Europium and Ytterbium in Rare Earth Mixtures Polarographic Determination H. A. LAITINEN
I
AND
W . A. TAEBEL, University of Illinois, Urbana, Ill.
T IS well known that europium and ytterbium differ from the rest of the rare earth metals in their relative ease of
Typical current-voltage curves for various concentrations of trivalent europium in 0.1 N ammonium chloride solution are shown in Figure 1. The values of the diffusion current, id, a t -0.9 volt are given in Table I, together with the values corrected for the residual current. -4satisfactory proportionality between the diffusion current and the europium concentration was observed.
reduction from the trivalent to the divalent state. Bruckl and Noddack ( 1 , l a ) have determined the reduction potentials of various rare earth sulfates in 0.01 M solution, in the absence of indifferent electrolyte, using the dropping mercury electrode. I n each case, two polarographic waves were obtained, indicating a two-step reduction, first to the divalent ion and then to the metal. Their results suggested the possibility of determining either europium or ytterbium, or both simultaneously, in rare earth mixtures. Holleck (3)has proposed an indirect polarographic method for the determination of europium, involving addition of a known concentration of zinc to the rare earth solution and calculation of the europium concentration from the relative wave heights obtained for europium and zinc. Such a method tacitly assumes that the zinc ion and the europic ion have equal diffusion coefficients. Owing to the larger size of the hydrated europic ion and its higher charge (8),its diffusion coefficient would be expected to be lower than that of the zinc ion. It is shown below that Holleck's method is only approximate. No polarographic methods have previously been suggested for the determination of ytterbium. It was first necessary to determine the current-voltage curves of pure europium and ytterbium in the presence of an excess of suitable indifferent electrolyte to establish a basis for the detection and determination of these elements. Ammonium chloride was chosen as the indifferent electrolyte, because it provides a sufficiently acid medium to prevent the hydrolysis of dilute rare earth chlorides without causing interference with the ytterbium wave due to hydrogen evolution. Moreover, ammonium chloride is very easily removed from the samples, allowing a simple recovery of the rare earth material.
I
I
I
I
I
I
I
I
I
POTENTIAL, VOLTS
FIGURE 1. REDUCTION OF EUROPIUM IN 0.1 N AMMONIUM CHLORIDE
A millimolar solution of europium yielded a diffusion current of 2.88 microamperes. With the same dropping electrode, a millimolar solution of zinc chloride in 0.1 N ammonium chloride gave a corrected diffusion current of 6.34 microamperes. Taking into account the fact that the reduction of a zinc ion requires two electrons, it is evident that a 10 per cent correction is necessary if zinc is used as a standard in europium determinations. It is apparent from Figure 1 that the diffusion current regions of europium, particularly with the highest europium concentration (curve IV), show a downward trend with increasing negative potential. This behavior, although d e
Current-Voltage Curves of Europium A sam le of pure europium oxide, one of a series prepared by fractiona?crystalliaation for use in atomic weight determinations, was kindly provided by B. S. Hopkins. A 0.1-gram sample, weighed after ignition t o 700' C. for 5 hours, was dissolved in 2 ml. of 6 N hydrochloric acid and made up t o 100 ml. in a volumetric flask. Various portions were pipetted into 50-ml. volumetric flasks, titrated with 0.5 N ammonium hydroxide using methyl red as an indicator, and adjusted t o the desired concentration of ammonium chloride by addition of 1 N solution. The electrolysis cell and dropping electrode were of the type described by Lingane and Laitinen (9). A saturated calomel electrode of large area was used as an anode. All potential measurements given below are referred to this electrode. The m and t values of the dropping electrode were, respectively, 1.931 mg. per second and 4.10 seconds with the capillary dipping into distilled water at 25' and an open electrical circuit. For most of the diffusion current readings, a Fischer Electropodewith the galvanometer scale calibrated t o read microamperes was used. For determining the equations of the rising portions of the curves and the half-wave potentials, a manual apparatus similar t o that of Lingane and Kolthoff (8)was used t o enable the potential to be determined with sufficient accuracy. All measurements were ma+ with the electrolysis cell immersed in a water thermostat at 25 , regulated to *0.02".
TABLE I. DIFFCSION CURRENT OF EUROPIUM IN 0.1 N AMMONIUM CHLORIDE id
C Millimotes/lite~ 0.693 1,386 3.466 6.932' a
825
In 0.2 N NHdC1.
id
Observed Corrected Microamperes 2.20 2.01 4.19 4.00 10.25 10.06 19.75 19.56
id/c
Microampers/millimole/lilrr 2.90 2.89 2.90 2.83 Av. 2.88
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 13, No. 11
(e),
scribed as anomalous by Holleck is to be expected from the Ilkovic (5) equation for the diffusion current id
=
0.63 nFCD'
l2m2
(1)
18
The decreasing value of the drop time, t, with increasing negative potential causes a decreasing diffusion current. The total observed current is the sum of the residual current (curve I) and the true diffusion current. Because of the upward slope of the residual current line, the observed current increases continually with very low concentrations of europium but decreases with increasing negative potential a t higher concentrations. The following data show that the effect is quantitatively predicted by Equation 1. At a potential of -0.9 volt, the diffusion current, corrected for residual current, is 10.06 microamperes, while at a potential of - 1.6 volts it is 9.53 microamperes. In this interval of potential the drop time changed from 4.06 to 2.98 seconds, while m remained practically constant. Multiplying the current of 9.53 microamperes by the factor (4.06/2.98)116,we have 10.04 microamperes, which is in excellent agreement with the observed value at -0.9 volt.
ANALYSISOF EUROPIUM WAVE. It can easily be shown (6,7, IS) that if the reduction of trivalent to divalent europium is reversible, the equation of the rising portion of the current-voltage curve should be given a t 25" by
-i = n,/t + 0.059 log id
P
POTENTIAL, VOLTS
FIQUFLE3. REDUCTION OF YTTERBIUM IN 0.1 N AMMONIUM CHLORIDE
(2)
where ?r is the potential of the dropping electrode and i is the current a t any point on the curve. The half-wave potential, ?r1/2, should be independent of the europium concentration, and the slope of the straight line obtained by plotting log id - i -against the potential should be 0.059 volt if the reduc-
i
tion is reversible. A typical logarithmic plot for a europium wave is shown in Figure 2. The half-wave potential is given by the potential a t which log (id - i)/i is zero, and was found to be -0.671 * 0.002 volt (os. saturated calomel electrode) for various concentrations of europium or -0.425 volt referred to the normal hydrogen electrode. The average slope of the logarithmic plot was found to be 0.079 volt rather than 0.059 volt as given by Equation 2, indicating that the dropping electrode does not behave as a strictly
reversible europic-europous ion electrode. However, the deviation from reversible behavior is only small, as can be seen by a comparison of the half-wave potential with the normal potential of the europic-europous ion electrode. It can easily be shown that the half-wave potential and the normal potential, T O , are related by Equation 3 (assuming reversible electrode behavior). (3) 7111and DIII are, respectively, the activity coefficient and diffusion coefficient of the trivalent ion and 7 1 1 and DII are the same quantities for the divalent ion. Although the values of y and D for these ions are not known, it is reasonable to assume that 711 > 7111. Also DII > DIIIbecause of the higher hydration of the trivalent ion. Thus the last term of Equation 3 cannot be large, and the half-wave potential should closely approach the normal potential if the electrode behavior is reversible. Actually the value of -0.425 volt for the half-wave potential is in close agreement with the value -0.43 volt for the normal potential (based on concentrations rather than activities) reported by McCoy (11).
C u r r e n t - V o l t a g e Curves of Y t t e r b i u m
08
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I
I l l
-062 -064 -0.66 -066
I
-om
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I
- O R -074
POTENTIAL, VOLTS
FIGURE2. ANALYSIS OF EUROPIUM REDUCTION CURVE
A sample of ytterbium oxide prepared by a threefold electrolytic reduction of a rare earth fraction high in ytterbium content was available. Spectroscopic analysis showed traces of thulium and lutecium. The sample was dissolved in hydrochloric acid and treated in the same way as the europium sample. Typical current-voltage curves for various concentrations of ytterbium in 0.1 N ammonium chloride are shown in Figure 3. The diffusion current values a t -1.6 volts are given in Table 11. By logarithmic plots similar to that shown in Figure 2, the half-wave potential was found to be -1.415 * 0.003 volts (os. saturated calomel electrode) or -1.169 volts (us. normal hydrogen electrode). The average slope of the logarithmic plots was found to be 0.066 volt.
ANALYTICAL EDITION
November 15, 1941
CURRENT OF YTTERBIUM IN 0.1 N TABLE 11. DIFFUSION AMMOSIFM CHLORIDE
c
Yb+++ Mzllimoles/liter
id
id
Observed
Corrected
Microamperes
1.93
2.32
0.623
1.559
5.21 8.32 9.89
2.595 3.110
id/C Microamper ea/ millinole/liter 3.10
3.09
4.82 7.93 9.50
Av.
3.06 3.06 3.08
Comparison of Diffusion Currents of Europium and Ytterbium Owing to the closely similar structure of the various trivalent rare earth ions, and particularly the nearly identical conductance values of rare earth salt solutions (6),a much closer agreement would be expected between the diffusion current constants of europium and ytterbium than is shown by Tables I and 11. In order to make an exact comparison it is necessary to correct the observed values to the same value of the capillary constant, m*l 3 t1 16. Correcting the observed value of 2.88 microamperes per millimole per liter for europium to the diffusion current region of ytterbium at -1.6 volts, we have 2.74 microamperes per millimole per liter. If the observed difference between europium and ytterbium were entirely due to a difference in the diffusion coefficient, a simple calculation from Equation 1 shows that the diffusion coefficients of ytterbium and europium would be in the ratio of 1.26 to 1. It is probable, however, that the diffusion coefficients of europium and ytterbium ions are nearly equal and that another explanation accounts for the inequality of the diffusion currents. It is well known that ytterbous ions have a pronounced tendency to react with hydrogen ions according to the equation 2Yb++
+ 2H+
+
+ Hz
21’b+T~
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by the polarographic method and by titration after reduction with the Jones reductor. PROCEDURE. For the polarographicanalysis, a weighed sample was dissolved in hydrochloric acid and treated as described above. The final electrolysis was carried out in a medium of 0.1 N ammonium chloride. From the diffusion current value at a potential of -0.9 volt, the percentage of europium was calculated,using the calibration value from Table I. A current voltage curve of 25 ml. of solution containing 0.00995 gram of rare earth as oxide is shown in Figure 4. The volumetric determination used was a modification of the method of McCoy (II), in which the reduced europium solution is passed into a standard iodine solution and back-titrated with standard thiosulfate. Difficulties due to volatilization of iodine were encountered, with the result that the determinations were not uniformly reproducible. However, the most closely checking values by the method of McCoy were in agreement with those given below. Substitutions of stannic chloride for iodine removed the possibility of error due t o volatilization and yielded the advantage of a direct titration over a back-titration. Forty-five milliliters of a solution containing 0.3498 gram of rare earth as oxide and 0.2 N in hydrochloricacid were passed through a Jones reductor into 30 ml. of 0.5 N stannic chloride solution. The reducing column was washed with 100 ml. of 0.06 N hydrochloric acid. Each 100 ml. of solution was treated with 20 ml. of concentrated hydrochloric acid and 2 ml. of 0.02 per cent solution of diphenylamine in concentrated sulfuric acid. The resulting solution was titrated with standard 0.1 N potassium dichromate. An atmosphere of nitrogen was used throughout the reducing column and the titrating vessel. Although the titration reaction was slow near the end point, a definite end point yielding reproducible results was observed.
RESULTS.Two polarographic determinations with widely differing rare earth concentrations gave results of 55.0 and 54.7 per cent of europium oxide, respectively. Two volumetric determinations yielded 56.6 and 56.5 per cent of europium oxide, respectively.
(4
while europous ions have a much smaller tendency to undergo a similar reaction in a medium of the same pH. Thus the diffusion current value of ytterbium would be increased because of the increased supply of ytterbic ions at the electrode surface due to the reaction represented by Equation 4. If this view is correct, a slightly increased diffusion current of europium would be expected in a very strongly acid medium, in which the rate of oxidation of europous ions by hydrogen ions is increased. Actually, in 1.2 N hydrochloric acid solution a 5 per cent increase in the diffusion current of europium was observed. At higher acid concentrations no further increase was obtained, but a shift of the half-wave potential of europium to more negative values indicated a formation of complexes of europium with a large excess of chloride, with a consequent change of the diffusion coefficient. A similar effect could not be directly determined in the case of ytterbium because of interfering hydrogen-ion discharge even in very slightly acid medium. However, even in the presence of 10-4 N hydrochloric acid added to 0.1 N ammonium chloride, the M ytterbium inobserved diffusion current of 1.5 X creased 7 per cent, partly because of hydrogen-ion discharge. Therefore great care must be taken to neutralize the excess acid used in dissolving ytterbium samples.
Determination of Europium Since europium occurs only in very small concentrations in natural rare earth minerals, the usual analytical problem involves its determination in rare earth mixtures in which the europium concentration has been greatly increased by partial separation. I n the present study, a synthetic mixture of europium and ytterbium oxides was analyzed for europium
bl
H 8-I l!d POTENTIAL, VOLTS
FIGURE 4. CURRENT-VOLTAGE CURVEOF EUROPIUMYTTERBIUM MIXTURE The results obtained by the two methods agree to an accuracy of 3 per cent. The polarographic procedure has the advantages of greater speed and simplicity and is applicable to samples containing only traces of europium. The volumetric method is probably more accurate with samples of high europium content, but requires much larger samples and involves a much more difficult recovery procedure for the rare earth material.
Determination of Ytterbium The successful polarographic determination of ytterbium in rare earth mixtures depends upon obtaining well-defined
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TABLE111. COMPOSITION OF ORES Spectroscopic Composition,
%
I
Euxenite
I1
I11 Samarskite
IV Xenotime
V Cerite 2.0 20.00 4.25 1.75 1.0 1.0 9.9
7.0
0.5
...
...
Fergusonite
Y Nd Sm Gd
DY
Yb La
Polarographic Percentage Yb
5.3
reduction waves of ytterbium in the presence of relatively large concentrations of the other rare earths. Preliminary experiments showed that if the ytterbium concentration was 25 per cent or more of the total rare earth content, well-defined waves having the appearance of those shown in Figure 3 were obtained. In order to determine its limits of applicability, the polarographic method was applied to the analysis of mixed rare earth oxides obtained from various representative classes of naturally occurring minerals. Oxide samples which had been prepared and analyzed spectroscopically by McCarty, Scribner, Lawrenz, and Hopkins ( I O ) mere available. The composition of the various ores, as determined spectroscopically, is given in Table 111, together with the polarographic values for ytterbium. Approximately 2 grams of the oxide mixture PROCEDURE. were dissolved in 15 ml. of 6 N hydrochloric acid and made up t o 100 ml. in a volumetric flask. A 10-ml. portion was pipetted into a 50-id. volumetric flask. After the addition of a drop of methyl red solution, the sample was titrated with 0.5 N ammonium hydroxide, using a color comparison standard of 25 ml. of 0.1 N ammonium chloride containing a drop of methyl red. Sufficient 1 N ammonium chloride was added to bring its concentration to 0.1 N after dilution to 50 ml. The current-voltage curve was determined in the usual way.
RESULTS. Samples I, 11, and I11 (Table 111) were found to yield well-defined ytterbium reduction waves. The diffusion current region was most nearly horizontal in the case of sample 11, and had an increasing upward slope with samples I and I11 in that order. Table I11shows that the spectroscopic value for the samarium content of these samples increases in the order of samples I1 < I
