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The pH-dependence of entropy changes of metal-edta complex formation
SHORT
872
COMMUNICATIONS
2. J. R. Delmastro 3. 4. 5. 6. 7.
and D. E. Smith, Anal. Chem., 1966, 38, 169. H. Matsuda S. Oka and P. Delahav. J. Am. Chem. Sot., 1959. 81, 5077. T. Bieeler and H. A. Laitinen. Anal. Chem.. 1965.,, 37. 512. v E. Levart and E. Poitier D’ange D’orsay, J. Electroanal. Chem.. 1966, 12, 277. J. P. Lingane, Anal. Chem., 1964, 36, 1723. H. Gerischer and W. Vielstich, Z. Physik. Chem. Frankfirt, 1955, 3, 16.
8. I. Shain and K. J. Martin, J. Phys. Chem., 1961, 65, 254. 9. I. Shain, K. J. Martin and J. W. Ross, ibid., 1961, 65, 259. 10. I. Shain and D. S. Polcyn, ibid., 1961, 65, 1949. 11. W. G. Stevens and I. Shain, ibid., 1966, 70, 2267. 12. Idem, Anal. Chem., 1966, 38, 865. 13. W. C. Cooner and N. H. Furman. J. Am. Chem. Sot.. 1952, 74, 6183. 14. M. Von Wogau, Ann. Physik., 1907, 23, 345. 15. N. I. Bashilova, Khim. Redkikh Elementor. 1957, 3, 105.
Summary-The technique of constant-potential electrolysis at a stationary spherical electrode (hanging mercury drop) was investigated for the determination of the diffusion coefficient of Tl(I) in sodium citrate-sodium hydroxide medium and of Tl in mercury. Current-time curves, at controlled potential, were obtained, covering periods from 1 to 25 set after the start of the electrolysis. The influence of applied potential, time of electrolysis, convection and shielding of the electrode was studied.
Tulanru, Vol 23. pp 872-873
THE
Pergamon
Press, 1976 Punted in Great Brltam
pH-DEPENDENCE
OF ENTROPY CHANGES COMPLEX FORMATION
OF
METAL-EDTA
KOTARO OGURA, KIYOSI TAKATU and TAKASI YOSINO Department of Applied Chemistry, Yamaguchi University, Ube, Yamaguchi-Ken 755, Japan (Received 3 February 1976. Accepted 31 May 1976)
Various methods for the determination of metal chelate stability constants have been developed, and reviewed.’ The structure of metal chelate complexes in solution has been discussed on the basis of thermodynamic and spectroscopic data.2 The chelate effect on the stability constant of the complex is of special interest. The temperature-dependence of the stabihty constant IS here studied at various pH values, and the pH-dependence of the entropy change of meta&EDTA complex formation is estimated and interpreted. EXPERIMENTAL
The stability constants were determined by means of potentiometric measurements with a mercury electrode.3 The concentrations of the mercury chelate, the metal ion and the metal chelate were 10e4. 10m3and lo-‘M, respectively, and the metal ions studied were Ca’+, Mn’+, Ni*+ and Pb* +. All chemicals were purified by recrystallization. Sodium perchlorate solution (O.lM) was used as the supporting electrolyte, and the ionic strength was kept at 0.11. The solution pH was controlled in the range s-8 by adding perchloric acid. The method could not be applied outside this pH range because of the formation of HgHY - in the acid region and Hg(OH)Y3- m the alkaline region. The solution was deaerated with purified nitrogen before measurement. The temperatures were kept constant within * 0.05”. RESULTS
AND
DISCUSSION
The reaction between a bivalent metal ion and EDTA in acidic solution is represented by M2+ + 2 Y4- + I H’ = MY’- + H,Y’-4
(1)
The conditional defined as
stability
constant
of this reaction
is
where Kuu and c+ are the thermodynamic stability constant for the metal-EDTA complex and the side-reaction coefficient, respectively. av is giien by c(y =
I + x[H]‘H&v.
According to the Schmid and Reilley methods3 a combination of the Nernst equation for a mercury electrode with the equations for the stability constants of mercuryEDTA and metal-EDTA complexes gives the equation E = E;1” + Rv2F
In [M] [HgY] K,,/[MY]I
(4)
where E;i”8= 612 mV (us. SCE), and CM]. [HgY] and .[MYI are the concentrations of metal ion, mercury-EDTA and metal-EDTA complexes, respectively. _ By substitution of equations (3) and (4) into (2). the conditional stability constant is given by A
K M’Y’= CMYI~H,YICMI C&y1 expC2F(E
- E;10,)/RTl (5)
From (5), KMpv. is obtained as a simple function of the potential of the mercury electrode, provided that the concentrations of the mercury-EDTA and metal-EDTA complexes, metal ion and hydrogen ion are kept constant, and hence, can be estimated from the experimental data and the stability constant for the mercuryyEDTA complex.
SHORT
873
COMMUNlCATlON.5
Table 1. Entropy changes of EDTA complex formation at various pH values Cation
PH
As”, cal.mole-‘.deg-’
3.71 4.40 4.90 5.51
21.0 33.4 41.1 49.4
Pb’+
3.00 4.81 5.10 5.47
16.3 17.3 18.1 18.9
Mn2+
3.00 3.66 5.86 6.36
29.9 34.1 34.6 34.8
Ca’ +
4.10 4.87 6.31 7.36
13.0 13.4 13.5 15.4
Ni2+
068
PH Fig. 1. The absorbance of the metal-EDTA complexes at ,I,,, us pH. A: PbEDTA (0.1 mM), 242 nm; 0, Mn-EDTA (0.5mM). 210nm; A, Ca-EDTA (0.5mM), 210nm; 0. Ni-EDTA (10 mM), 985 nm.
The standard free energy change for reaction (1) is related to the conditional stability constant by I&.,.. = exp(-AG’IRT)
(6)
From the temperature-dependence of I&v.. the entropy change of reaction (1) can be calculated. The values found for the entropy change in formation of the complexes studied are shown in Table 1. The entropy changes for the Ca, Mn and Pb complex-formation reactions are practically independent of the pH, whereas for Ni there is a large change in AS with pH. The absorption spectra of the metal-EDTA complexes were measured at various pHvalues, and the absorbances at the wavelength of maximum absorption are plotted against pH for each metal in Fig. 1. The absorbance of each metal complex decreases with pH. This decrease is ascribed to the side-reaction of EDTA with protons. However the drop in absorbance is much smaller for the Ni-EDTA complex, and this difference in behaviour must be related to the structure of this complex in solution. The Ni-EDTA complex should be octahedral, with two amino and four carboxylate groups co-ordinated
to the central metal ion. This configuration may be sensitive to pH, since with decreasing pH the carboxylate group is prevented by protons from co-ordination with the metal ion. which would result in the formation of mixed-ligand (EDTA, water) complexes of nickel. Such a situation should lead to a decrease in the entropy change. Consideration of the log K and AH values for the complexes supplies an indication of the explanation. For the Ni, Ca and Mn complexes AH is comparatively small (-7.6, -6.5, -4.6 kcal/mole respectively) but the much bigger entropy change makes the nickel complex much the strongest of the three, and thus much less susceptible to the side-reactions of EDTA with protons. The lead complex has about the same stability as the nickel complex, but the AH is much more negative (- 14.9 kcal/mole) and AS correspondingly less positive. REFERENCES
F. J. C. Rossotti and H. S. Rossotti, Determination of Stability Constants, McGraw-Hill. New York, 1961. 2. S. Kirshner, J. Am. Chem. Sot., 1956, 78, 2372. 3. R. W. Schmid and C. N. Reilley, ibid.. 1956, 78, 5513. 1.
Summary-The entropy changes of Ca’+-, Mn’+-, Ni*+- and Pb*+-EDTA complex-formation reactions were measured at various pH-values. The entropy changes for the Ca. Mn and Pb complexes were practically independent of pH, but that of the Ni complex decreased considerably with pH. It is suggested that mixed-ligand (EDTA, water) complexes of Ni are formed at low pH.
872
COMMUNICATIONS
2. J. R. Delmastro 3. 4. 5. 6. 7.
and D. E. Smith, Anal. Chem., 1966, 38, 169. H. Matsuda S. Oka and P. Delahav. J. Am. Chem. Sot., 1959. 81, 5077. T. Bieeler and H. A. Laitinen. Anal. Chem.. 1965.,, 37. 512. v E. Levart and E. Poitier D’ange D’orsay, J. Electroanal. Chem.. 1966, 12, 277. J. P. Lingane, Anal. Chem., 1964, 36, 1723. H. Gerischer and W. Vielstich, Z. Physik. Chem. Frankfirt, 1955, 3, 16.
8. I. Shain and K. J. Martin, J. Phys. Chem., 1961, 65, 254. 9. I. Shain, K. J. Martin and J. W. Ross, ibid., 1961, 65, 259. 10. I. Shain and D. S. Polcyn, ibid., 1961, 65, 1949. 11. W. G. Stevens and I. Shain, ibid., 1966, 70, 2267. 12. Idem, Anal. Chem., 1966, 38, 865. 13. W. C. Cooner and N. H. Furman. J. Am. Chem. Sot.. 1952, 74, 6183. 14. M. Von Wogau, Ann. Physik., 1907, 23, 345. 15. N. I. Bashilova, Khim. Redkikh Elementor. 1957, 3, 105.
Summary-The technique of constant-potential electrolysis at a stationary spherical electrode (hanging mercury drop) was investigated for the determination of the diffusion coefficient of Tl(I) in sodium citrate-sodium hydroxide medium and of Tl in mercury. Current-time curves, at controlled potential, were obtained, covering periods from 1 to 25 set after the start of the electrolysis. The influence of applied potential, time of electrolysis, convection and shielding of the electrode was studied.
Tulanru, Vol 23. pp 872-873
THE
Pergamon
Press, 1976 Punted in Great Brltam
pH-DEPENDENCE
OF ENTROPY CHANGES COMPLEX FORMATION
OF
METAL-EDTA
KOTARO OGURA, KIYOSI TAKATU and TAKASI YOSINO Department of Applied Chemistry, Yamaguchi University, Ube, Yamaguchi-Ken 755, Japan (Received 3 February 1976. Accepted 31 May 1976)
Various methods for the determination of metal chelate stability constants have been developed, and reviewed.’ The structure of metal chelate complexes in solution has been discussed on the basis of thermodynamic and spectroscopic data.2 The chelate effect on the stability constant of the complex is of special interest. The temperature-dependence of the stabihty constant IS here studied at various pH values, and the pH-dependence of the entropy change of meta&EDTA complex formation is estimated and interpreted. EXPERIMENTAL
The stability constants were determined by means of potentiometric measurements with a mercury electrode.3 The concentrations of the mercury chelate, the metal ion and the metal chelate were 10e4. 10m3and lo-‘M, respectively, and the metal ions studied were Ca’+, Mn’+, Ni*+ and Pb* +. All chemicals were purified by recrystallization. Sodium perchlorate solution (O.lM) was used as the supporting electrolyte, and the ionic strength was kept at 0.11. The solution pH was controlled in the range s-8 by adding perchloric acid. The method could not be applied outside this pH range because of the formation of HgHY - in the acid region and Hg(OH)Y3- m the alkaline region. The solution was deaerated with purified nitrogen before measurement. The temperatures were kept constant within * 0.05”. RESULTS
AND
DISCUSSION
The reaction between a bivalent metal ion and EDTA in acidic solution is represented by M2+ + 2 Y4- + I H’ = MY’- + H,Y’-4
(1)
The conditional defined as
stability
constant
of this reaction
is
where Kuu and c+ are the thermodynamic stability constant for the metal-EDTA complex and the side-reaction coefficient, respectively. av is giien by c(y =
I + x[H]‘H&v.
According to the Schmid and Reilley methods3 a combination of the Nernst equation for a mercury electrode with the equations for the stability constants of mercuryEDTA and metal-EDTA complexes gives the equation E = E;1” + Rv2F
In [M] [HgY] K,,/[MY]I
(4)
where E;i”8= 612 mV (us. SCE), and CM]. [HgY] and .[MYI are the concentrations of metal ion, mercury-EDTA and metal-EDTA complexes, respectively. _ By substitution of equations (3) and (4) into (2). the conditional stability constant is given by A
K M’Y’= CMYI~H,YICMI C&y1 expC2F(E
- E;10,)/RTl (5)
From (5), KMpv. is obtained as a simple function of the potential of the mercury electrode, provided that the concentrations of the mercury-EDTA and metal-EDTA complexes, metal ion and hydrogen ion are kept constant, and hence, can be estimated from the experimental data and the stability constant for the mercuryyEDTA complex.
SHORT
873
COMMUNlCATlON.5
Table 1. Entropy changes of EDTA complex formation at various pH values Cation
PH
As”, cal.mole-‘.deg-’
3.71 4.40 4.90 5.51
21.0 33.4 41.1 49.4
Pb’+
3.00 4.81 5.10 5.47
16.3 17.3 18.1 18.9
Mn2+
3.00 3.66 5.86 6.36
29.9 34.1 34.6 34.8
Ca’ +
4.10 4.87 6.31 7.36
13.0 13.4 13.5 15.4
Ni2+
068
PH Fig. 1. The absorbance of the metal-EDTA complexes at ,I,,, us pH. A: PbEDTA (0.1 mM), 242 nm; 0, Mn-EDTA (0.5mM). 210nm; A, Ca-EDTA (0.5mM), 210nm; 0. Ni-EDTA (10 mM), 985 nm.
The standard free energy change for reaction (1) is related to the conditional stability constant by I&.,.. = exp(-AG’IRT)
(6)
From the temperature-dependence of I&v.. the entropy change of reaction (1) can be calculated. The values found for the entropy change in formation of the complexes studied are shown in Table 1. The entropy changes for the Ca, Mn and Pb complex-formation reactions are practically independent of the pH, whereas for Ni there is a large change in AS with pH. The absorption spectra of the metal-EDTA complexes were measured at various pHvalues, and the absorbances at the wavelength of maximum absorption are plotted against pH for each metal in Fig. 1. The absorbance of each metal complex decreases with pH. This decrease is ascribed to the side-reaction of EDTA with protons. However the drop in absorbance is much smaller for the Ni-EDTA complex, and this difference in behaviour must be related to the structure of this complex in solution. The Ni-EDTA complex should be octahedral, with two amino and four carboxylate groups co-ordinated
to the central metal ion. This configuration may be sensitive to pH, since with decreasing pH the carboxylate group is prevented by protons from co-ordination with the metal ion. which would result in the formation of mixed-ligand (EDTA, water) complexes of nickel. Such a situation should lead to a decrease in the entropy change. Consideration of the log K and AH values for the complexes supplies an indication of the explanation. For the Ni, Ca and Mn complexes AH is comparatively small (-7.6, -6.5, -4.6 kcal/mole respectively) but the much bigger entropy change makes the nickel complex much the strongest of the three, and thus much less susceptible to the side-reactions of EDTA with protons. The lead complex has about the same stability as the nickel complex, but the AH is much more negative (- 14.9 kcal/mole) and AS correspondingly less positive. REFERENCES
F. J. C. Rossotti and H. S. Rossotti, Determination of Stability Constants, McGraw-Hill. New York, 1961. 2. S. Kirshner, J. Am. Chem. Sot., 1956, 78, 2372. 3. R. W. Schmid and C. N. Reilley, ibid.. 1956, 78, 5513. 1.
Summary-The entropy changes of Ca’+-, Mn’+-, Ni*+- and Pb*+-EDTA complex-formation reactions were measured at various pH-values. The entropy changes for the Ca. Mn and Pb complexes were practically independent of pH, but that of the Ni complex decreased considerably with pH. It is suggested that mixed-ligand (EDTA, water) complexes of Ni are formed at low pH.