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Stability constants and thermodynamic functions of molybdenum and uranium chelates formed with dl -α-aminobutyric acid
Tdm~cr. Vol. 21. pp. 763 lo 765 Pergamon Press Ltd 1980.Printed in Great Britain
ANALYTICAL
DATA
STABILITY CONSTANTS AND THERMODYNAMIC FUNCTIONS OF MOLYBDENUM AND URANIUM CHELATES FORMED WITH DL-a-AMINOBUTYRIC ACID J. P. N. SRIVASTAVA* and M. N. SRIVASTAVA* Chemical Laboratories, University of Allahabad, Allahabad 211002, India (Received 28 February 1980. Accepted 25 March 1980) Summary-The stability constants and thermodynamic functions involved in the formation of Mo(VI) and U(W) chelates with DL-a-aminobutyric acid have been determined potentiometrically. It is observed that in the case of MO(W) system three chelates ML, ML2 and ML3 are formed stepwise, whereas in the U(W) system only two chelates ML and ML2 are formed before precipitation occurs, and both steps occur almost simultaneously. Results show that entropy makes a predominant contribution to the stability of both MO(W) and U(W) chelates.
The metal chelates of a-aminobutyric acid with some transition metal ions were studied earlier by various workers’-” and their stability constants and thermodynamic data are available. In previous publications’ 1--l3 from this laboratory the stability constants of some amino-acid chelates were reported. Their formation was studied potentiometrically by the Irving and Rossotti method. I4 This paper gives the results of a similar study of molybdenum(W) and uranium(W) chelates formed with DL-a-aminobutyric acid. Their stability constants, and thermodynamic parameters have been determined. RESULTS AND DISCUSSION
The protonation constants at 25” and O.lM ionic strength (KNOa) are log Ki = 9.50 and log K2 = 2.30. The pH-titrations show that in the U(W) chelate systems, precipitation starts from pH 5 5.5, whereas for Mo(V1) the solutions remain clear throughout the titration. The formation curves (E us. pL) show that for MO(W) Ii approaches a maximum value of 3 but for U(W) Z approaches a maximum value of 2 (before precipitation occurs). Analysis of the formation curve shows that for the MO(W) chelates the K1 and K2 values are rather close (K1/K2 - 10) whereas K3 may be taken to be almost independent of K, and K2 (KI/KJ 5 10s7). Hence, the whole formation curve can be resolved into two regions (0 < ii < 2 and 2 < E < 3) which *Postal addresses: Dr. J. P. N. Srivastava, S-7/202, R. K. Puram, New Delhi-110022, India. Dr. M. N. Srivastava, Ramanand, Nagar (Allapur), Allaha28B/122D, bad-2 I 1006, India. 763
can be treated separately. The value of K3 can be read directly from the curve [log K3 equals_RL,+,.,J. It can also be computed by the average-value method in the second region. For calculation of K, and K2 the first region can be taken as a system for which N = 2, and the values of K1 and K2 computed by the correction-term method. Alternatively the approximate constants obtained by the half-E method can be further refined by successive approximation. For the U(VI) chelates the K, and K2 values are very close, log K1 - log K2 being ~0.5. Albert is has pointed out that under such conditions K, and K2 cannot satisfactorily be computed separately; instead, log /I2 values may be obtained from the relation: log /I2 = log n - log(2 - ii) - 2 log[L] Hence in this case log & values were calculated in this way. However, attempts were also made to compute K1 and K2 separately by the use of the leastsquares method, which gave quite satisfactory results, the value of log K, + log K2 thus obtained being in good agreement with log fi2 calculated from Albert’s equation. In Tables 1-3, the protonation and stability constant data at different temperatures and ionic strengths are reported. Stability constants at zero ionic strength were obtained by extrapolation (log K vs. ,/p). The results show that the stability constants decrease with increasing temperature and ionic strength. Thermodynamic parameters for the chelate systems are reported in Table 4. AH values were obtained by the temperature-coefficient method and the values of AG and AS were calculated in the usual way, The results show that the entropy term makes
764
ANALYTICAL
DATA
Table 1. Stability constants of MO(W) and U(W) chelates with DL-a-aminobutyric acid (p = O.lM KNO,, temp. 25”) pH-range for ii-calculations
Cation MO(W)
3.2-9.5
U(VI)
3.7-5.3
Computational method Half-ii values Average-value method for Ks only (2
log Kl
log K2
log K3
8.31
7.30
3.62
-
Half-Z values Albert’s equation Least-squares method
-
3.62
8.15
7.46
8.16
7.45
3.62
7.78
7.27 -
-
8.59
-
6.48
log B”
-
19.23 f 0.01
15.09 15.07 + 0.02
Table 2. Stability constants of Mo(V1) and U(V1) DL-a-aminobutyrate chelates at different temperatures (n = O.lM KNOs) Cation H+ MoWI)
U(VI)
20
25”
30
35”
40”
log Kt log K2 log Ki log K2 log K,
9.63 2.34 8.21 7.48 3.64
9.50 2.30 8.16 7.45 3.62
9.34 2.25 8.11 7.42 3.59
9.19 2.22 8.05 7.40 3.56
9.05 2.20 8.00 1.37 3.54
Iog Ki log KZ
6.54 8.62
6.48 8.59
6.42 8.55
6.36 8.52
6.31 8.48
Table 3. Stability constants of Mo(V1) and U(VI) DL-a-aminobutyrate chelates at different ionic strengths (KNO,; 25”) Cation H+ MoWI)
U(VI)
/I = 0.2
/l= 0.1
,n = 0.05
jl = 0.02
log K, log K2 log K, log K2 log K3
9.52 2.30 8.15 7.44 3.62
9.50 2.30 8.16 7.45 3.62
9.45 2.28 8.18 7.46 3.63
9.36 2.25 8.21 7.49 3.65
8.27 7.55 3.70
log Ki Iog K2
6.46 8.58
6.48 8.59
6.51 8.61
6.55 8.65
6.64 8.73
/J = o.OO* -
* Extrapolated values at zero ionic strength (log K vs. Jr).
Table 4. Thermodynamic
parameters for Mo(V1) and U(V1) DL-a-aminobutyrate chelate systems (temp. 25”, p = O.lM KNOs)
Cation MO(W) U(VI)
4.5 4.9
6.7 7.8
8.9 -
11.14 8.84
AH and AC in kcaI/mole; AS in cal. mole- I. deg- ‘.
21.30 20.56
26.24 -
25 13
53 43
64 -
ANALYTICAL
dominant lates.
contribution
to the stability
of both
che-
REFERENCES I. A. Albert, Biochem. J., 1950, 47, 531. 2. D. J. Perkins. Biochem. .I., 1953, 55, 649. 3. D. D. Perrin, J. Gem. Sot., 1958, 3125; 1959, 290. 4. R. Ralea and A. Pooa. An&l Stiint. Unio. A.I. Cuza, lasi, Sect. I, 1960, 6: 351. 5. M. J. Ellis and R. L. Sykes, J. Am. Leather Chemisrs’ Assoc., 1963, 58, 346. 6. V. S. Sharma, H. B. Mathur and A. B. Biswas, .I. Inorg. Nucl. Chem., 1964, 26, 382. 7. V. S. Sharma, H. B. Mathur and P. S. Kulkarni, Indian J. Chem., 1965, 3, 146.
DATA
765
8. E. V. Raiu and H. B. Mathur. J. Inora. Nucl. Chem., 1968,30;2181. 9. J. L. Meyer and J. E. Bauman, Jr., J. Chem. Eng. Data, 1970, 1s. 404. 10. A. Gergely, J. Mojzes and Zs. Kassai-Bazsa, J. Inorg. Nucl. Chem., 1972,34, 1277. 11. M. K. Singh and M. N. Srivastava, ibid., 1972, 34, 567, 2067, 2081; Talanta, 1972, 19, 699. 12. R. C. Tewari and M. N. Srivastava. J. Inorg. Nucl. Chem., 1973, 35, 2441, 3044; Talanta, 1973, u), 133, 360. 13. J. P. N. Srivastava and M. N. Srivastava, Indian J. Chem., 1976. 14, 818; 1977, 15, 1109; Vij. Pari. Anu. Patrika, 1976, 19, 117; Rev. Chim. Min., 1977, 14, 263; J. lnorg. Nuci. Chem., 1978, 40, 2076. 14. H. Irving and H. S. Rossotti, .I. Chem. SOC.~1953, 3397; 1954, 2904. 15 A. Albert, Biochem. J., 1953,54,6&i.
ANALYTICAL
DATA
STABILITY CONSTANTS AND THERMODYNAMIC FUNCTIONS OF MOLYBDENUM AND URANIUM CHELATES FORMED WITH DL-a-AMINOBUTYRIC ACID J. P. N. SRIVASTAVA* and M. N. SRIVASTAVA* Chemical Laboratories, University of Allahabad, Allahabad 211002, India (Received 28 February 1980. Accepted 25 March 1980) Summary-The stability constants and thermodynamic functions involved in the formation of Mo(VI) and U(W) chelates with DL-a-aminobutyric acid have been determined potentiometrically. It is observed that in the case of MO(W) system three chelates ML, ML2 and ML3 are formed stepwise, whereas in the U(W) system only two chelates ML and ML2 are formed before precipitation occurs, and both steps occur almost simultaneously. Results show that entropy makes a predominant contribution to the stability of both MO(W) and U(W) chelates.
The metal chelates of a-aminobutyric acid with some transition metal ions were studied earlier by various workers’-” and their stability constants and thermodynamic data are available. In previous publications’ 1--l3 from this laboratory the stability constants of some amino-acid chelates were reported. Their formation was studied potentiometrically by the Irving and Rossotti method. I4 This paper gives the results of a similar study of molybdenum(W) and uranium(W) chelates formed with DL-a-aminobutyric acid. Their stability constants, and thermodynamic parameters have been determined. RESULTS AND DISCUSSION
The protonation constants at 25” and O.lM ionic strength (KNOa) are log Ki = 9.50 and log K2 = 2.30. The pH-titrations show that in the U(W) chelate systems, precipitation starts from pH 5 5.5, whereas for Mo(V1) the solutions remain clear throughout the titration. The formation curves (E us. pL) show that for MO(W) Ii approaches a maximum value of 3 but for U(W) Z approaches a maximum value of 2 (before precipitation occurs). Analysis of the formation curve shows that for the MO(W) chelates the K1 and K2 values are rather close (K1/K2 - 10) whereas K3 may be taken to be almost independent of K, and K2 (KI/KJ 5 10s7). Hence, the whole formation curve can be resolved into two regions (0 < ii < 2 and 2 < E < 3) which *Postal addresses: Dr. J. P. N. Srivastava, S-7/202, R. K. Puram, New Delhi-110022, India. Dr. M. N. Srivastava, Ramanand, Nagar (Allapur), Allaha28B/122D, bad-2 I 1006, India. 763
can be treated separately. The value of K3 can be read directly from the curve [log K3 equals_RL,+,.,J. It can also be computed by the average-value method in the second region. For calculation of K, and K2 the first region can be taken as a system for which N = 2, and the values of K1 and K2 computed by the correction-term method. Alternatively the approximate constants obtained by the half-E method can be further refined by successive approximation. For the U(VI) chelates the K, and K2 values are very close, log K1 - log K2 being ~0.5. Albert is has pointed out that under such conditions K, and K2 cannot satisfactorily be computed separately; instead, log /I2 values may be obtained from the relation: log /I2 = log n - log(2 - ii) - 2 log[L] Hence in this case log & values were calculated in this way. However, attempts were also made to compute K1 and K2 separately by the use of the leastsquares method, which gave quite satisfactory results, the value of log K, + log K2 thus obtained being in good agreement with log fi2 calculated from Albert’s equation. In Tables 1-3, the protonation and stability constant data at different temperatures and ionic strengths are reported. Stability constants at zero ionic strength were obtained by extrapolation (log K vs. ,/p). The results show that the stability constants decrease with increasing temperature and ionic strength. Thermodynamic parameters for the chelate systems are reported in Table 4. AH values were obtained by the temperature-coefficient method and the values of AG and AS were calculated in the usual way, The results show that the entropy term makes
764
ANALYTICAL
DATA
Table 1. Stability constants of MO(W) and U(W) chelates with DL-a-aminobutyric acid (p = O.lM KNO,, temp. 25”) pH-range for ii-calculations
Cation MO(W)
3.2-9.5
U(VI)
3.7-5.3
Computational method Half-ii values Average-value method for Ks only (2
log Kl
log K2
log K3
8.31
7.30
3.62
-
Half-Z values Albert’s equation Least-squares method
-
3.62
8.15
7.46
8.16
7.45
3.62
7.78
7.27 -
-
8.59
-
6.48
log B”
-
19.23 f 0.01
15.09 15.07 + 0.02
Table 2. Stability constants of Mo(V1) and U(V1) DL-a-aminobutyrate chelates at different temperatures (n = O.lM KNOs) Cation H+ MoWI)
U(VI)
20
25”
30
35”
40”
log Kt log K2 log Ki log K2 log K,
9.63 2.34 8.21 7.48 3.64
9.50 2.30 8.16 7.45 3.62
9.34 2.25 8.11 7.42 3.59
9.19 2.22 8.05 7.40 3.56
9.05 2.20 8.00 1.37 3.54
Iog Ki log KZ
6.54 8.62
6.48 8.59
6.42 8.55
6.36 8.52
6.31 8.48
Table 3. Stability constants of Mo(V1) and U(VI) DL-a-aminobutyrate chelates at different ionic strengths (KNO,; 25”) Cation H+ MoWI)
U(VI)
/I = 0.2
/l= 0.1
,n = 0.05
jl = 0.02
log K, log K2 log K, log K2 log K3
9.52 2.30 8.15 7.44 3.62
9.50 2.30 8.16 7.45 3.62
9.45 2.28 8.18 7.46 3.63
9.36 2.25 8.21 7.49 3.65
8.27 7.55 3.70
log Ki Iog K2
6.46 8.58
6.48 8.59
6.51 8.61
6.55 8.65
6.64 8.73
/J = o.OO* -
* Extrapolated values at zero ionic strength (log K vs. Jr).
Table 4. Thermodynamic
parameters for Mo(V1) and U(V1) DL-a-aminobutyrate chelate systems (temp. 25”, p = O.lM KNOs)
Cation MO(W) U(VI)
4.5 4.9
6.7 7.8
8.9 -
11.14 8.84
AH and AC in kcaI/mole; AS in cal. mole- I. deg- ‘.
21.30 20.56
26.24 -
25 13
53 43
64 -
ANALYTICAL
dominant lates.
contribution
to the stability
of both
che-
REFERENCES I. A. Albert, Biochem. J., 1950, 47, 531. 2. D. J. Perkins. Biochem. .I., 1953, 55, 649. 3. D. D. Perrin, J. Gem. Sot., 1958, 3125; 1959, 290. 4. R. Ralea and A. Pooa. An&l Stiint. Unio. A.I. Cuza, lasi, Sect. I, 1960, 6: 351. 5. M. J. Ellis and R. L. Sykes, J. Am. Leather Chemisrs’ Assoc., 1963, 58, 346. 6. V. S. Sharma, H. B. Mathur and A. B. Biswas, .I. Inorg. Nucl. Chem., 1964, 26, 382. 7. V. S. Sharma, H. B. Mathur and P. S. Kulkarni, Indian J. Chem., 1965, 3, 146.
DATA
765
8. E. V. Raiu and H. B. Mathur. J. Inora. Nucl. Chem., 1968,30;2181. 9. J. L. Meyer and J. E. Bauman, Jr., J. Chem. Eng. Data, 1970, 1s. 404. 10. A. Gergely, J. Mojzes and Zs. Kassai-Bazsa, J. Inorg. Nucl. Chem., 1972,34, 1277. 11. M. K. Singh and M. N. Srivastava, ibid., 1972, 34, 567, 2067, 2081; Talanta, 1972, 19, 699. 12. R. C. Tewari and M. N. Srivastava. J. Inorg. Nucl. Chem., 1973, 35, 2441, 3044; Talanta, 1973, u), 133, 360. 13. J. P. N. Srivastava and M. N. Srivastava, Indian J. Chem., 1976. 14, 818; 1977, 15, 1109; Vij. Pari. Anu. Patrika, 1976, 19, 117; Rev. Chim. Min., 1977, 14, 263; J. lnorg. Nuci. Chem., 1978, 40, 2076. 14. H. Irving and H. S. Rossotti, .I. Chem. SOC.~1953, 3397; 1954, 2904. 15 A. Albert, Biochem. J., 1953,54,6&i.