VOl. 60
500
NOTES CONCENTRATION EFFECTS ON SOLVENT HMXI species and as dissociated H'+ and MXI). EXTRACTION COEFFlCIENTS OF SOME ions (neglecting the details of h y d r a t i ~ n ~ , ~The chemical potentials of the HMX4 species in the TRIVALENT METAL HALIDESlanb aqueous and organic phases are the same at equiBYJEROME SALDICK~
Contribution f rom the Department of Chamialr , Brookhaden National Laboratory, Upton, N . Received August 17, 1066
4.
The deviation from the Nernst distribution law shown by the hydrochloric acid : ferric chloride :isopropyl ether extraction system (i.e., increase of extraction coefficient with increase of ferric concentration)a has been clearly explained4s6in terms of ion triplet formation in the organic phase. Conductivity phenomena observed in the organic phase and viewed in the light of the work of Fuoss and Krause form the basis for the ion association viewpoint. Other trivalent metals whose halides extract together with one molecule of HC1 or HBr, such as gold, gallium, thallium and indium, behave in a similar manner a t high concentrations.a The conductivity data of Campbell4 also show that the HFeCI4 species dissociates into ions in the organic phase in very dilute solution. This dissociation should produce an increase in the over-all metal extraction coefficient at very low ferric concentrations by decreasing the activity (per unit of total iron concentration) of the undissociated HFeC14 species in the organic phase, as shown below. I n the case of In(Br), Au(Cl), Tl(C1) and Ga(C1 and Rr), recent work by Irvine and co-workers' has shown that the extraction coefficients Eorg/aq of the metals in ether extraction from strQng HCI and HBr solutions do increase as the metal concentrations in the aqueous phase decrease below apM , approaching an upper limit at proximately concentrations below 10-6 M . The addition of extractable acids (such as perchloric) to these systems suppresses this increase of E o r g / a q at low metal concentrations. If, as believed, these effects are due to ionic dissociation in the organic phase, the fer:ic system should behave in a similar fashion. The Extraction Coefficient, EOrg/,,.-Consider the organic phase to contain metal as undissociated (1) (a) Research performed under the auspices of the U. 8. Atomic Energy Commission. (b) Presented before the Division of Physical and Inorganic Chemistry at the 126th National Meeting of the A.C.S., September, 1954. (2) General Electric Co.. ANPD. Cincinnati, Ohio. (3) N. H. Nachtrieb and R. E. Fryxell, J . Am. Chcm. Soc., 1 4 , 897 (1952). This paper also contains referencos to earlier work in this field. (4) D . E. Campbell, AECU 2313. Ph.D. thesis, Reneselaer Polytechnic Institute, T r o v . N. Y.. 1052. (5) A. H. Laurene, AECU 2484. Ph.D. thesis, Rensselaer Polytechnic Institute, Troy, N. Y., 1952. (6) R. M. Fuoss and C. A. Kraus, J . A m . Chem. SOC.,66, 21, 476, 1019, 2387, 3614 (1933). (7) (a) R. H. Herber and J. W . Irvine, Jr., dbid., 76, 987 (1954): (b) R. H. Herber, W. E. Bennett, D . R. Bentz, L. C. Bogar, R. J. Dietx, Jr., G. S. Golden and J. W. Irvine, Jr., paper presented before the Division of Physical and Inorganic Chemistry a t the 126th National Meeting of the A.C.S., September, 1954.
librium. For a given activity of HMX4 species in the organic phase, the total metal concentration in that phase will be greater, the greater the extent of dissociation. For the sake of simplification, assume that the total concentration of HX in the aqueous phase is constant, and so large that changes in the metal concentration will not significantly affect the ionic strength. Under these conditions, the activity of the HMX4 species in the aqueous phase is proportional to the sum of the concentrations of all metal-containing species present, Z(M) w, regardless of what they may be, provided only that each contains only one metal atom. Then KiZ(M)w = (HMXdo
(1)
Parentheses refer to concentrations. The subscripts w and 0 refer to the aqueous and organic phases. K1,defined as written, is not a true equilibrium constant; it is a product of thermodynamic equilibrium constants, appropriate activity coefficients, and, when necessary, also the activities of those species in the aqueous phase, such as H+, X- and HX, which are assumed to be conslant. For example, K 1 includes the thermodynamic equilibrium constants of all the pertinent equilibria in the aqueous solution, the pertinent activity coefficients, the (constant) concentrations of H + and X- in the aqueous phase, and also the activity coefficient of undissociated HMX4 (which may be an ion pair) in the organic phase. The effect of an extractable acid HY in the system (present in concentrations small enough that the ionic strength and acidity in the aqueous phase, fixed by the HX present, are not affected) comes into consideration when the important dissociations in the organic phase
+
HMX, H + MXIH X Z H + + X HY;H++Y-
are described by equilibrium equations ( H M Z )o = Kz(H+)o(MXd-)o (HX)O= K8(H+)O(X-)O (HY)o = Kd(Hf)o(Y-)o
(2)
(3) (4)
The partition coefficient is The relation (H+h
= (Mx-410
+ (x-10+ (y-10
(6)
is a statement of over-all electrical neutrality. Then
Assuming that (HX)odepends only on HX in the aqueoiis phase, ant1 is constant, K i = (HX)O/Ka.
NOTES
April, 1956 Similarly, since the partition equilibrium of HY is described by equation 8 Ks(H+)w(Y-)w = (HY)o
(8)
and since (H+)wis controlled by the HX, we may write Ki = K6(H+),. Solving for (H+)o (9)
and substituting in (5) gives
which is of the form
where A = K1,
dz))
At the point of inflection, in the absence of Y-
The numerator describes the partition of HMX4, the denominator of HX. Both are moderately weak acids, hence the ratio and consequently the aqueous metal ion concentration a t the point of inflection should not vary much with changes of the organic solvent. The experimental findings of Irvine, et al., have shown that this is, indeed, descriptive of the behavior of these metals.8 KS, K 4 and K; are independent of the metal present, and equation 10 contains only two parameters which depend on the metal species present. For a given solvent, the ratio B/C is a function of H X concentration only. The ratio KL/K4 = (H+)o(Y-)~/(Y-)wwhich appears both in equation 10 describing the effect of (Y-), on E at constant Z(M)w and in equation 11 describing the effect of (Y-), on Z(M), a t the point of inflection may become accessible from data not involving metal extractions.9 Under certain conditions (e.g., lower HX concentration) it may be also necessary to consider the equilibrium (MXa)w J _ (MXs)o
and K:Z(M)w
in addition to eq. 1. This seems to be true in the case of thallium, l o for instance. Activity coefficients of ions in the organic phase are factors in Kt, K t and K4; these may not reasonably be expected to remain constant as the ionic strength of the organic phase changes. (The major contribution to the ionic strength in the organic phase comes from the H+ and MX4- ions themselves in the absence of extractable HY). This effect may be expected to complicate quantitative application of equation 10, but is independent of solvent extraction processes and can in principle be understood. The treatment above formally ascribes the variation in E to changes in the acidity in the organic phase. A thermodynamically similar development formally based on the equilibrium (MX4-)0
= K2, B =
Equation 10a shows that Eo/, rises to a "high value" a t low metal and Y-, conof (K1 K1/(KP centrations, and decreases when these increase. The point of inflection in the graph of E vs. log I;(M)woccurs a t
+
501
(MXJIO
(8) J. W. Irvine. Jr., personal communication.
(9) Experiments performed by Dr. R. H. Herber ahow that the equivalent conductivity of HClO, extracted into ethers from aqueous rolutiona is distinctly higher than that of HCI.
+
(MXS)~ (x-10
and formally stressing the effect of halide ions in the organic phase, leads to an equation identical with (10). However, other evidence3-, indicates that HMX, and MX4- are the significant species in the organic phase. Acknowledgment.-The author wishes to thank Professor J. W. Irvine, Jr., for his cooperation in making the unpublished results of his group available and for helpful discussions, and Dr. R. W. Dodson for his encouragement and advice. (IO) R. W. Dodson and R. D. Stoenner, private communication.
A NOTE ON THE OSMOTIC COEFFICIENTS OF AQUEOUS POTASSIUM CHLORIDE SOLUTIONS AT 25' BY R. A. ROBINSON DepaTt?"
of Chemistry, University of Malaya, Singapore Received October 10, 1066
Brown and Delaneyl have recently described an ingenious method for measuring the vapor pressure lowering of an aqueous potassium chloride solution : the solution is maintained a t 25' whilst the temperature of a sample of the solvent is progressively lowered until it registers the same vapor pressure, the equality of vapor pressure being judged by means of a sensitive interconnecting bellows pressure gage. Their experimental measurements were therefore temperature differences which were then converted into terms of vapor pressures by means of one of two relations between the temperature and the vapor pressure of pure water. Brown and Delaney expressed their results as osmotic coefficients, cp = -(55.5062/2m) In PIP", and gave a set of such coefficients over the range 0.025 to 2.38 M calculated by each method. Thus a t 2.3783 M they found cp = 0.9205 or 0.9204 but at 0.02515 M ,0.9588 or 0.9659. This may suggest that the form of the relation between the vapor pressure of water and the temperature has great influence on the result. Nevertheless, because of the way in which the osmotic coefficient is defined, the effect of experimental error is considerably exaggerated for dilute solutions. Since an experi(1) 0.L. I. Brown lcnd C. M. Delaney, THISJOURNAL, 68, 255 (1954).
,
