Aug. 5, 1952
POLAROGRAPHY OF
ETHYLENEDIAMINE TETRAACETATE COMPLEXES OF EUROPIUM 3773
with each other much as do perfluoro-n-pentane and peduoroisopentane. The isomers are much 1093.77 alike but they differ from each other slightly in loglo fimm = 6.8825 215.59 t( " C ) directions opposite to those for the corresponding (for perfluoro-2-methylpentane) hydrocarbons. Evidence appears to be accumulatThe normal boiling points for the two compounds as ing that chain branching in fluorocarbons results in shown by vapor pressures are 57.23 and 57.74; respectively. higher density, higher viscosity, higher boiling Their molar heats of vaporization at the boiling points are point, higher index of refraction and higher surface 7307 and 7273 cal., and at 20' they are 7793 and 7740 cal. tension. Apparently the branched molecules can (Data for %-C6F14 are given first in each case.) When calculating the heats of vaporization by the Clapeyron equa- pack together more closely than the normal ones. tion it was assumed that the vapors were ideal. Molar Acknowledgment.-This work was performed volumes of the liquids were obtained from their densities. under contract with the Office of Naval Research, Discussion U. S. Navy Department. These two isomeric perfluorohexanes compare SEATTLE 5, WASHINGTON
+
[CONTRIBUTION FROM
THE
LOS ALAMOSSCIENTIPIC LABORATORY]
Polarography of Ethylenediamine Tetraacetate Complexes of Europium1s2 BY E. I. ONSTOTT RECEIVED FEBRUARY 11,1952 In the presence of a large excess of anions of ethylenediamine tetraacetic acid, europium(II1) is reversibly reduced to europium(I1) a t the dropping mercury electrode in the p H range of 6 to 13.6. Complex ions identified from half-wave potential and diffusion current data are: pH 6 t o 8, [EuHYI-'; pH 11 to 13.6, [EuY]-*; pH 6 to 8, [ E U ( H Y ) ~ ] -PH ~; 11 t o 12, [EuY2]-'; pH 13 t o 13.6, (EUY(OH)]-~. Evidence is given for the,existence of [EuHY]-'and [EuY(HY)]-', in the pH range of 8.5 to 10.3. The ratios of dissociation constants for several of the complex ions are calculated. The equi[EuY(OH)]-* Y-4 is calculated t o be approximately 2. librium constant for the reaction [EuY*]* f OH-
+
Few of the known complexing agents are coordinated strongly to rare earth ions. Citrate has been used extensively in ion exchange separations,a and recently it has been shown that anions of ethylenediamine tetraacetic acid form stable complexes with rare earth ions.4-8 Of the rare earths which show oxidation states other than (111), europium and ytterbium appear to be the most suitable for studying complex formation by the polarographic method, since they are reduced to the (11) state at the dropping mercury electrode.Y-ll This report gives the results obtained in investigating the polarography of europium complexed by anions of ethylenediamine tetraacetic acid. Laitinen and Blodgett12 have used the dropping mercury electrode as an indicator electrode to show the presence of ytterbium(I1) and absence of neodymium(I1) when complexed by ethylenediamine tetraacetate. (1) Work done under the auspices of the A.E.C. (2) Data used in plotting the graphs in this article can be obtained by writing the author or by ordering Document 3612 from American Documentation Institute, 1719 N Street, N . W., Washington 6, D. C., remitting 11.00for microfilm (images 1 inch high o n standard 36 mm. motion picture film) or $1.00 for photocopies (6 X 8 inches) readable without optical aid. (3) F. C.Nachod. rl al., Academic Press, Inc.. New York, N. Y., p. 167. (4) H.Britzinger, H.Thiele and U. Muller, 2. anorg. allgcm. Chem., aai, 286 (1943). (5) T.Moeller and J. C. Brantley, THIS JOURNAL, 72, 6447 (1960). (6) G. Schwarzenbach and E. Frietag, Relv. Ckim. Acla, 84, 1603 (1951). ( 7 ) J. K.Marsh, J . Chcm. Soc., 1819 (1950). ( 8 ) R. C. Vickery, ibid., 1817 (1961). (9) W.Noddsck and A. Bruckl, Angcw. Ckem., 60, 382 (1937). (IO) H.A. Laitinen and W. A. Taehel, Znd. Eng. Ckem., Anal. Ed., 13, 825 (1941). (11) H.A. Laitinen, THIS JOURNAL, 64, 1133 (1942). (12) H.A. Laitinen and Eva Blodgett, ibid., 71, 2260 (1949).
For simplicity, the symbol proposed by Schwarzenbach and Ackermannla for ethylenediamine tetraacetic acid, H*Y, will be used. Experimental Spectrographically pure europium oxide, obtained from Johnson, Matthey and Co., Limited, was weighed and dissolved in dilute hydrochloric acid to make a stock solution. The concentration of europium was based on the weight of the oxide after igniting at 900'. Ethylenediamine tetraacetic acid was precipitated from a solution of the sodium salt (Bersworth Chemical Co.) with concentrated hydrochloric acid, then recrystallized twice from 6 N hydrochloric acid, washed with alcohol and dried under a heat lamp. This product was weighed and dissolved in sodium hydroxide t o give a stock solution. Sodium hydroxide was used for the reason that potassium hydroxide and tetrabutylammonium hydroxide gave irreversible reductions. Boric acid was used as a buffer for solutions having a pH of 8 t o 10, and sodium dihydrogen phosphate was used for the pH range of 6 to 8. Citrate was not a suitable buffer, as irreversible behavior was observed when it was used. The ionic strength of the solutions was adjusted with sodium chloride. Sodium nitrate was tried as an indifferent electrolyte, but a reduction wave at about -1.5 volts when europium was present precluded its use. A modified H-cell having a middle compartment separated by 20 mm. fine sintered glass discs was used. The first compartment and middle compartment contained the solution to be analyzed and in the third compartment was placed saturated potassium chloride, used as a bridge solution to a bulb type S.C.E. The a.c. resistance of the four cells used was 710,990, 1500 and 1750 ohms, as measured with a General Radio impedance bridge in conjunction with the dropping mercury electrode and S.C.E. A Sargent Model X X I polarograph was used to record polarograms. Potential measurements were made with a Rubicon Type B potentiometer. A Beckman Model G pH meter was used for p H measurements. For highly alkaline solutions, a Type E electrode was used. The polarograph was operated manually. Average current data were taken along the polarographic wave and in (13) G.Schwarzenbach and H. Ackermann, Hclu. Ckim. Acta, S O , 1796 (1947).
E. I. ONSTOTT
3774
the diffusion current region. All current values were corrected for residual current and potential values were cori ) / i us. rected for d? drop of the cell. Plots of log (;* were made t o determine the reversibility of the electrode reaction. Half-wave potentials were taken directly from the logarithmic plots. Nitrogen, which Measurements were made at 25 f0.1'. was passed through a chromous chloride solution t o free it of oxygen, was used t o purge the solutions of oxygen. A maximum suppressor was not used, as no maxima were observed. The capillary used had a m value of 1.471 mg. set.-' for a mercury column height of 52.1 cm.
-
Discussion Reversibility of the electrode reaction is demonstrated by the values of the slopes for plots of log (id - i)/ivs. E shown in Table I. TABLE I No. of log plots
pH rauge
6 to 8 8 to 11 11 to 12 >12
r
Av. value
16
Slope of log plot Low value High value
0.065 .065 .066 .065
8 9
10
0.063
0.008
.064 .061 .056
.067 .074 .071
These values are close to the theoretical value of 0.059 volt for a one-electron reduction. The diffusion current constant (Fig. 5) also is of the right order of magnitude for a single electron.reduction. In Figs. 1, 2, 3 and 4 are plots of log Cx vs. E1/2. Values of C,, the concentration of complexing anion, were calculated from the acid dissociation constants measured by Schwarzenbach and Ackermann13 (k1 = 0.01; kz = 2.14 X ks = 6.92 x 10-7; k4 = 5.50 x 10-11). Log C, is the parameter which determines of the following electrode reaction [Eu X p ] W f 3 ) * e- + [Eu X q ] ( Q b + 2 ) + + ( p - q ) X b The equation relating C, and El/2 i d 4for 25'.
+
- (E1/z)m
i=
-
0.05915 log Ka/Kz ( p - 4) 0.05915 log
(1)
Cxfx
Superscript b denotes the charge on the ligand X, while 9 and q are symbols for the number of ligands attached t o europium(II1) and europium(II), respectively. The subscript c refers to the complex ion and s to the simple ion. The dissociation con-
r '
I
'
I
1
1
/
1
I . 1
Vol, 74
stant of the europium(II1) complex is designated by Ks, that of the europium(I1) complex by Kz. C, expresses the concentration of complexing agent, and jxis the activity coefficient of the complexing agent. Although equation (1) is not exact due to the assumption that the ratio of the diffusion coefficients and the ratio of activity coefficients of the europium(II1) and europium(I1) complex ions are unity, it can be used as a good first approximation. #H11to 12.-At a 9H of 11 to 12 the complexing ion is ethylenediamine tetraacetate ion. Figure 1 shows that varies linearly with the logarithm of the ethylenediamine tetraacetate concentration. The reciprocal slope of the plot is 0.055 volt, a value close to the theoretical value of 0.050 volt for ( p q) in equation (1) equal to one. Therefore, europium(II1) must have one more ligand attached than does europium(I1). Since europium(I1) should behave like barium(I1) in complexing ethylenediamine tetraacetate'aJ6 and coordinate one ligand, then two ligands are attached to europium(111) ion. No report has been made in the literature of trivalent rare earths coordinating more than one ethylenediamine tetraacetate group. However, the experiments that have been carried O U ~ ~were - ~ not of such a nature as to employ a large excess of the complexing ion, as is used in polarographic measurements. #H > 12.8.-Above a pH of about 12.8, the halfwave potential becomes essentially independent of the ethylenediamine tetraacetate concentration (Fig. l), but varies linearly with the logarithm of the hydroxide ion activity (Fig. 2).
-
0.5 E 0 0.9 a 1.3 I
I
I
1.18
I
I
I
I
I
I
1.20
1.22 1.24 -Elf2 VS. S.C.E. Fig. 2.-Plot of pOH vs. E112. Open circles, variable pH, constant Y-4; l i e d circles, p H 13.2 0.1, variable Y-'.
*
Equation (1) is not valid for interpreting these data, since it is apparent that the europium is complexed by two different ligands. The correct equation can be derived by using the same line of reasoning as was used by Lingane14in deriving equation (1). Consider the electrode reaction
I
i
I
I
I
I
I
I
1
1
[Eu X,,&]W+ud+@ 1
+
+ e- + + ( p - p)xb+ (U -
PLl &Zv](Qbb+Dd+2)*
1
The derived equation for 25' is 1.19 1.21 1.23 S.C.E. (EI/,), = 0.05915 log Ks/Kz (p Fig. 1.-Plot of log CY-4os. EI/2: open circles, pH 11 t o 12, q ) 0.05915 log Cxfx 2 (U - V ) 0.05915 log C.f. variable Y-'; lined circles, pH 13.2 f 0.1, variable Y-*. 1.15
1.17
-&/z
YS.
(14) J. J. Lingane, Chcm. Revs., 29, l(1941).
-
-
V)Zd
-
(2)
(15) G. Schwarzenbach and H. Ackermann, Hclu. Chirn. Acto, 81, 1029 (1948).
Aug. 5, 1952
POLAROGRAPHY OF
ETHYLENEDIAMINE TETRAACETATE
COMPLEXES OF E U R O P I U M
3775
Here the symbols have the same meaning as those volt, indicating that europium(II1) is complexed outlined for equation (1). The second ligand is by one more ligand than is europium(I1). If the designated as 2; u and v are the number of attached same reasoning is used as was used in establishing the gormulas of the complexes present at a PH of groups and d is the charge on 2. Independence of El/, with respect to the ethyl- 11 to 12, it follows that two ligands should be enediamine tetraacetate concentration shows that attached to europium(II1) and one to europium ( p - q) in equation (2) is zero, and thus the same (11) * pH 8.5 to lO.d.-Above a pH of about 8.5 but number of ethylenediamine tetraacetate ions is coordinated to europium(II1) as is coordinated to less than 10.3, the behavior of Et/, with respect to europium(I1). The most probable number is one. log Cx is not well defined. The five vertical lined If no group were coordinated, the europium(111) circles which deviate from the straight line in Fig. would precipitate as the hydroxide, and if two or 3 are also plotted in Fig. 4. A straight line is obmore groups were coordinated, the charge probably would be too high for stahility of such a complex 1.4 ion. From the reciprocal slope of 0.066 volt for the plot in Fig. 2, it is ascertained that the predominant 1.8 species of europium(II1) coordinates one more *. hydroxide ligand than does europium(11). The I electrode reaction can be written 0" 2.2
+
[EUY(OH)]-~ e-
-+- [ E U Y ] - ~+ OH-
3M
I 2.6
I-
,/
This reaction appears to be the most logical, since / -) -. europium(II), like barium(II), should not be sufficiently strong as an acid to coordinate hydroxide ions readily. Hydroxo-ethylenediamine G I f lI ! I I . ~ tetraacetate complexes are not unusual; Schwarzenbach and co-workers have shown the existence of 1.15 1.16 1.17 1.18 transition metal complexes of this type. 13,16-18 Quite --E112 VS. S.C.E. recently Schwarzenbach and Heller18 have given Fig. 4.-Plot of log us. E I ~ zPH ; 8.5 to 10.3. quantitative interpretation to the study of iron(I1) and iron(II1) complexed by both hydroxo and tained by plotting the logarithm of the concentraethylenediamine tetraacetate ligands. tion of ethylenediamine tetraacetate ion as the pH 6 to %--In the pH range of 6 to 8 the complex- ordinate, but the reciprocal slope of 0.018 volt is not ing anion appears to be monohydrogen ethylenedi- indicative of definite complex species. Such deviaamine tetraacetate. Figure 3 shows the variation tion from ideality would be expected if the euroof Ell, with the logarithm of the ligand concentra- pium(II1) complex and/or europium(I1) complex tion. The straight line is drawn with the five were being neutralized in this pH region. A conclosed circles as a foundation, these circles repre- sideration of the magnitude of El/, helps to estabsenting data taken a t a fiH of 6.7 h 0.1 with vari- lish the principal electrode reaction. For the same able total ethylenediamine tetraacetic acid concen- concentration of ethylenediamine tetraacetate ion, tration. The reciprocal slope of the plot is 0.057 El/, a t a p H of 8.5 to 10.3 is more negative than it is a t a pH of 11 to 12. This can be explained by - . , assuming the following electrode reaction to be thc principal one a t a pH of 8.5 to 10.3 L-
4
1
!-
[Eu(HY)Y]-*
-
1.14
1.16
(3)
+
--+ [ E U Y ] - ~+ Y-4 (4) for the reason that &/Kz for reaction (3) would be [ E u Y ~ ] - ~ e-
-1
63
+ e- + [EuHYI-' + Y-4
The half-wave potential of this reaction should be more negative than that of the reaction
t'
1.12
-1
__ , -r-;---;
I
0.8
~
1.18
S.C.E. of log C ~ y - 3vs. E I / z . Open circles, pH 6 to -E1/2 VS.
Fig. 3.-Plot 8; crossed circles, pH 6.15 f 0.05; closed circles, pH 6.7 A 0.1; lined circles, gH 8.5 t o 10.2.
(16) 0. Schwarzenbach a n d W. Biedermann, Helu. Chirn. A c t a . , 31, 459 (1948). (17) G. Schwarzenbach, ibid., 32, 839 (1949). (18) G. Schwarzenbach a n d J. Heller, ibid., S4, 57G (1951).
expected to be a smaller number than KB/K2 for reaction (4). The addition of one hydrogen ion to (ethylenediamine tetraacetate) europate(I1) ion should make it proportionately more unstable than the addition of one hydrogen ion to bis-(ethylenediamine tetraacetate) europate(II1) ion. Diffusion Current Data.-The diffusion coefficient of an ion varies directly with its equivalent ionic conductance and inversely with its charge.lg Thus the diffusion current constant often can be used to distinguish different ionic species diffusing .to a dropping mercury electrode. Two ions of approximately the same structure and size but having different charges have similar equivalent ionic (19) J J Liny.ine dnd I M (1839)
Kolthoff, THISJ O U R N A L , 61, 826
H. F. HOLTZCLAW, JR., E;. W. R.
JOHNSON AND
conductances due to the fact that the higher charge is 111 part compensated for by more hydration. Since a diffusion coefficient varies with the degree oi hydration of the ion, the diffusion current constant indirectly measures the charge on the ion. 111Fig. 5 is plotted the diffusion current constant the p H of solutions having a constant total cthylenediamine tetraacetic acid concentration m d constant europium concentration. There are three distinct regions of constancy corresponding to the europiuni(II1) complex ions postulated for these fiH ranges. The relative order of magnitude i d the diffusion current constant that is expected lor these ions is: [Euk-2]-; < [Eu(HY)~]--~ < [Eu T(OH) The complex ion, (ethylenediamine tetraxetatep(monohydrogen ethylenediarnirie tetraacetate) europate(II1) ion, is not shown distinctly by the plot in Fig. 5, but neither was it irieirtifietl positivelv b y El/, data. '8).
u
F. W. HENGEVELD
Vol. T4
could possibly be observed if the electrode reaction were reversible and the polarographic wave were not obliterated by discharge of other ions. Several experiments were performed in which tetrabutylammonium ion was used as the indifferent cation and the pH was kept high to prevent a hydrogen wave, but no wave for the reduction to the amalgam was observed. Probably the reduction process is irreversible under the conditions tried, since the europium( 111) to europium(11) reduction was found to be irreversible when tetrabutylaminonium ion was present. Calculations.-Data obtained in this investigation are not of such a nature so as to permit calculation of individual dissociation constants of the complex ions identified] but the ratio K3/K2 can be calculated from equations (1) and ( 2 ) . In Table V are listed the complex ions, the extrapolated i G ; Z values, and calculated values of K3/K2. In making these calculations the value of -0.671 volt us. S.C.E. measured by Laitinen and TaebellO was used for E l / , of the simple ion reduction. TABLE 11 -EI/I
6.5
S5
10.3
12.5
pri. Fig. L-Plot of diffusion current constant :s. pH: total H4Y, 0.05 molar; europium, 0.46 inillimolar; p , 0.6 for p1-I 12, 1.0 for p H > 1%
Reduction to the Amalgam.-Further reduction of the europium(I1) complexes to the amalgam
[COSTRIBUTION FROM THE AVERY
Eu(I11) complex
Eu(I1) complex
[Eu (HY)21-3 [Eu YZl-6 [Eu Y (OH)]-z CSi% = 1.
[Eu HY]-* [Eu YI-2 [Eu Y ] - '
at 1 M Cx
vs. S.C.E.
Ka/Kz X 10'0
1.250
4.7 1.6
1.270'
0.74
1.223
By combining the values of Ka/Kz for the second and third entries in Table V and eliminating K,, the equilibrium constant] K,,, for the following reaction can be calculated
+
[ E u ~ ; z ] - ~ OH-
T? [EuY(OH)]-*
+ Y-*
A value of KeqE 2 is found. Los ALAMOS, YEWMEXICO
LABORATORY OF THE
UNIVERSITY O F XEBRASKA]
Polarographic Reduction of the Copper Derivatives of Several 1,3-Diketones in Various Solvents' BY HENRY F. HOLTZCLAW, JR., KARLW. R. JOHNSON AND FRANK W. HENGEVELD RECEIVED JANUARY 31, 1962 The polarographic reductions of the copper chelate derivatives of 2,4-pentanedione, l-phenyl-1,3-butanedione,1,3diphenyl-1.3-propanedione, 3-methyl-2,4-pentanedione, 3-ethyl-2,4-pentanedione and 3-n-propyl-2,4-pentanedioneare 5tudied. Solvents used are water and aqueous solutions of ethanol, 2-methoxyethanol and dioxane. The effects of the solvents upon the half-wave potential and upon the diffusion current constant ant1 the trends within each solvent are discussed.
Introduction The choice of solvent is difficult in the polarographic study of metal chelate compounds of the 1,3-diketones because of their insolubility. A satisfactory solvent must not only dissolve the chelate in satisfactory concentration but must also fulfill the other requirements for polarographic analysis. Calvin and Bailes2 have reported haif-wave potentials for a considerable number of copper chelate ( I ) Abstracted from t h e X I A 'I heses of Karl 'A' R Johnson, Aug1050 a n d r r a n k XV Iiengeveld, August, 1951, University of h ebrd-ka ( 2 A I C a l \ ~ nand R I1 B.riles, T m s J O U K ~ A L ,68, 918 (IUiG)
u.t,
compounds in 507, aqueous pyridine solution Evidence of two reduction steps in pyridine was shown. These authors, in reporting qualitative trends of half-wave potential in that solvent, point out that the cobrdinating ability of the pyridine constitutes an obstacle to any interpretation other than the observation of orders of magnitude. The purpose of the present investigation is to study polarographic reductions of copper derivatives of several 1,3-diketones in water and in aqueous solutions of ethanol, 2-methoxyethanol and dioxane.
