols have their usual meaning, Data for testing this equation are given in Table 111 and plotted in Figure 4. Both cobalt chloride and nickel chloride solutions were used, the latter with both stationary and rotating electrodes. The equation predicts a shift of the half-wave potential N.1
441
c
o
y
j
in the positive direction of 67 mv. a t 407 C. for a tenfold change in concentration. There was fair agreement between observed and calculated values as shown in Table 111. A comparison can be made between the half-wave potentials of the ions studied and the standard molal potentials in aqueous media. In Table IV are listed the half-wave potentials found in this investigation for solutions approximately 1.2 x 10-4 mole fraction a t 407" C. as well as the molal potential us. the standard hydrogen electrode a t 25 O C. as found in the literature. A limited number of experiments were performed in the melt consisting of lithium nitrate, sodium nitrate, and potassium nitrate in the mole ratio 1.00 to 0.567 to 1.768 ( 7 ) . Crystallization begins a t 145" C. for this melt and the eutectic is a t 120" C. Both rotating and stationary platinum and silver microcathodes were used. Silver-silver chloride electrodes mere used as anode and therefore potassium chloride to the extent of 0.001 mole per mole of mixture was added to the melt. Copper sulfate was investigated and the wave height was proportional to the concentration in the range from 3 to 8 millimoles per liter. The wave commenced a t about -0.2 volt with an irregular trend to more positive values for increasing concentrations of copper sulfate. Thermal decomposition in the melt was one of the problems which discouraged further investigation. ACKNOWLEDGMENT
The authors wish to express their gratitude to the Atomic Energy Commission for funds which made this research program possible. LITER4TURE CITED
(1) Kambara, T., Tsukakoto, T., and Tachi, I., J . Electrochem. SOC. Jaman. 19. 199 (1951). (2) Karpachev, S., Stromberg, A, and Podtschainova, V. N., Zhur. Obshchel Khim., 5 , 1517 (1935). (3) Kolthoff, I. M., and Lingane, J. J., "Polarography," 2nd ed., p. 203, Interscience, New York, 1952. ( 4 ) Lyalikov, Y. S.,Zhur. Anal. Khim., 5 , 323 (1950); Ibid., 8, 38 (1953). ( 5 ) Lyalikov, Y. S., and Karmazin, V. I., Zavodskaya Lab., 14, 144 (1948). (6) Xachtrieb, N.H., and Steinberg, >I., J . Am. Chem. Soc., 70, 2613 (1948). (7) Steinberg, XI., and Sachtrieb, S . H., Ibid., 72, 3558 (1950). ~I
LOG N
Figure 4. Change of half-wave potential with mole fraction for cobalt chloride and nicliel chloride Cobalt chloride.
Stntionary cathode (platinum. No. 27), 407' C. Nickel chloride (a). Rotating cathode (platinum, No. 301, 264 r.p.m., 407' C. Nickel chloride ( b ) . Stationary cathode (platin u m , No. 30), 411° C .
RECEIVED for review November 10, 1954. Accepted January 17, 1955, Presented before the Division of Analytical Chemistry a t the 125th lleeting of the . ~ E R I C A N CHEMICAL SOCIETY.Kansas City, 310...*Fril 1954.
