Jan., 1957
VARIATIONOF DISTRIBUTION QUOTIENT WITH METAL ION
75
THE SOLVENT EXTRACTION BEHAVIOR OF INORGANIC COMPOUNDS.’ 111. VARIATION OF THE DISTRIBUTION QUOTIENT WTIH METAL ION CONCENTRATION BY R. M. DIAMOND Contribution from the Department of Chemistry and the Laboratory for Nuclear Studies, Cornell University, Ithaca, N . Y. Received July $6, I966
The metal ion concentration dependence of the distribution quotient expression previously derived2 has been tested for niolybdenum(V1) at concentrations from 10-9 to 10-2 M distributing between aqueous HC1, HBr and mixed HCl-HNOa and HBr-HC104 acids and the following oxygenated organic solvents: diethyl, p,p’-dlchlorodiethyl and dibutyl ethers, methyl isobutyl and methyl amyl ketones and 2-ethylhexanol. These results are interpreted in terms of the expression derived for b log D / b log Q and show that both polymerization of the metal-containing species and the “common ion” effect can cause a variation of D with molybdenum(V1) concentration. Some information on the molybdenum(V1) species is also obtained. It is shown that polymeric forms can exist in the aqueous solutions a t hydrohalic acid concentrations below 6 hf (and 7 is Lowering the p H causes the formation of polymeric species5; some, - , well such as the paramolybdate ion, M o ~ O ~ Jare characterized.6 Around pH 0.9 the isoelectric point O C C U ~ S and ,~ precipitation of hydrous Moos takes place. Increasing the acidity still further (1) Supported in part by the joint program of the Office of Naval Research and the Atomic Energy Commission. (2) R. M. Diamond, THISJOURNAL, 61, 69 (1957). (3) H. Irving, F. J. C. Rossotti and R. J. P. Williams, J . Chem. Soc., 1906 (1955). (4) H. J. Emelbus and J. S. Anderson, “Modern Aspects of Inorganic Chemistry,” D. Van Nostrand Co., Inc., New York, N. Y., 1954, p. 215.
(5) G.Jander, 2.anoyg. Chem., 194, 383 (1930). ( 8 ) (a) J. Sturtevant, J . A m . Chem. Soc., 59, 630 (1937); Lindqvist, Acta Chem. Scand., 8, 88 (1948). (7) (3. Carpeni, Bull. soc. chim. Prance, 1 4 , 4 9 6 (1947).
(b) I.
results in solution of the oxide precipitate if the acid used has an anion such &ssulfate or chloride which forms complex ions with m$als; perchloric acid is not very effective in dissolving Moo3. There have been suggestions that the resulting acid solutions contain the cation MOO?++,similar to the UOz*+ ion, or even Moo++++,but there is little evidence for the existence of such ions. Resin-exchange studies performed in the course of the present work showed little or no absorption of molybdenum(V1) by a cation resin (Dotvex-50) from solutions 1-12 IM in HC1 or 1-6 M in HBr, HF, HzS04 or “08, while anion-exchange resin (Dowex-1) gave marked absorption from the same solutions. Only solutions 1-6 M in HC1O4 showed very low distribution quotients for the anion resin and some slight absorption on cation resin. These results indicate that if cationic molybdenum(V1) species exist in these sohtions (other than perchloric), the species are probably polymeric and large. Except in the perchloric acid solutions, however, there must be anionic species small enough to exchange. Dimeric species have been suggested for both molybdenum(V) and molybdenum(V1) in hydrochloric acid solutions below 6 M in concentration,8 and the monomerdimer equilibrium has been demonstrated for the former. In 2 M HCl, 0.2-0.3 M molybdenum(V) is predominantly dimeric, but with increasing HCl concentration the equilibrium shifts in favor of the monomer so that in 6 M HCl, the monomer predomi n a t e ~ . Such ~ behavior seems to indicate that the dimer contains more oxygen or less chorine, or both, than the monomer. No proof of such a monomerpolymer equilibrium has yet been given in the molybdenum(V1)- HCl system, but evidence for I t comes from the results of the present work. Since the formula of this proposed polymer is not known, it will be assumed, as in the molybdenum(V) case, to be a dimer and will be represented by the symbol G in the following equations. Previous worklo has shown that the species extracting into diethyl and diisopropyl ethers from 3, (8) (a) A. R. Tourky and H. K. El-Shamy, J . Chrm. Xoc., 140 (1949); (b) H. K. El-Shamy and A. h4. El-Aggan, J. Am. Chem. Soc., ‘75, 1187 (1953). (9) L. Saoconi and R. Cini, i b i d . , ‘76, 4239 (1954). Actually, this
experiment does not rule o u t the possibility t h a t the dimer is really a higher polymer. (lo) L. Nelidow and R. M. Diamond, T H I e ~ o u R N a b6, 9 , 7 1 0 (1955)
76
R. M. DIAMOND
6 or 9 M HCl or 6 M HBr solutions have the composition MoOzXz(H20),(Ether),, where X is C1 or Br. Conductivity studies“ on the ether extracts indicate that the molybdenum-containing species is either molecular or, if ionic, highly ion-paired in these solvents of low dielectric constant. The same empirical formula holds for the molybdenum(V1) species extracted into p,p‘-dichlorodiethyl ether from 9 M HCl or from 7.5 M LiC1-2.5 M HCl, but conductivity studies in this solvent, as well as in methyl isobutyl and methyl amyl ketones, indicate that the molybdenum-containing species is no longer mainly ion-paired, a t least not as much as HC1, although more so than perchloric acid. Also, since other metal halides which extract into oxygenated organic solvents from aqueous hydrohalic acid solutions do so as the anions of acids stronger
than HC1, i.e., HFeCL, HAuC14, HGaC14, HTICll HInBr4, etc., it is plausible to ascribe the ionization of the molybdenum species to an acid ionization into H +.o~vated and a molybdenum containing anion, possibly MoOs(OH)X2(H20)- or Mo02(0H)X~ (Ether) -, which shall be represented by the symbol A-. The anion-exchange resin work briefly mentioned above shows that anionic molybdenum species also exist in the aqueous phase, and these include, as is to be expected, dihalide species of the form represented by A-.12 Since solid compounds of the type M[MoOZC~(HZO)] and Mz[MoOzC14] (M = NH4+, K+, Rb+, Cs+) can be precipitated from solutions,l3 the existence in aqueous solution of anionic species containing three and four halogen atoms and carrying a dinegative charge is possible; such anions will be represented by the symbol B-. Since it is not known whether the molybdenum dimer present in solutions less than 6 M in HC1 dissociates directly to form anionic species or also is in equilibrium with a neutral monomer, the possibility of the latter will also be considered for completeness. It will be given the symbol N, although its existence in moderate amounts, or lack of existence, does not change the results to be described. Then the possible species to be considered in the aqueous and organic phases, including the hydrohalic acid, HX, and the possible presence of a foreign strong acid, HZ, are Aqueous phase: G, N, A-, B; H+, X-, ZOrganic phase: N, A-, H+, X-, Z-, HA, HX, HZ’4
I n the organic phase, the last three entities may be true molecular species, but, except for possibly HX, are most likely ion-pairs. Higher ion associations (11) R. M. Diamond, unpublished results. (12) H. M. Neumann and Nancy C. Cook, Abstracts of the 129th Meeting of the American Chemical Eooiety, Dallas, Texas. April, 1956. (13) J. W. Mellor, “A Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. XI, Longmans, Green and Co., London, 1931, p. 632. (14) On purely eleotrostatib grounds. small polynegative ions, represented by B‘, are not expected to extract significantly into these organic solvents of moderately low dielectric constants and actually dinegative ions such as ZnClr- and CoCIa- do not extract appreciably while the similar uninegative ions FeClr-, AuClr; GaCb-, TlClr-. InBrc- do.
Vol. 61
such as ion-triplets, quadruplets, etc., are also possible,15 but apparently are not important in the molybdenum concentration range up to M covered in this work. SG the molybdenum distribution quotient can be expressed as
where ( )o and ( ) indicate concentration of the enclosed species in the organic and aqueous phase, respectively, and charges on ions are not being shown. Using the notation of reference 2 and the derivations given there one obtains16
IC
and in terms of aqueous concentrations
In this paper we are concerned with the variation of D caused by variations in the molybdenum(V1) concentration. The experimental data will be presented in graphs of log D us. log Q and compared with the expression derived from eq. 3 b-=----logD b l o g P blog(A) blog& a i o g ~ b i o g ( ~ )a i o g ~ ~ I O = -( A - ) bp (A) - -bQ
&
W)/ Q W)
P
-
~
- l / J o A - [ a ~ (X) ~
+
CYEA HZ
(A) (Z)
+
HA
3 -fa
(A)
1 +fa
(4)
wherefOA- is the fraction of organic phase molybdenum present as A- and f~ is the fraction of aqueous phase molybdenum present as G, i.e. fa=----2 ( G ) - 2Pa (A)’g‘’(H,X)
Q
I
-Q
Experimental Tracer and Reagents .-The radioactive tracer used was 6.7 hour Mo93m. It was produced by irradiating Nbgs metal foil with deuterons in the Massachusetts Institute of Technology cyclotron and separating Carrie:-free Mogsm from the target. The niobium metal contained 0.004% molybdenum, which determined the lower l i m t ?n the concentration of molybdenum usable in the extraction experiments. Samples of the tracer were followed for decay over a several week period, so that the amount of long-lived (nonextracting) impurity present a t the time of the distribution experiments could be determined. It was negligible during the course of the experiments. Reagent grade nitric, hydrochloric, hydrobromic and perchloric acids were used without further treatment, as were reagent grade diethyl ether and C.P.dibutyl ether, methyl isobutyl and methyl amyl ketones, and practical grade 2-ethylhexanol. In contrast to these solvents, two different samples of C.P. D,p’-dichlorodiethyl ether were used, and were found to give different distribution quotients. Both samples were distilled under reduced pressure and a ( 1 5 ) (a) R. M. Fuoss and C. A. Kraus, J . Am. Chsm. Soe.. 66, 2387 (1933); (b) N. H. Nachtrieb and R. E. Fryxell, ibid., 70, 3552 (1948): 74, 897 (1952); (c) R. J. Myers and D. E. Metzler, ibid., 72, 3772
-
(1950). (N)a (16) The relationships needed in this case are a N ( N ) aHX(H)(X) (H)a(X)o, ~ H Z ( H ) ( Z )= (H)D(Z)D,w~A(H)(A)= (H)o(A:@,dHA(H)n(A)o = (HA)o, &(A)g(H,X) = (B), BN(A)g’(H,X)=, (N), Ba(A)WH,X) = (G), and (H)D = (X)O (2). ( A h where a,8, a are concentration constants and 0 , a’. and 0” are functiona of (HI and (X) b u t not (A).
-
+
+
I
I
similar two degree cut taken, but these cuts still yielded unchanged, differing, quotients. The cause of thls is not known and only the results with the better extracting ether are presented. (The shape of log D us. log Q plots are similar for the two, differing only in magnitude of D . ) The various molybdenum( VI ) solutions were prepared by weighing out reagent grade Na~iCZoOa.2H~0 into the appropriate volume of acid to make a 0.100 M solution. From these solutions aliquols were taken and diluted with more of the acid, and the process repeated down to 10-8 M molybdenum. Lower concentrations were obtained using the tracer alone. Procedure.-Five or ten milliliters of aqueous acid were added to equal volumes of the organic solvents in 2 oa. glassstoppered bottles. (The solvents were not pre-treated with the aqueous acids.) The tracer was added in a volume of 5-20 mcl., and the bottles were shaken for 40 minutes pi1 a mechanical shaker. (Distribution quotients determined after 5, 10, 20, 40,80 minutes, and 6 and 20 hours, ahowed that constant values were obtained in less than 20 minutes.) The bottles were then allowed to stand for 10-20 minutes; satisfactory phase separation usually occurred immediately, but for a few difficult cases the mixture was centrifuged. These experiments were all perforomed in an air-conditioned room a t a temperature of 21 f 1 . Two-milliliter aliquots from each phase were then pipetted into screw-cap vials and ?-counted using a well-type NaI( T1) scintillation counter. The ratio of the counting rate, corrected for background, of the sample from the organic phase to that from the aqueous phase gives directly the value of the distribution quotient, D,of molybdenum(V1) between the two phases. Duplicate trials showed that reproducibility was usually 5% or better; exceptionally difficult systems might show differences as large as 20%, but these were rare.
Results and Discussion The experimenta.1 results for the changes in the distribution quotient, D , with variation in the total aqueous molybdenum(V1) concentration, Q, for extraction from various aqueous acid solutions into the solvents are given in Figs. 1-5. The data are plotted as curves of log D us. log Q for a variation of Q from to M , and for a variety of HCI and HBr concentrations, as well as for some mixed HC1-HNO, and HBr-HC104 solutions. From eq. 3 it can be seen that the only species which can lead to a change of D with variation of the aqueous molybdenum concentration are the dimer, G, in the aqueous phase and the anion, A-, in the organic phase. If a dimer can exist in the aqueous phase, its formation with increasing molybdenum concentration should cause a decrease in D which is independent of the nature of the organic I
"
-0
I
I
-8
-6
log
0.
I
-8
I
I
I
-6
I
-4
I
\t II I
-2
P. Fig. 1.-Plot of log D us. log Q (equilibrium aqueous molybdenum concentration) for diethyl ether and (initial concentrations) 2.0 M HCI, A; 3.1 A t HCl-1.0 d l "03, B; 4.1 M HCl, C; 7.1 hf HC1, D ; for dibutyl ether and 8.1 M HCl, E; 10.1 M HCI, F ; 9.0 A f HC1-2.0 Al "03, G; 11.9 M HC1, H; 5.8 M HBr-1.0 M HC104, I; 7.7 M HBr, J. log
-2
.-4
1
Fig. 2.-Plot of log D us. log Q for 2-ethylhexanol and 2.0 ill HCl, A ; 2.0 M HBr, B ; 1.5 M HBr-0.5 M HClOa, C; E; 3.1 M HCl-1.0 M "03, D ; 9.0 A f HC1-2.0 M "03, 4.1MHC1,F; 11.9MHC1,G; 10.1AfHC1,H; 7.1MHC1, I; 5.8 M HBr-1.0 M HClOI, J ; 7.7 M HBr, K. 0
0 o -
J
I
1
I
I
I
I
I
I
-o-o-o---a
\0 H
0G c1)
-O
EF D
c
-21
I
-8
I
I
-6
I
log 9.
I
-2
-4
Fig. 3.-Plot of log D us. log Q for DDE and 3.1 M HCl1.0 M HN03, A (the D-values have been multi lied by 10); 4.1 M HCI, B; 6.2 hf HC1, C; 4.0 M HC1-2.4 if HNOJ, D ; 7.1 M HC1, E;(5.1 A f HC1-2.0 f i f "03, F ; 9 M HC1-2.0 M "03, G; 8.1 M HCl, H; 11.9 M HCI, I; 10.1 M HCl, J.
solvent, being an aqueous phase phenomenon. This decrease should be of the form b log D / b log Q = - f ~ / ( l fc), with a limiting slope of - '/z when f~ + 1 a t high Q. (The slope mould have a somewhat larger negative value if higher polymers are involved, i e . , - z / / 3 for trimers and - 1 for very high polymeric species.) On the other hand, if no dimer exists but the anionic species, A-, occurs in the organic phase, b log D / b log Q would take the form -l/2.foA~ H A ( A ) / ~ ~ H A ( aHX(X) A) aHZ(Z)] and so would depend on the nature of the organic solvent through the different relative solubilities and ionization of HA, H X and HZ in the organic phase when shaken with the same initial aqueous solution. Thus the common ion" effect described in the preceding paper can be differentiated from the effect of aqueous phase polymerization. Consider first the solvents, diethyl and dibutyl ethers, representative of the aliphatic ethers. These two solvents have low dielectric constants, 4.34 a t 20" and 3.06 at 25O, respectively,17 5 0 that small and moderate sized ions extracting into them
+
+
-21
7'7
VARIATION OF DISTRIBUTION QUOTIENTWITH METALION
Jan., 1957
+
(17) A. A. Maryott and E. R. Smith, "Tables of Dielectric Constants of Pure Liquids," National Bureau of Standarda Circular 514, 1951.
78
R. M. DIAMOND
I
' 0 I
I
I
-8
-6
I
loo 0.
I \
I
-2
-4
Fig. 4.-Plot of log D us. log Q for methyl isobutyl ketone and 1.6 M HC1-0.4 M HN03, A; 3.2 M HBr-1.0 M HC104, B; 1.2 M HC1, C; 4.3 M HBr, D; 2.0 M HC1, E . 3.1 M HC1-1.0 M "Os, F; 4.1 M HCl, G; 4.6 M Hdl-1.5 AT "03, H ; 6.2 M HCI, I; 8.1 M HC1, J.
IF
II
O-
"
I I E
D
c
I
1
0 -0 -0
I
-8
0 0
I
I
-6
-0
I
log 9.
I
-4
I
I
-2
i I
I
Fig. 5.-Plot of log D us. log Q for methyl amyl ketone and 1.5 M HC1-0.5 A 1 "08, A ; 2.0 M HCl, B; 3.1 Af HCI1.0 M "03, C ; 4.1 A1 HCI, D; 7.5 M HC1-2.0 M "03, E; 10.1 M HC1, F; 8.1 M HCl, G.
will largely form ion-pairs except a t vanishingly small concentrations. The term in (A) in the numerators of eq. 3 and 4 will thus be negligible, so that any molybdenum concentration dependence of D will have to come from the presence of the dimer. The dimer is expected to exist a t low HC1 concentrations and, in fact, the curve of log D vs. log Q for 2 M HC1 and diethyl ether (Fig. 1) is flat from lo-* to M Q, and then D does begin to decrease, the slope reaching a value of about -l/z. Simito belarly, the curve for 4 M HCl is flat from M Q, and then again shows a decrease. yond The fact that the decrease starts at a higher equilibrium molybdenum concentration in 4 M HC1 than in 2 M HCI indicates that the monomer-dimer equilibrium involves hydrogen and chloride j ons, as suggested earlier. On going t o higher HC1 concentrations, the formation of the dimer requires still higher molybdenum concentrations and so the curve for 7 M HCl is flat from to M. Changing t o dibutyl ether a t 8 M HC1 again yields
Vol. 61
a flat curve, as expected, as does 10 and 12 M HCI and 8 M HBr. A check on the above reasoning is obtained by studying the concentration dependence of D when a foreign strong acid, HC104, or HN03, is present. Since the extraction of molybdenum(V1) from HClO4 or HNOI solutions alone is small, the assumption that no appreciable amounts of new extractable molybdenum-containing species have been created is reasonable. Then, except with the anionic species, A-, the presence of the foreign acid will have little effect on the distribution, lowering the value of D perhaps, but leaving the shape of the log D vs. log Q plots relatively unchanged, including the decrease due to formation of the dimer. If the added acid is more soluble and highly ionized in the organic solvent than HC1, it will, however, have an effect on the extraction of A- as seen by the term in CYHZ(Z)in eq. 3 and 4; the added acid will repress the extraction of A- through the common ion effect described in ref. 2 and D will become small and independent of Q. Experimentally, the plot for 3 M HC1+ 1M H N 0 3 and diethyl ether is very similar to the 4 M HCl plot, except for having slightly smaller values of D. M Q, inI n particular, D still decreases above dicating that the decrease is due to the formation of molybdenum dimers (polymers) in the aqueous phase. The curves from 9 M HC1-2 M HN03 and 6 M HBr-1 M HC104 and dibutyl ether are flat from lo-* to M Q and practically unchanged by the addition of the H N 0 3 or HC104; the latter acids have little effect on the distribution of the H+A- ion-pairs existing as the predominant molybdenum-containing species in the ether. Next, consider extraction into 2-ethylhexanol. This solvent has a higher dielectric constant than diethyl and dibutyl ethers and with its OH group is more water-like in its solvating abilities. HC1 extracts into 2-ethylhexanol quite well, and is much more highly ionized there than in the two ethers, as is indicated by conductivity studies." So in this solvent, representative of the higher, immiscible alcohols, the fraction ~ O H A might be expected to be smaller and FA-larger, than with the ethers, but there will also be much more ionized HC1 dominating the organic phase. The experimental results are shown in Fig. 2. to With 2 M HC1 log D is constant from AI Q and then decreases. This curve is superposable on that for 2 M HC1 and diethyl ether by a translation along the ordinate axis, as it must be if the decrease is due, as suggested, to a common aqueous phase phenomenon, namely dimerization. The same is true of the 4 M HCl plot which is similar to the 4 M HC1 and diethyl ether curve. The 2 M HBr plot for 2-ethylhexanol also shows a decrease a t high Q due to molybdenum(VI) dimerization. From the diethyl ether results a t 7 M HC1, no decrease of D due to dimerization is expected up to M Q with 2-ethylhexanol and 7 M HCI and none is found. But the two situations are nevertheless somewhat different. I n the former solvent, the molybdenum(V1) and HC1 species exist predominantly as ion-pairs, FA-
