Analytical Separation of Rhenium and Molybdenum by Ion Exchange V l L L l E R S W. M E L O C H E and A.
F. PREUSS'
Department o f Chemistry, University o f Wisconsin, Madison, W i s .
An improvement has been made in the procedure for the separation of perrhenate from molybdate using Amberlite IR.4-400 (Clod-) wherein the volume of eluting agents and the time required have been considerably reduced. Potassium oxalate is recommended for the separation and elution of the molybdate and perchloric acid for the subsequent elution of the perrhenate.
I
K rl previous paper Fisher and ,2leloche ( 4 )successfully sepa-
rated mixtures of perrhenate and molybdate ions by use of ion exchange resins. Their procedure consisted of adsorbing both molybdate and perrhenate on Amberlite IRA-400, removal of the molybdate selectively with 2.5N sodium hydroxide solution, and finally elution of the retained perrhenate with 7'V hydrochloric acid. The removal of molybdate by sodium hydroxide and perrhenate by hydrochloric acid was timeconsuming because of the large volume of eluting agents required. The present paper describes the work undertaken to find more efficient developing and eluting agents for the separation, so the time required might be reduced and the concentration of the rhenium and the molybdenum might be increased in the respective eluants.
molarity of sodium hydroxide. An increase in the sodium hydroxide concentration would have had little effect on the elution of molybdate ion and it would have increased the danger of removing rhenium prematurely. Distribution Coefficients of Molybdate and Perrhenate in Hydrochloric Acid. Fisher ( 3 ) had noted that 800 ml. of 3 M hydrochloric acid could be passed through a column with 10 grams of resin containing 22.T mg. of rhenium before any rhenium n-as found in the effluent. Tet, by using 7 M hydrochloric acid removal of rhenium from a column was quantitative. In an attempt to develop B chromatographic separation of molybdenum and rhenium based on the use of hydrochloric acid, the distribution coefficients of riiolybdenum and rhenium as a function of hydrochloric acid concentration were determined. Figure 2 is a plot of Kd for molybdate and perrhenate us. molarity of hydrochloric acid. The rhenium distribution appears normal but the molybdenum distribution shows a minimum between 1.0 and l.5.V hydrochloric acid.
1
EXPERIMENTAL
Materials. The Amberlites IRA-400, IRA-401 (formerly XE75), and IRA-410 were obtained from the Rohm & Haas Co., and the Permutit S2 from the Permutit Co. All resin weights are on the air-dried basis. The potassium perrhenate (99.5%) was obtained from A. D. Jlackay, Inc. All other reagents were analytical grade except for the acids q-hich were of C . P . grade. Apparatus, column operation, and the analytical procedures are the same as those described by Fisher and Meloche ( 4 ) .
Table I.
Adsorption and Elution of Molybdenum with Hydrochloric Acid Solutions Volume,
Fraction
MI. 200 200 400 400
1 M HC1 249.0 115.5 119.1 37.1
M o in Effluent, 1Ig. 23f 3M HC1 HCl 399.0 224.6 106.6 105.7 39.2 120.8 4.2 50.3
RESULTS
amount of substance in resin phase/gram of resin K d = amount of substance in solution phase/ml. of solution Figure 1 is a plot of Kd for molybdenum and rhenium us. address, Rohm & Haas Po.. Philadelphia. P a .
I
I
I(
RHENIUM
0
MOLYBDENUM
I
e 4 6 8 MOLARITY OF SODIUM HYDROXIDE
7M HC1 175.9 88.5 113.2 70 1
Behavior of Molybdate and Perrhenate in Sodium Hydroxide Solutions. To determine whether IO!& (2.5-M) sodium hydroxide was the most effective concentration of reagent to use, the distribution coefficients of perrhenate and molybdate were determined as a function of sodium hydroxide concentration. The distribution coefficient ( K d ) is defined by Tompkins and Mayer ( 7 ) as follows:
: Present
IO
I
IO
Figure 1. Distribution Coefficientsfor Molybdenum and Rhenium in Sodium Hydroxide with Amberlite IR.4-400 (OH-)
Because of the low Kd value for molybdate in this region, column studies were made to determine whether the molybdate could be eluted with a volume of acid so small that no significant amounts of perrhenate would be removed from the column. Accordingly, 555.8 mg. of molybdenum as sodium molybdate was diluted to 200 ml. in a final concentration of 1, 2, 3, and 7 M hydrochloric acid and passed over columns of 10 grams of Amberlite IRA-400 (OH-). After a water rinse of 208 ml., the columns were eluted with their respective molarities of hydrochloric acid. Table I summarizes the results of these elutions. The data of Table I appears to verify the shape of the curve in Figure 2, although the minimum would be shifted to a higher acidity if calculated from the column data. Even when elutions were carried out to 2 liters, molybdenum still appeared in the effluents. Although, projecting the data of Fisher ( 3 ) , this volume of 2M hydrochloric acid is smaller than that required to begin the elution of perrhenate, it represents no advantage, volume-
1911
1912
ANALYTICAL CHEMISTRY
wise, over the amount of sodium hydroxide required to perform the s;tme elution. Moreover, the elution peak is sharper in the case of the sodium hydroxide with a corresponding decrease in the concentration of molybdenum in the tail fractions. Even if the molybdate could be quantitatively removed from the column with 2iM acid, an additional large volume of this acid would be required to elute the perrhenate. I n fact, it actually requires 800 ml. of 7 M hydrochloric acid to perform this elution. Therefore, other approaches to the problem seemed necessary. The same type of columnar study was made with sulfuric acid solutions. No minimum was found in the three concentrations studied, 1 , 3 , and 4.5M.
Table 11. Adsorption of Reduced Molybdenum by Amberlite IRA-400 Sample Conditions 5 5 5 . 8 mg. Mo 0,2.Y H2SOa 2 7 7 . 9 mg. M o Z M HCI 1 4 2 . 1 mg. h l o 211-1 HC1 555 8 mg. hlo 0 , 2 M NaH?POI 1 . 2 g. NazSzOa 5 5 5 . 8 mg. Mo la\' K2SOa
Reducing Agent Saturated sodium sulfite Mercury reductor Walden reductor (silver) Lykopon (Na~Sz04) Jones reductor (zinc)
0
MOLYBDENUM
Eluting Solution NaOH NaOH NaxCOs Na?C03 KzCzOa
I
I
4
MOLARITY OF
I
I
0 8 HYDROCHLORIC ACID
C1-
91 ti
CI-
95.5
HtPOc
92.1
SO1--
81.1
Effluent for Complete Removal of Mo, M I . 1600
Concn. 10%
ioon
05% 625M 1 25.M 1.Ti
1 e
%
lifi,3
Table 111. Elution of Molybdenum from Amberlite IRA-400 with Various Reagents
X RHENIUM
10000
110 in Fiist 400 M I . I.>fflnent,
Resin Form SOn--
1000 600
I
RHENIUM
0
MOLYBDENUM
I
IO'
Figure 2. Distribution Coefficients for Molybdenum and Rhenium in Hydrochloric .4cid with Amberlite IRA-400 (Cl-)
Adsorption of Reduced Molybdenum from Solutions by Amberlite IFU-400. The nest approach to an efficient separation from rhenium and molybdenum w s that of differential reduction. I n general, sesivalent molybdenum can be more easily reduced than septivalent rhenium. By using a suitable reducing agent it was hoped that molybdenum could be reduced to a cation and thus be free to pass through an anion exchanger. Table I1 lists the reducing agents and the percentage of molybdenum which passed through 10 grams of Amberlite IRA-400 (C1-) in 400 ml. of effluent. The influent volume was 200 ml. of sample followed by 200 ml. of water. I n each of these cases there was some retention of molybdenum by this resin. None of these methods would lead to a quantitative separation of rhenium from molybdenum. Where molybdenum was reduced to lIo(II1) with the Jones reductor there appeared to be some separation of the red and green isomers. I n this experiment, the effluent appeared a dark green whereas the resin retained a pinkish hue. Elution of the column with2M hydrochloric acid removed most of the red isomer. Choice of Eluting Agent for Molybdate. Because of the failure to achieve complete removal of molybdenum from the resin in acidic systems, a return to basic media was considered. In basic systems, molybdenum would exist as the simple molybdate ion in contrast to the acid systems where it exists as a poly acid or complex ion ( 2 ) . The reagents considered as eluants were mainly those which contained bivalent anions which showed some chemical similarity to molybdate and which would not interfere in the subsequent analysis for molybdenum and rhenium. Accordingly, elutions were performed with sodium hydroxide, sodium carbonate, and potassium oxalate solutions. Table I11
l
o
o
0
l
Kd
I
0.1
I
1.0
0
1.:
MOLARITY OF POTASSIUM OXALATE
Figure 3. Distribution Coefficients for Molybdenum and Rhenium in Potassium Oxalate Solutions with Amberlite IRA-400 (CzOd--)
lists the resuits of the elution of 555.8 mg. of molybdenuni from 10 grams of -4mberlite 1R.l-400 (OH-). The elutions were considered complete when the effluent contained less thau 0. I y of molybdenum per ml., t,he limit of the analyt,ical method ( 4 ) . Table I11 s h o w nearly a threefold decrease in elution volume for 1.11 potassium oxalate over 2 . 3 1 sodium hydroxide. TVlien 19.8 mg. of perrhenate were presrnt on a similar column of resin, no rhenium was removed by the same volunies of the respective. reagents. Figure 3 is a plot of the distribution coefficients for molybdate and perrhenate as a function of the molarity of potassium oxalate. rln increase in potassium oxalate concentration has a very small effect on the Kd for molybdenum and hence on the elution of molybdenum. Thus, ldf potassium oxalate would have the greatest efficiency in terms of obtaining complete elution with a minimum volume and weight of reagent. Choice of Eluting Agent for Perrhenate. After the select'iori of 1X potassium oxalate for the preferential removal of molybdate from -4mberlite IRA-400, the nest study was that of the elution of perrhenate. Fisher and lleloche ( 4 ) used 7-%f hydrochloric acid to remove the perrhenate quantitatively. Atteberry and Boyd ( 1 ) used a mixture of ammonium sulfate and ammonium
1913
V O L U M E 26, NO. 12, D E C E M B E R 1 9 5 4 thiocyanate. Both of these procedures required large volumes of reagents and, in addition, thiocyanate ion interfered with the analytical method for perrhenate. In the search for a more efficient eluting agent the perchlorate ion, whose chemical behavior approsirnates that of the perrhenate ioii, n as the leading possibility. Figure 4 shows a plot of the Kd for perrhenate as a function of the perchloric acid concentration. Comparing this with Figure 2, the values of Kd are smaller in the case of perchloric acid, indicating that this material should be R more efficient eluting agent for perrhenate. - _-
-
Table IV.
Effect of Resin Type on Retention and Elution of Perrhenate Volume, hfl
Reagent Sample plus wash HClO4, 1M
200 100 100 100 100
Amberlites IRA400
Re, Mg. per C u t Amber- rlmber- Amberlite" litea liteb IRAIRAIR.4410 401 400
Iti 8
2.37 0.28
10.3
?.34 0.04 0.93 Re recoyered, 70 98.5 89 0 Perrhenate pel sample, 19 8 iiie. Re. h Perrhenate per sample, 9 92 nig. Re.
0.17 14.5 3.08 1.07 0.35
Permutitb 5-2
...
...
8.46
7.26 1.72 0.81 0.12
1.00 0.24 0.0;
B(i.8
!l7 0
Y9.U
Table Y. Effect of Resin Form on Elution of lllolybdate and Perrhenate from Amberlite IRA-400 ____._~
Resin I'orni Volume, A y d r o s i d e . Throughput MI. hlo Re Sample plus wash 400 h'one Son? &CzOa, 1.U 200 831 1 None 0 94 None HCIOI, ].If 100
Eluted, Cu~ t ~ _ _ _ ~ ~ ~ _hlg. _ per _ _ Chloride hIo Re ~
Perrlilorate . hlo Re
~
403 9 123 9 0 1
None None None 14 3 15 6 5 20 3 87 0 26 0 16 0 01 0 04 These d a t a represent individual studies on Re and 310 rather than the behavior of niixtures. Molybdate. 535 mg. of RIo, and perrhenate, 19.8 mg. of Re, were used in each case.
e
Effect of Resin Type and Form on Elution of Perrhenate and Molybdate. Previously all reported work had utilized the chlcride or hydroxide forms of Amberlite IR.4-400. There was a possibility that other resin types or ionic forms might increase the effectiveness with which perrhenate is separated from molybdate. Using columns containing 10 grams of resin and a flow rate of 2 ml. per minute the data in Tables IV and V were obtained. From these data it was concluded that the perchlorate form of Amberlite IRA-400 was the best of the materials tested for this separation. Summary of Optimum Conditions for Separating Perrhenate and Molybdate. As a result of these experiments and those previously reported (3, 4),the following conditions were selected for the separation of perrhenate and molybdate: Resin type, perchlorate form of Amberlite IR.4-400. Sample content, less than 300 mg. of rhenium as perrhenate in an alkaline solution. There is no maximum or minimum set on the amount of concentration of molybdate in the influent sample but the sample volume must be sufficient to dissolve both components. Variations in the hydroxide concentration have no effect on the separation. Rate of flow, for a 5-gram column of resin rates as high as 2 ml. per minute have been used for all steps. Elution of molybdate, 300 ml. of 1X potassium oxalate for a 5-gram column. Elution of perrhenate, 300 ml. of 1M perchloric acid for a 5-gram column. Separation of Mixtures of Perrhenate and Molybdate. One hundred milliliters of solution containing 0.98 mg. of rhenium as perrhenate, and 0.1044, 1.044, and 10.44 grams of molybdenum as molybdate "ere placed on the column. The potassium salt of the molybdate was used in these experiments because of its greater solubility. These columns were eluted according t o the above procedure. The results are summarized in Table VI. From these results it was concluded that the separation of perrhenate and molgbdate was complete.
Table YI. Separation of Mixtures of Rhenium and Molybdenum
When a column of 10 grams of Amberlite IRA400 (Cl-) cwitaining 19.8 mg. of rhenium as perrhenate was eluted with 131 perchloric acid, the perrhenate was completely removed by 300 ml. of this reagent. Under similar conditions, the amount of illf hydrochloric acid required was at least 800 ml. From this it was concluded that perchloric acid as an effective eluant for pcrrhenate in that it removed it quantitatively from a column in a much smaller volume and 7%ith the expenditure of lcss moles of reagent than had heretofore been possible.
cut
I
I1 I11 IV
\
VI VI1
Reagent
+
Sample Hz0 1.v I(zC204 1.M KsC20.1 1-V RtCtOI H?O Total, >Io
+
100 1111. 1 M HClOi 100 1111. 1.11 HC104
100 ml 1.M HClOa Total, R e
Re recovered, %
1000
-
-
Kd
1.0 1.5 MOLARITY OF PERCHLORIC A C I D
-
e .O
Figure 4. Distribution Coefficients for Rhenium in Perchloric Acid Solutions with Amberlite I R 4 400 (ClOa-)
Volume, Ml. 200 100 100 100 30
1:lOO >IO,
Re/hIo Ratio 1:lOOO 1:10,000 hIn./CUt JICJ, G./Cllt
a7.7 4.5.0 1.40 0.04 ___
104 1
0 90 0 05
