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Steric and hydrophobic effects of substituents in extraction of metal complexes with O,O-dialkyldithiophosphoric acids
Talonto, Vol. 34, No. 1, pp. Printed in Great Britain
211-214, 1987
0039-9140/87 $3.00 + 0.00 Pergamon Journals Ltd
STERIC AND HYDROPHOBIC EFFECTS OF SUBSTITUENTS IN EXTRACTION OF METAL COMPLEXES WITH O,O-DIALKYLDITHIOPHOSPHORIC ACIDS V. F. TOROWVA, A. R. GARIFZYANOV and I. E. PANPJLOVA Faculty of Chemistry, U.I.Ul’yanov-Lenin State University, Lenina, 18 Kaxan, 420008, USSR (Received 15 October 1985. Revised 25 March 1986. Accepted 9 August 1986) Summary-The two-phase stability constants (equilibrium constants) of metal complexes MA,, w+ = Ni(II), Zn(II), Cd(II), Pb(II), n(I), In(III)] with a series of 0,Odialkyldithiophosphoric acids in the system water-organic solvent) have been determined. By use of correlation analysis the role of the steric and hydrophobic effects of the substituents at the phosphorus atom on the stability constant Il. and distribution constant P of the complexes has been elucidated. The data obtained are of use for determining the relationships in the influence of structure of a reagent on its extraction properties and on the conditions for practical application of 0,Odialkyldithiophosphoric acids for metal extraction.
A great many organic compounds are suitable for extraction of metals from aqueous solution, but the search for new extractants continues, owing to the growing role of extraction methods in analysis and technology. In this context data on the interrelation between the structure of organic reagents and their extraction properties acquire especial significance.’ The O,O-dialkyldithiophosphoric acids (DTPA), first proposed by Busev and Ivaniutin* as analytical reagents, have found wide application in various methods of analyses. The availability of a wide variety of dithiophosphoric acid derivatives leads to choice of these compounds as an ideal model system for investigation of the influence of substituents in ligand molecules on the stability and distribution of the corresponding metal chelates. To examine the possibility of applying the principle of linear freeenergy change for the quantitative estimation of substituent effects on the extraction properties of DTPA, the extraction of metal ions by a series of DTPA of different structure was investigated. In extraction in a two-phase system of water and organic solvent there exists the equilibrium: (M”+ )p9+ n(A- ).q= (MA,),,
(1)
where M = Ni(II), Zn(II), Cd(II), Pb(II), Tl(I), In(II1); A- = anion of DTPA, (R0)2P(S)S-. The equilibrium constant (or two-phase stability constant) is equal to the product of the stability constant of the complex MA, in the aqueous phase, /I,, and the distribution constant P: pB
=
”
MU, W+l,[A-I&
count: (H + )aq+ (A- )aq= (HA),,
EXPERIMENTAL Reagents O,O-Dialkyldithiophosphoric acids were synthesized and purified by a literature method.” Heptane, benzene, toluene, carbon tetrachloride and chloroform were used as the solvents. Procedure Extraction was performed in test-tubes at constant temperature (25 f 0.2”) by shaking for 1 hr equal volumes (8 ml) of the aqueous and organic phases, the concentration of metal in the aquous phase being 5 x 10-4-10-3M. The ionic strength and acidity of the solutions were maintained by adding the appropriate amounts of perchloric acid and sodium nerchlorate or. in the case of PNII) and Cd(H). hydrochloric acid and. sodium chloride. % equilibrium concentration of free metal ions in the aqueous phase was determined by complexometric titration or polarographically. RESULTS AND DISCUSSION
The values of &/IL, were obtained by the method of Wingefors.4 The values of P/l, were calculated by using the equation log D = n log[A-] + log(Pj?,)
(2)
When the extraction proceeds in acid solutions, the distribution of DPTA must also be taken into ac-
(3)
The equilibrium constant of this process (extraction constant of DTPA) is equal to K/K,, where & is the distribution constant of HA between the aqueous and organic phases and K. is the acid dissociation constant of HA in the aqueous phase.
(4)
where D is the distribution ratio for the metal. The dependence of log D on log[A-] is a straight line with slope equal to the charge on the metal ion 211
212
V. F. TOROFWAet al.
Table 1. Logarithms of the two-phase stability constants (log P/J) of complexes M[S(S)P(OR),k in the systems 1M HClO,/NaClO,-organic solvent, at 25 f 0.2” (the error in log PB, is estimated to be kO.05) log W” Ni(I1) R
C,H,, Me Et 1.68 Pr 4.17 i-Pr 4.82 Bu 6.93 i-Bu 6.53 s-Bu 1.90 t-Bu n-C,H,, 9.50 i-C,H ,, 9.13 (C*H,),CH 10.56 n-C&H,, -
Ccl,
toluene
0.18 3.34 5.93 6.65 8.68 8.20 9.32 11.08 10.38 12.08 -
0.59 3.73 6.34 6.91 9.14 8.62 10.08 11.77 11.06 12.31 -
C,H,
CHCl,
4.06 6.87 1.43 9.41 9.01 10.52 11.49 12.02 11.69 14.63
1.58 4.45 7.11 1.98 9.91 9.32 10.92 11.99 12.59 12.02 15.07
Zn(II) CC& 0.09 2.70 5.13 5.44 1.12 1.49 8.29 8.85 10.37 10.04 10.88 13.06
In(III) Ccl, 2.15 6.88 10.80 11.92 14.78 14.36 15.55 19.00 17.81 19.44 23.19
WI) Cd(H) toluene toluene CHCl, 2.51 5.22 7.55 1.95 10.32 10.03 10.56 11.78 12.83 12.48 13.01 15.58
1.61 2.90 4.22 4.14 5.50 5.29 5.53 6.36 6.40 6.52 -
1.69 3.00 4.28 4.20 5.62 5.38 5.60 6.48 6.71 8.11
Pb(II)b ccl, 3.55 6.33 9.15 9.34 11.93 11.58 12.30 13.81 14.55 14.30 14.80 17.29
toluene 8.84 11.62 11.78 14.37 13.83 14.52 16.36 17.17 16.69 19.96
“The ionic strenath of the aqueous phase (U = 1) were maintained with HCl and NaCl. “p = 2 (HCl/NaCl). _ _ (n). Since the total concentration of metal in the solution did not exceed 5 x 10e4A4, any possibility of forming polynuclear complexes could be disregarded. The values found for log K,/K, and log P/In are given in Tables 1 and 2. As can be seen, the two-phase stability constants depend on the length and the branching of the alkyl groups. In all cases the values of P/i,, increase with increase in the carbon atom number of the substituents. The dependence on the branching is more complicated. Since P/I,, is a composite value, the interpretation of the results obtained requires consideration of the substituent effects on both the stability constant and the distribution constant. Since the generally accepted opinions,6 is that the induction effect of alkyl groups is constant, it may be supposed that changing an alkyl substituent should not seriously affect the energy of the metal-ligand bond. It might then be concluded that any change in the stability constants would be mainly due to steric effects. The influence of alkyl groups on the value of P is connected to a considerable degree with the solvent effects: the influence of substituents on the solvation of the ligand and of the complex. It is well
known that an increase in anion size leads to an increase in solvation free. energy. The effect on the solvation of the neutral chelates will largely depend, as a rule, on the entropy effects: increasing the size of the alkyl groups and, consequently, the size of the complex molecules, results in destruction of the water structure and thus in a free energy gain. In this case, both factors will promote increase of the stability constants with increase in alkyl group size. Asregards the influence of substituents on the distribution constant of the complex, a large number of data on the distribution of homologous series of organic compounds are present in the literature.’ As a rule, the higher members of a series are more easily transferred into the organic phase. Since it was shown in our previous communication’ that the infhtence of substituents on the stability constant of DTPA complexes can be described by a one-parameter correlation equation using Taft Es constants, characterizing the steric effects of substituents, log 8, = a0 + u1ZE,
Table 2. Logarithms of the extraction constants (J&/K,) of O,O-dialkyldithiophosphoric acids (RO),P(S)SH in the systems IM HClO,/NaClO, (or HCl/NaCl)-organic solvent (estimated uncertainty f 0.02) R Pr i-Pr Bu i-Bu s-Bu t-Bu
n-W,, i-C,H, , GH, ),CH c-C,H,,* *Cyclohexene.
Ccl, 1.71 1.99 3.03 2.96 3.39 4.11 4.23 4.02 4.64 4.68
C6H6 1.99 2.22 3.22 3.14 3.56 4.49 4.19 4.71 -
toluene 1.88 2.11 3.15 3.03 3.44 4.16 4.42 4.11 4.68 4.70
(5)
and as the hydrophobic constants’ (n) can be applied for description of the influence of substituents on the
CHCI, 2.40 2.71 3.66 3.56 4.01 4.66 5.27
w&6
-
2.10 2.00 2.48 3.36 3.10 3.80 -
-SE,
0.78 1.86 0.80 0.70 1.94 3.48 0.80 0.80 1.94 1.96
Zn
1.06 0.86 2.06 1.86 1.86 1.66 3.06 2.86 2.86 2.82
Extraction with O,O-dialkyldithiophosphoric acids
213
Table 3. Parameters of the correlation equations log PB. = an + n(u, XE. + u&c) Organic M”+
an
Ni(II) Ni(II) Ni(II) Ni(II) Ni(I1) Zn(I1) Cd(H) In(W) n(I) TW) PbGI)
CHCI, GH, toluene CCL C7H,6
ccl, toluene ccl, CHCl, toluene toluene
pbinj
ccl,
3.65 3.30 2.99 2.52 0.80 2.16 4.61 6.09 2.83 2.78 8.17 5.76
-6
0.54 4 0.04 0.54 f 0.04 0.46 * 0.06
0.50 f 0.04 0.50 f 0.05
0.34 4 0.03 0.36 & 0.05 0.37 + 0.04 0.19*0.09 0.16 f 0.07 0.44 & 0.06 0.43 f 0.06
a2
R*
1.28f 0.03 1.29f 0.03 I .30 f 0.04 I .27 + 0.03 1.30+ 0.03 1.27f 0.02 1.26f 0.04 1.30f 0.02 1.25f 0.04 1.22kO.03 1.34* 0.04 1.32 f 0.04
0.9980
0.9978 0.9983 0.9992 0.9985 0.9991 0.9973 0.9987 0.9972 0.9975 0.9964 0.9965
St 0.26 0.23 0.24 0.16 0.18 0.18 0.29 0.32 0.16 0.14 0.37 0.36
*Regression coefficient TStandard deviation.
distribution constant P, we use a two-parameter correlation equation amounting for both the steric and hydrophobic effects of the substituents: log(PB,)=u~+n(a,ZE*+a,Z:n)
(6)
where ZE, is the sum of the steric constants of the substituents on the phosphorus atom, Y&ris the sum of the hydrophobic constants of the substituents, and n is the stoichiometric coefficient in equation (1). It turns out that equation (6) well describes the influence of alkyl groups on PB, for DTPA complexes and on &/PC. for DTPA. The parameters of the correlation equation are given in Tables 3 and 4. The coefficient a*, which reflects the sensitivity of the series to the hydrophobic effects of the substitu-
ents, is virtually independent of the central ion and of the organic solvent, whereas the coefficient a,, which reflects the sensitivity of the series to the steric effects of the substituents, shows an appreciable dependence on the nature of the metal ion. Hence, it follows that by variation of the substituents, the conditions for preconcentration and separation of metals can be optimized to a certain degree with DTPA as the extracting agent. The separate determination of the P and /?,,values in difficult, as the equilibrium concentration of the complex in the aqueous phase is very low, owing to the high value of f. At the same time, the stability constants of the DTBA complex in aqueous solutions cannot readily be determined, owing to their low
Table 4. Parameters of correlation equations a, ZE, + a,Zn Organic solvent W,,
ccl, GWH, GH, CHCl,
a0 -0.90 -0.11 0.06 0.18 0.59
--ai 0.42 f 0.05 0.41 f 0.02 0.50 f 0.03 0.55 f 0.02 0.58 f 0.03
oz 1.30 & 0.03 1.28 f 0.02 1.27 f 0.02 1.26 f 0.01 1.26f 0.02
log &/1r; =a,+
R
s
0.9978 0.9912 0.9955 0.9993 0.9992
0.09 0.08 0.07 0.04 0.05
Table 5. Logarithms of stability constants (log &) in the water-propan2-01 solutions and of the distribution constants (log P) in the system 1M WClO,/NaClO,-CCl, for the nickel 0,Odialkyldithiophosphates Ni[S(S)P(OR)& at 25 f 0.2” 100% H,O log P 25% H,O 40% H,O 60% H,O (extrapoi.) R Me Et Pr i-Pr Bu i-Bu S-BU
t-Bu n-W%* i-C,H, ,
(C,H<),CH . . ..-_
n-W%
2.04 2.73 3.01 4.71 3.19 3.44 5.51 8.90 3.20 3.27 6.51 3.20
0.76 1.49 2.06 3.59 2.11 2.40 4.47 7.65 1.93 2.12 5.35 2.00
-0.28 0.47 1.01 2.49 1.13 1.49 3.65 6.49 1.11 1.42 4.37 -
-1.62 -0.88 -0.47 1.09 -0.37 0.13 2.08 5.23 -0.37 0.03 2.93 -0.39
1.80 4.22 6.40 5.56 9.05 8.07 7.24 11.45 10.35 9.15 -
214
V. F. TOROP~VA et al.
solubility. The only way to solve this problem consists in determining the /I. values in aqueous organic solutions containing different concentrations of water, and extrapolating the results to 100% water. In that way, we have determined the fir values of nickel-DTPA complexes in water-propan-2-01 solutions containing 20, 40 and 60% v/v water. For all cases investigated, the change in /I2 from one medium to another is approximately the same, and the equilibrium constant of the complexation reaction (K = jIz[HzO]6) is only slightly affected by the composition of the aqueous alcohol solvent, i.e., the change observed for a series of solvents is considered to be due to the dilution effect.’ The values of jz found by extrapolation to purely aqueous medium are presented in Table 5. It is interesting that the influence of alkyl groups on the distribution constants P of the complexes examined can also be described by a two-parameter correlation equation log P = 4.53 + 2(0.49 ZE, + 1.25 En)
(7)
with a standard deviation of 0.4 and regression coefficient 0.995. The coefficient u, has a positive value, i.e., unlike the stability constants, the P values
decrease with increase in the steric effect of the alkyl groups. In conclusion, it should be noted that there is a possibility of using these data in analytical chemistry. Knowledge of the extraction equilibrium constants and correlations between the extraction properties of DTPA and their structure allows a quantitative approach to the choice of proper reagents and estimation of the optimal conditions for preconcentration or separation of different metal ions.
REFERENCES
1. V. S. Shmidt, Usp. K&I., 1978, 47, 1755. 2. A. I. Busev and M. I. Ivaniutin, Tr. Komks. po Analit. Khim. Akad. Nauk SSSR, 1960, 11, 172. 3. P. S. Pishchimuka, Zh. Russ. Fiz-Khim. Obshcbest. 1912, 44, 1406. 4. S. Wingefors, Acta Chem. Stand., 1980, A34, 289. 5. D. F. DeTar. J. Am. Chem. Sot.. 1980. 102. 1988. ’ 6. M. Charton,‘ibid., 1979, 99, 5687. 7. A. Leo, C. Hansch and D. Elkins, Ckem. Rev., 1971, 71, 525. 8. V. V. Gvchinnikov, A. R. Garifzyanov and V. F. Toropova, Zh. Obshch. Khim., 1983, 53, 1262. 9. V. I. Belevantsev and V. A. Fedorov, Koord. Khim., 1977, 3, 638.
211-214, 1987
0039-9140/87 $3.00 + 0.00 Pergamon Journals Ltd
STERIC AND HYDROPHOBIC EFFECTS OF SUBSTITUENTS IN EXTRACTION OF METAL COMPLEXES WITH O,O-DIALKYLDITHIOPHOSPHORIC ACIDS V. F. TOROWVA, A. R. GARIFZYANOV and I. E. PANPJLOVA Faculty of Chemistry, U.I.Ul’yanov-Lenin State University, Lenina, 18 Kaxan, 420008, USSR (Received 15 October 1985. Revised 25 March 1986. Accepted 9 August 1986) Summary-The two-phase stability constants (equilibrium constants) of metal complexes MA,, w+ = Ni(II), Zn(II), Cd(II), Pb(II), n(I), In(III)] with a series of 0,Odialkyldithiophosphoric acids in the system water-organic solvent) have been determined. By use of correlation analysis the role of the steric and hydrophobic effects of the substituents at the phosphorus atom on the stability constant Il. and distribution constant P of the complexes has been elucidated. The data obtained are of use for determining the relationships in the influence of structure of a reagent on its extraction properties and on the conditions for practical application of 0,Odialkyldithiophosphoric acids for metal extraction.
A great many organic compounds are suitable for extraction of metals from aqueous solution, but the search for new extractants continues, owing to the growing role of extraction methods in analysis and technology. In this context data on the interrelation between the structure of organic reagents and their extraction properties acquire especial significance.’ The O,O-dialkyldithiophosphoric acids (DTPA), first proposed by Busev and Ivaniutin* as analytical reagents, have found wide application in various methods of analyses. The availability of a wide variety of dithiophosphoric acid derivatives leads to choice of these compounds as an ideal model system for investigation of the influence of substituents in ligand molecules on the stability and distribution of the corresponding metal chelates. To examine the possibility of applying the principle of linear freeenergy change for the quantitative estimation of substituent effects on the extraction properties of DTPA, the extraction of metal ions by a series of DTPA of different structure was investigated. In extraction in a two-phase system of water and organic solvent there exists the equilibrium: (M”+ )p9+ n(A- ).q= (MA,),,
(1)
where M = Ni(II), Zn(II), Cd(II), Pb(II), Tl(I), In(II1); A- = anion of DTPA, (R0)2P(S)S-. The equilibrium constant (or two-phase stability constant) is equal to the product of the stability constant of the complex MA, in the aqueous phase, /I,, and the distribution constant P: pB
=
”
MU, W+l,[A-I&
count: (H + )aq+ (A- )aq= (HA),,
EXPERIMENTAL Reagents O,O-Dialkyldithiophosphoric acids were synthesized and purified by a literature method.” Heptane, benzene, toluene, carbon tetrachloride and chloroform were used as the solvents. Procedure Extraction was performed in test-tubes at constant temperature (25 f 0.2”) by shaking for 1 hr equal volumes (8 ml) of the aqueous and organic phases, the concentration of metal in the aquous phase being 5 x 10-4-10-3M. The ionic strength and acidity of the solutions were maintained by adding the appropriate amounts of perchloric acid and sodium nerchlorate or. in the case of PNII) and Cd(H). hydrochloric acid and. sodium chloride. % equilibrium concentration of free metal ions in the aqueous phase was determined by complexometric titration or polarographically. RESULTS AND DISCUSSION
The values of &/IL, were obtained by the method of Wingefors.4 The values of P/l, were calculated by using the equation log D = n log[A-] + log(Pj?,)
(2)
When the extraction proceeds in acid solutions, the distribution of DPTA must also be taken into ac-
(3)
The equilibrium constant of this process (extraction constant of DTPA) is equal to K/K,, where & is the distribution constant of HA between the aqueous and organic phases and K. is the acid dissociation constant of HA in the aqueous phase.
(4)
where D is the distribution ratio for the metal. The dependence of log D on log[A-] is a straight line with slope equal to the charge on the metal ion 211
212
V. F. TOROFWAet al.
Table 1. Logarithms of the two-phase stability constants (log P/J) of complexes M[S(S)P(OR),k in the systems 1M HClO,/NaClO,-organic solvent, at 25 f 0.2” (the error in log PB, is estimated to be kO.05) log W” Ni(I1) R
C,H,, Me Et 1.68 Pr 4.17 i-Pr 4.82 Bu 6.93 i-Bu 6.53 s-Bu 1.90 t-Bu n-C,H,, 9.50 i-C,H ,, 9.13 (C*H,),CH 10.56 n-C&H,, -
Ccl,
toluene
0.18 3.34 5.93 6.65 8.68 8.20 9.32 11.08 10.38 12.08 -
0.59 3.73 6.34 6.91 9.14 8.62 10.08 11.77 11.06 12.31 -
C,H,
CHCl,
4.06 6.87 1.43 9.41 9.01 10.52 11.49 12.02 11.69 14.63
1.58 4.45 7.11 1.98 9.91 9.32 10.92 11.99 12.59 12.02 15.07
Zn(II) CC& 0.09 2.70 5.13 5.44 1.12 1.49 8.29 8.85 10.37 10.04 10.88 13.06
In(III) Ccl, 2.15 6.88 10.80 11.92 14.78 14.36 15.55 19.00 17.81 19.44 23.19
WI) Cd(H) toluene toluene CHCl, 2.51 5.22 7.55 1.95 10.32 10.03 10.56 11.78 12.83 12.48 13.01 15.58
1.61 2.90 4.22 4.14 5.50 5.29 5.53 6.36 6.40 6.52 -
1.69 3.00 4.28 4.20 5.62 5.38 5.60 6.48 6.71 8.11
Pb(II)b ccl, 3.55 6.33 9.15 9.34 11.93 11.58 12.30 13.81 14.55 14.30 14.80 17.29
toluene 8.84 11.62 11.78 14.37 13.83 14.52 16.36 17.17 16.69 19.96
“The ionic strenath of the aqueous phase (U = 1) were maintained with HCl and NaCl. “p = 2 (HCl/NaCl). _ _ (n). Since the total concentration of metal in the solution did not exceed 5 x 10e4A4, any possibility of forming polynuclear complexes could be disregarded. The values found for log K,/K, and log P/In are given in Tables 1 and 2. As can be seen, the two-phase stability constants depend on the length and the branching of the alkyl groups. In all cases the values of P/i,, increase with increase in the carbon atom number of the substituents. The dependence on the branching is more complicated. Since P/I,, is a composite value, the interpretation of the results obtained requires consideration of the substituent effects on both the stability constant and the distribution constant. Since the generally accepted opinions,6 is that the induction effect of alkyl groups is constant, it may be supposed that changing an alkyl substituent should not seriously affect the energy of the metal-ligand bond. It might then be concluded that any change in the stability constants would be mainly due to steric effects. The influence of alkyl groups on the value of P is connected to a considerable degree with the solvent effects: the influence of substituents on the solvation of the ligand and of the complex. It is well
known that an increase in anion size leads to an increase in solvation free. energy. The effect on the solvation of the neutral chelates will largely depend, as a rule, on the entropy effects: increasing the size of the alkyl groups and, consequently, the size of the complex molecules, results in destruction of the water structure and thus in a free energy gain. In this case, both factors will promote increase of the stability constants with increase in alkyl group size. Asregards the influence of substituents on the distribution constant of the complex, a large number of data on the distribution of homologous series of organic compounds are present in the literature.’ As a rule, the higher members of a series are more easily transferred into the organic phase. Since it was shown in our previous communication’ that the infhtence of substituents on the stability constant of DTPA complexes can be described by a one-parameter correlation equation using Taft Es constants, characterizing the steric effects of substituents, log 8, = a0 + u1ZE,
Table 2. Logarithms of the extraction constants (J&/K,) of O,O-dialkyldithiophosphoric acids (RO),P(S)SH in the systems IM HClO,/NaClO, (or HCl/NaCl)-organic solvent (estimated uncertainty f 0.02) R Pr i-Pr Bu i-Bu s-Bu t-Bu
n-W,, i-C,H, , GH, ),CH c-C,H,,* *Cyclohexene.
Ccl, 1.71 1.99 3.03 2.96 3.39 4.11 4.23 4.02 4.64 4.68
C6H6 1.99 2.22 3.22 3.14 3.56 4.49 4.19 4.71 -
toluene 1.88 2.11 3.15 3.03 3.44 4.16 4.42 4.11 4.68 4.70
(5)
and as the hydrophobic constants’ (n) can be applied for description of the influence of substituents on the
CHCI, 2.40 2.71 3.66 3.56 4.01 4.66 5.27
w&6
-
2.10 2.00 2.48 3.36 3.10 3.80 -
-SE,
0.78 1.86 0.80 0.70 1.94 3.48 0.80 0.80 1.94 1.96
Zn
1.06 0.86 2.06 1.86 1.86 1.66 3.06 2.86 2.86 2.82
Extraction with O,O-dialkyldithiophosphoric acids
213
Table 3. Parameters of the correlation equations log PB. = an + n(u, XE. + u&c) Organic M”+
an
Ni(II) Ni(II) Ni(II) Ni(II) Ni(I1) Zn(I1) Cd(H) In(W) n(I) TW) PbGI)
CHCI, GH, toluene CCL C7H,6
ccl, toluene ccl, CHCl, toluene toluene
pbinj
ccl,
3.65 3.30 2.99 2.52 0.80 2.16 4.61 6.09 2.83 2.78 8.17 5.76
-6
0.54 4 0.04 0.54 f 0.04 0.46 * 0.06
0.50 f 0.04 0.50 f 0.05
0.34 4 0.03 0.36 & 0.05 0.37 + 0.04 0.19*0.09 0.16 f 0.07 0.44 & 0.06 0.43 f 0.06
a2
R*
1.28f 0.03 1.29f 0.03 I .30 f 0.04 I .27 + 0.03 1.30+ 0.03 1.27f 0.02 1.26f 0.04 1.30f 0.02 1.25f 0.04 1.22kO.03 1.34* 0.04 1.32 f 0.04
0.9980
0.9978 0.9983 0.9992 0.9985 0.9991 0.9973 0.9987 0.9972 0.9975 0.9964 0.9965
St 0.26 0.23 0.24 0.16 0.18 0.18 0.29 0.32 0.16 0.14 0.37 0.36
*Regression coefficient TStandard deviation.
distribution constant P, we use a two-parameter correlation equation amounting for both the steric and hydrophobic effects of the substituents: log(PB,)=u~+n(a,ZE*+a,Z:n)
(6)
where ZE, is the sum of the steric constants of the substituents on the phosphorus atom, Y&ris the sum of the hydrophobic constants of the substituents, and n is the stoichiometric coefficient in equation (1). It turns out that equation (6) well describes the influence of alkyl groups on PB, for DTPA complexes and on &/PC. for DTPA. The parameters of the correlation equation are given in Tables 3 and 4. The coefficient a*, which reflects the sensitivity of the series to the hydrophobic effects of the substitu-
ents, is virtually independent of the central ion and of the organic solvent, whereas the coefficient a,, which reflects the sensitivity of the series to the steric effects of the substituents, shows an appreciable dependence on the nature of the metal ion. Hence, it follows that by variation of the substituents, the conditions for preconcentration and separation of metals can be optimized to a certain degree with DTPA as the extracting agent. The separate determination of the P and /?,,values in difficult, as the equilibrium concentration of the complex in the aqueous phase is very low, owing to the high value of f. At the same time, the stability constants of the DTBA complex in aqueous solutions cannot readily be determined, owing to their low
Table 4. Parameters of correlation equations a, ZE, + a,Zn Organic solvent W,,
ccl, GWH, GH, CHCl,
a0 -0.90 -0.11 0.06 0.18 0.59
--ai 0.42 f 0.05 0.41 f 0.02 0.50 f 0.03 0.55 f 0.02 0.58 f 0.03
oz 1.30 & 0.03 1.28 f 0.02 1.27 f 0.02 1.26 f 0.01 1.26f 0.02
log &/1r; =a,+
R
s
0.9978 0.9912 0.9955 0.9993 0.9992
0.09 0.08 0.07 0.04 0.05
Table 5. Logarithms of stability constants (log &) in the water-propan2-01 solutions and of the distribution constants (log P) in the system 1M WClO,/NaClO,-CCl, for the nickel 0,Odialkyldithiophosphates Ni[S(S)P(OR)& at 25 f 0.2” 100% H,O log P 25% H,O 40% H,O 60% H,O (extrapoi.) R Me Et Pr i-Pr Bu i-Bu S-BU
t-Bu n-W%* i-C,H, ,
(C,H<),CH . . ..-_
n-W%
2.04 2.73 3.01 4.71 3.19 3.44 5.51 8.90 3.20 3.27 6.51 3.20
0.76 1.49 2.06 3.59 2.11 2.40 4.47 7.65 1.93 2.12 5.35 2.00
-0.28 0.47 1.01 2.49 1.13 1.49 3.65 6.49 1.11 1.42 4.37 -
-1.62 -0.88 -0.47 1.09 -0.37 0.13 2.08 5.23 -0.37 0.03 2.93 -0.39
1.80 4.22 6.40 5.56 9.05 8.07 7.24 11.45 10.35 9.15 -
214
V. F. TOROP~VA et al.
solubility. The only way to solve this problem consists in determining the /I. values in aqueous organic solutions containing different concentrations of water, and extrapolating the results to 100% water. In that way, we have determined the fir values of nickel-DTPA complexes in water-propan-2-01 solutions containing 20, 40 and 60% v/v water. For all cases investigated, the change in /I2 from one medium to another is approximately the same, and the equilibrium constant of the complexation reaction (K = jIz[HzO]6) is only slightly affected by the composition of the aqueous alcohol solvent, i.e., the change observed for a series of solvents is considered to be due to the dilution effect.’ The values of jz found by extrapolation to purely aqueous medium are presented in Table 5. It is interesting that the influence of alkyl groups on the distribution constants P of the complexes examined can also be described by a two-parameter correlation equation log P = 4.53 + 2(0.49 ZE, + 1.25 En)
(7)
with a standard deviation of 0.4 and regression coefficient 0.995. The coefficient u, has a positive value, i.e., unlike the stability constants, the P values
decrease with increase in the steric effect of the alkyl groups. In conclusion, it should be noted that there is a possibility of using these data in analytical chemistry. Knowledge of the extraction equilibrium constants and correlations between the extraction properties of DTPA and their structure allows a quantitative approach to the choice of proper reagents and estimation of the optimal conditions for preconcentration or separation of different metal ions.
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