NOTES
Sept., 1963
1941
.+ MgClz +MgU04 + UO&lz
equal to 1.73 A., corresponding to a maximum radivs of 2.95 8.,the a-parameter is eel-tainly a t least 4 A. and one would expect negligible associations of this salt in water, on the basis of the sphere-in-continuum model. A possible explanation is that the potassiun ion is about the right size to fit into one of the triangular faces of the PFe- ion, thereby reducing significantly the center-to-center distances between cationic and anionw charges. With a larger cation, association to the PFsXlgUO4 MgCL +UOzClz 2MgO (3) ion should not occur. The purpose of this note is to and its subsequent decomposition by reaction 1. present some exploratory measurements which confirm Attempts to compare the new spectrum with those this expeatation. of known uranium(V) compounds, UC15 and u o c l ~ , Tetrabu ty lammonium and tetramethylammonium in molten chloride solutions were unsuccessful. At temhexafluorophosphates were prepared ; their solubility in peratures above the melting pointE of the chloride mater is very low, but they are sufficiently soluble in mixtures, UClli volatilized from the solution and coc13 acetonitrile to permit conductance measurements. was unstable. However, the spectrum of a petrolatum Acetonitrile has a dielectric constant of 36.01, about mull of UOCla was obtained which showed maxima corhalf that of water; therefore association is more probresponding to those of the uranium(V) species in soluable for a given electrolyte in acetonitrile than in water. tion. In addition, there were two major peaks in the Our data show that the tetramethyl salt is slightiy UOc13 spectral a t 1070 and 1270 mp which were not associated in acetonitrile, while association of the tetrapresent in the uranium(V) spectrum in molten salts. butyl salt is negligible. This spectrum is shown in Fig. 1. Experimental Molar absorptivities have been obtained for the Tetramethylammonium hexafluorophosphate was prepared by uranium(V) species in the LiC1-KC1 and in the ITaClmixing aqueous solutions of tetramethylammonium bromide and potassium hexafluorophosphate (Ozark-Mahoning Company). KCl-MgCl2 eutectics at 650'. These were obtained The precipitate was thoroughly washed with ice-water and was from absorbance measurements and the corresponding recrystallized from water. Tetrabutylammonium hexafluoroUOz+ concentrations calculated according to reaction phosphate was prepared in a similar way, starting with tetra1from the amount of chlorine remwed from the system. butylammonium bromide. I t s solubility in water is not over 5 X The values range from 5 to 20 depending 011 wave 10 -4; the butyl salt was recrystallized from 50-50 water-methanol. Acetonitrile was purified as described by Berns.4 The length and solvent. conductance cells used had constants equal to 0.3911 and 0.12448, Isosbestic points, at 590 mp in the NaC1-KCl-MgCl2 determined by comparison with a standard cell calibrated with system and at, 565 m p in the LiC1-KC1 system, were potassium chloride solutions.5 Electrical equipment and techgenerated when several spectra were taken of the same nique have already been described.6 The condqctance data are UOzClz solution at the same temperature after varying given in Table I. amounts of chlorine were removed. This behavior is TABLE I characteristic of concentration changes between only CONDUCTANCE IN ACETONITRILE two absorbing species in a solution, in this case uraA 104 e AA nium(V) and uranium(T'1). MedXPF6 Equilibrium constants for reaction 1 as calculated 14.010 181.85 2.44 from the preasures of chlorine in equilibrium with 10.533 183.84 1.86 various concentrations of UOnCl appear to be of the 7.703 185.72 1.36 order of atm a t 650'. 5.151 187.71 0.86 Acknowledgment.-The authors wish to express 2.680 190.36 0.53 their appreciation to Dr. D. M. Gruen for helpful 0.000 (196.75) 0.00 discussions on this problem. 2uo3
(2) Apparently UOzClz is also formed and is converted rapidly to the uranium(V) species as shown in reaction 1. MgU04 is slightly soluble in these salt mixtures, producing again uranium(V) solutions. This can be explained by formation of UOzClzas shown in reaction 3,
+
+
CONDl JCTANCE OF QUATERNARY AMMONIUM HEXAFLUOROPHOSPHATES I N ACETONITRILE BYJEHUDAH ELIASSAF,~ RAYMOND M. FUOSS, AND JOHNE. LISD, JR. Contrzbutzon X o . lY88 from the Steilzng Chemistry Laboratory of Yale Unzverszty, X e w Haven, Connecticut Receaved M a y 9, 1966
Potassium hexafluorophosphate has been shown to be associated ( K A = 2.42) in water, both by conductance and by vapor pressure measurements.2 Since the PFsion is octahedral, with P-F internuclear distances* (1) Grateful acknowledgment is made for a postdoctoral fellowship from a research grant made t o Yale University by the California Research Corporation. (2) R. A. Robinson, J. M. Stokes, and R. H. Stokes, J . Phgs. Cl*em., 65, 542 (1961). (3) H.Bode and G. Teufer, Acta CTgst, 8, 611 11955); H. Bode and H. Clausen, Z. a n o w C'lem ,266, 228 (1951); 268, 20 (1952).
Bu~NPFB 154.40 154.62 155.83 158.93 158.31 159.52 (164.75)
9.497 8.876 6.840 5.137 3.412 2,213 0.000
Discussion For slightly associated electrolytes (KA conductance equation simplifies to the form A
= ho
- XC'"
2.38 2.15 1.70 1.26 0.86 0.56 0.00
>
5 ) the
+ EC log c + ( J - KAAO)C+ O(C"')
because y, the fraction of solute present as unpaired ions, becomes practically unity. A plot of
h' = A
+ SC'/' - Ec log c
(4) D.S. Berns and R. 3%.Fuoss, J , A m . Chem. Soc., 82, 5585 (1960). (5) J. E. Lind, Jr., J. J. Zwolenik, and R. & Fuoss, 'I. zbzd., 81,1557 (1959). (6) J. E.Lind, Jr., and 11. h1. Fuoss, J . Phys. Chem., 65 999 (1961).
COMMUNICATIONS TO THE EDITOR
1942
against concentration is linear and extrapolates to A,. For the data of Table I the A’-c plots are linear and give the values of limiting conductances shown in parentheses. The last column of the table gives AA = A’ - A0
which is seen to be proportional to concentration, within experimental error. The slope of the A’-c plot for Bu&PF6 is 2500; if K A = 0, J ( a ) = 2500, whence d = 5.44. This value seems reasonable; if d were 6.00, K A would be 1.3. Hence we conclude that association is
Vol. 67
a t most slight for this salt. On the other hand, the slope for Me4NPF6is 1730; if K A is set equal to zero, J ( a ) = 1730, a value which leads to (& = 2.90, which definitely seems too small. If d = 5.00, the slope 1730 leads to K A = 5.0 for this salt. Our conclusions, based on these results, are that Me4NPFBis slightly associated in acetonitrile and that Bu4NPF6shows negligible association. Both conclusions are those expected on the basis of the structure of the salts and the dielectric constant of the medium. Therefore the association2 of KPF6 in water represents a special case of specific ionic interaction.
COMMUNICATIONS TO THE EDITOR THE FORMAL REDUCTION POTENTIAL OF THE EUROPIUM(II1)-EUROPIUM(I1)SYSTEM Sir: The formal reduction potential of the Eu3+-Eu2+ system was determined by McCoy1 in a potentiometric study of the cell: Pt I Eu(COOH), Eu(COOH)~ HCOOH N KC1In.c.e. The value obtained was -0.43 v. 11s. n.h.e. During a chronopotentiometric study of the europium couple in various supporting electrolytes, we obtained evidence for preferential complex formation between europium(II1) and formate ion in aqueous solution. The formation of such a complex would shift the equilibrium potential to more negative values. This leads us to believe that McCoy’s value is in error. I n order to estimate the true formal reduction potential we carried out a chronopotentiometric study with M europium in the noncurrent reversal of 3 X complexing medium, 1M sodium perchlorate-perchloric acid (pH 2.08) a t 25.0’. The Eu3+-Eu2+ system is chronopotentiometrically irreversible in this medium, and the absolute rate theory must be applied according to the method of Delahay,2in order to obtain the formal potential. This technique requires a linear plot of E, the observed potential, us. a log term in t, the time of electrolysis. The resulting straight line is extrapolated to the potential at which the time equals zero (E2=,0). The standard heterogeneous rate constants, k o f , h and kob,h, for the reduction and oxidation, respectively, may then be obtained using the equations
+
+
kof,h = (io/nFCo)exp(n,FE+o/RT) kob,h =
(iO’/n’FC0)/eXp((l - CX’)na’Ff?’+O/RT)
ErO = RT/nF In
(kr,ho/kb,ho)
(3)
The results are summarized in Table I. TABLE I STANDARD FORMAL REDUCTION POTENTIAL OF THE Eu3+-EuP+ COUPLEIN NONCOMPLEXINQ MEDIUM
+
Run no.
I I1 I11 IV
kor.h, om. sec.-1
6.03 x 10-lo 1.42 x 10-9 1.60 X l o w 8 6.64 x
k b , om. sec.-1 2.26 X 10” 5.47 x l o o 1.81 X 10’ 1.29 X 10’
ErO us. Ag-AgC1-NaC1 (satd.)
-0.566 V. -0.566 V. -0.535 V. -0.549 V . average = -0.554 v.
On the basis of these results, we would recommend the use of the value of -0.55 v. us. Ag-AgC1-NaC1 (satd.) (-0.35 v. us. n.h.e.) for the formal reduction potential of the Eu3+-Eu2+ couple in noncomplexing medium. Further studies of the electrochemical behavior of the Eu3+-Eu2+ system in sodium perchlorate-perchloric acid medium, as well as in other supporting electrolytes, are in progress. A more detailed paper describing experimental techniques and results will be forthcoming. CHEMISTRY DEPARTNEXT LARRYB. ANDERSON SYRACUSE UNIVERSITY DANIELJ. MACERO SYRACUSE 10, X. Y. RECEIVED JULY 15, 1963
THE SQUARE WAVE APPROXIMATIOK AND
(1)
RADICAL LIFETIME MEASUREMEKTS
(2)
Sir: Rotating sector techniques are widely used for determining individual rate constants and/or radical lifetimes. The method is particularly well suited for photochemical work where pure square wave pulses of radiation are readily obtainable. The greater penetration of ionizing radiation, particularly yradiation, makes it virtually impossible to obtain a square wave form in a rotating sector experiment. Thus, penumbral effects distort the result. The magnitude of the distortion is unknown. Ghormley’s results on the electron beam decomposition of water, for instance, show steady-state peroxide concentrations
where io is the current density, Co the original concentration of the oxidized species in the solution, n the number of electrons in the over-all electrode reaction, and n, the number of electrons in the rate-determining step; F , R, and T have their usual significance. The primes in eq. 2 indicate that the appropriate parameter is for the oxidation process. Using these values, the formal reduction potential of the system may be calculated from the relationship (1) H. N. McCoy, J . Ana. Chem. Soc., 58, 1577 (1936). (2) P. Delahay, “New Instrumental Methods in Electrochemistry,” Interscience Publishers, Inc., New York, N. Y., 1954, Chapter 8.
