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Polarographic study of the composition and stability constants of thallium(I) cyclohexylthioglycolate complexes
Polyhedron Vol. 6, No. 3, pp. 40-402, Rioted in Great Britain
1987 0
0277-5387/87 S3.00 + .OO 1987 Pergnmoo Joumalls Ltd
POLAROGRAPHIC STUDY OF THE COMPOSITION AND STABILITY CONSTANTS OF THALLIUM(I) CYCLOHEXYLTHIOGLYCOLATE COMPLEXES USHA GUPTA and A. L. J. RAO* Department of Chemistry, Punjabi University, Patiala 147002, India (Received 20 March 1986; accepted after revision 7 July 1986)
Abstract-The complexation of thallium(I) by cyclohexylthioglycolate (CyHTG) has been studied polarographically. The reduction of Tl+ in cyclohexylthioglycolate solution has been found to be reversible and diffusion controlled involving a one-electron transfer process. Potential vs concentration data at 0.5 M ionic strength are interpreted on the basis of the formation of two complex species, TlA and TlA;. The logarithms of the stability constants of these complexes are 1.73, 3.176 at 2O”C, 1.77,3.342 at 30°C and 1.87, 3.398 at 4O”C, respectively. The values of AC*, AH* and ASo have been calculated at 30°C.
Mercapto compounds containing active SH and COOH groups have a wide variety of applications. The polarographic behaviour of Tl+ has been studied in various media with different complexing reagents such as ethylene bis-(3-mercaptopropionate),’ ethylthioglycolate,2 ethane-1,2-dithiol,l butylthioglycolate,3 furfuryl mercaptan,4 2-mercapto ethanol’ and /?-mercaptopropionic acid.6 In the present investigations the complexation of cyclohexylthioglycolate with thallium(I) has been studied polarographically to determine the composition and stability constants of the complex ions formed. The values of AC*, AHo and AS* have also been calculated and are reported in this paper.
EXPERIMENTAL Cyclohexylthioglycolate (CyHTG) was used as metal binding agent. It was synthesized by the method given by Gambarov,’ and was standardized. A stock solution of the reagent was prepared in pure ethanol. A 2 M solution of KNO, was used as the supporting electrolyte and 0.1% Triton X100 as the maximum suppressor. All solutions used in the polarographic measurements had in addition to CyHTG, a Tl+ concentration of l.OmM, 0.5 M *Author to whom correspondence should he addressed. 401
KNO, and 0.002% Triton X-100 in 50% ethanolic media at pH6.0. The CyHTG concentration was varied from 0.005 to 0.03 M. A manual Toshniwal polarograph (CL02 type) and saturated calomel electrode were used. The necessary correction for the iR drop and residual current were applied in determining the half-wave potential and diffusion current data, respectively. The plot of id vs fi and id vs C (C = concentration of thallium) is linear, indicating the diffusion-controlled nature of the reduction wave. The values of slopes from log plots (i.e. plot of log i/id - i vs E - d *e) agreed with the theoretical values for one-electron transfer. The half-wave potential shifted toward more negative values with increasing CyHTG concentration indicating complex formation. The diffusion current and half-wave potential values are recorded in Table 1. A plot of El12 as a function of log (CyHTG) showed the curvature indicating the formation of more than one complex. The method of Deford and Hume* was applied to the calculation of stability constants for the two complexes TlA and TlA;. Thermodynamic parameters AC*, AH* and AS* were also evaluated at 30°C for the two complexes and the results are summarized in Table 2. The percentage distribution of thallium present in different forms as a function of log (CyHTG) has been calculated and the results are presented in Fig. 1.
U. GUPTA and A. L. J. RAO
402
Table 1. Half-wave potentials, diffusion current various functions at 20°C Concentration ligand (M) 0.000 0.005 0.010 0.015 0.020 0.025 0.030
of -El,,
jd
(CIA) 0.554 4.048 3.774 3.542 3.238 3.036 2.834
SCE
us
(v) 0.460 0.480 0.490 0.502 0.510 0.520 0.530
F,(X) 1.288 1.640 2.135 2.673 3.377 4.285
Table 2. Stability constants and their thermodynamic 8
Stability constants Composition 1:l 1:2
and values of the
20°C
30°C
54 1.5 x 103
60 2.2 x 103
(kJ:l-‘) - 10.5 - 19.2
F,(X) 57.6 64.9 65.7 83.7 95.0 109.5
F,(X) 720 1090 1440 1484 1640 1850
functions at 30°C AH* (kJ mol-‘)
ASe (J K-’ mol-‘)
6.8 29.3
57 159
Acknowledgement-Financial assistance of CSIR to one of the authors (U.G.) is gratefully acknowledged. REFERENCES
03 2.4
2.2
2.0
1.8
1.6
1.4
1.2
Loit G Fig.
1.
Distribution diagrams for cyclohexylthioglycolate system.
thallium-
1. R. S. Saxena and U. S. Chaturvedi, J. Inorg. Nucl. Chem. 1972,34, 2964. 2. R. S. Saxena and U. S. Chaturvedi, J. Inorg. Nucl. Chem. 1972,34,913. 3. R. S. Saxena and M. C. Saxena, Indian J. Chem. 1976, 14A, 628. 4. R. S. Saxena and S. S. Sheelwant, Labdev. 1974, 12, 124. 5. R. S. Saxena and G. L. Khandelwal, J. Indian Chem. Sot. 1976, 53, 970. 6. R. S. Saxena, K. C. Gupta and M. L. Mittal, Indian J. Chem. 1969,7, 374. 7. D. G. Gambarov, K. Z. Guseinov and R. Fati-Zade, Org. Reagentry Anal. Khim. Tezisy. Dokl. Vses. 4th Knof. 1976,1, 111; Chem. Abatr. 1977,87, 193132X. 8. D. D. DeFord and D. N. Hume, J. Am. Chem. Sot. 1951,73, 5321.
1987 0
0277-5387/87 S3.00 + .OO 1987 Pergnmoo Joumalls Ltd
POLAROGRAPHIC STUDY OF THE COMPOSITION AND STABILITY CONSTANTS OF THALLIUM(I) CYCLOHEXYLTHIOGLYCOLATE COMPLEXES USHA GUPTA and A. L. J. RAO* Department of Chemistry, Punjabi University, Patiala 147002, India (Received 20 March 1986; accepted after revision 7 July 1986)
Abstract-The complexation of thallium(I) by cyclohexylthioglycolate (CyHTG) has been studied polarographically. The reduction of Tl+ in cyclohexylthioglycolate solution has been found to be reversible and diffusion controlled involving a one-electron transfer process. Potential vs concentration data at 0.5 M ionic strength are interpreted on the basis of the formation of two complex species, TlA and TlA;. The logarithms of the stability constants of these complexes are 1.73, 3.176 at 2O”C, 1.77,3.342 at 30°C and 1.87, 3.398 at 4O”C, respectively. The values of AC*, AH* and ASo have been calculated at 30°C.
Mercapto compounds containing active SH and COOH groups have a wide variety of applications. The polarographic behaviour of Tl+ has been studied in various media with different complexing reagents such as ethylene bis-(3-mercaptopropionate),’ ethylthioglycolate,2 ethane-1,2-dithiol,l butylthioglycolate,3 furfuryl mercaptan,4 2-mercapto ethanol’ and /?-mercaptopropionic acid.6 In the present investigations the complexation of cyclohexylthioglycolate with thallium(I) has been studied polarographically to determine the composition and stability constants of the complex ions formed. The values of AC*, AHo and AS* have also been calculated and are reported in this paper.
EXPERIMENTAL Cyclohexylthioglycolate (CyHTG) was used as metal binding agent. It was synthesized by the method given by Gambarov,’ and was standardized. A stock solution of the reagent was prepared in pure ethanol. A 2 M solution of KNO, was used as the supporting electrolyte and 0.1% Triton X100 as the maximum suppressor. All solutions used in the polarographic measurements had in addition to CyHTG, a Tl+ concentration of l.OmM, 0.5 M *Author to whom correspondence should he addressed. 401
KNO, and 0.002% Triton X-100 in 50% ethanolic media at pH6.0. The CyHTG concentration was varied from 0.005 to 0.03 M. A manual Toshniwal polarograph (CL02 type) and saturated calomel electrode were used. The necessary correction for the iR drop and residual current were applied in determining the half-wave potential and diffusion current data, respectively. The plot of id vs fi and id vs C (C = concentration of thallium) is linear, indicating the diffusion-controlled nature of the reduction wave. The values of slopes from log plots (i.e. plot of log i/id - i vs E - d *e) agreed with the theoretical values for one-electron transfer. The half-wave potential shifted toward more negative values with increasing CyHTG concentration indicating complex formation. The diffusion current and half-wave potential values are recorded in Table 1. A plot of El12 as a function of log (CyHTG) showed the curvature indicating the formation of more than one complex. The method of Deford and Hume* was applied to the calculation of stability constants for the two complexes TlA and TlA;. Thermodynamic parameters AC*, AH* and AS* were also evaluated at 30°C for the two complexes and the results are summarized in Table 2. The percentage distribution of thallium present in different forms as a function of log (CyHTG) has been calculated and the results are presented in Fig. 1.
U. GUPTA and A. L. J. RAO
402
Table 1. Half-wave potentials, diffusion current various functions at 20°C Concentration ligand (M) 0.000 0.005 0.010 0.015 0.020 0.025 0.030
of -El,,
jd
(CIA) 0.554 4.048 3.774 3.542 3.238 3.036 2.834
SCE
us
(v) 0.460 0.480 0.490 0.502 0.510 0.520 0.530
F,(X) 1.288 1.640 2.135 2.673 3.377 4.285
Table 2. Stability constants and their thermodynamic 8
Stability constants Composition 1:l 1:2
and values of the
20°C
30°C
54 1.5 x 103
60 2.2 x 103
(kJ:l-‘) - 10.5 - 19.2
F,(X) 57.6 64.9 65.7 83.7 95.0 109.5
F,(X) 720 1090 1440 1484 1640 1850
functions at 30°C AH* (kJ mol-‘)
ASe (J K-’ mol-‘)
6.8 29.3
57 159
Acknowledgement-Financial assistance of CSIR to one of the authors (U.G.) is gratefully acknowledged. REFERENCES
03 2.4
2.2
2.0
1.8
1.6
1.4
1.2
Loit G Fig.
1.
Distribution diagrams for cyclohexylthioglycolate system.
thallium-
1. R. S. Saxena and U. S. Chaturvedi, J. Inorg. Nucl. Chem. 1972,34, 2964. 2. R. S. Saxena and U. S. Chaturvedi, J. Inorg. Nucl. Chem. 1972,34,913. 3. R. S. Saxena and M. C. Saxena, Indian J. Chem. 1976, 14A, 628. 4. R. S. Saxena and S. S. Sheelwant, Labdev. 1974, 12, 124. 5. R. S. Saxena and G. L. Khandelwal, J. Indian Chem. Sot. 1976, 53, 970. 6. R. S. Saxena, K. C. Gupta and M. L. Mittal, Indian J. Chem. 1969,7, 374. 7. D. G. Gambarov, K. Z. Guseinov and R. Fati-Zade, Org. Reagentry Anal. Khim. Tezisy. Dokl. Vses. 4th Knof. 1976,1, 111; Chem. Abatr. 1977,87, 193132X. 8. D. D. DeFord and D. N. Hume, J. Am. Chem. Sot. 1951,73, 5321.