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Synergistic effect of pyridine bases on the solvent extraction of manganese(II) with 2-thenoyltrifluoroacetone
J. inors,nucl.Chem.,1968,Vol.30, pp. 3065to 3073. PeqlamonPress. Printedin Great Britain
SYNERGISTIC EFFECT OF PYRIDINE BASES ON THE SOLVENT EXTRACTION OF MANGANESE(II) WITH 2 - T H E N O Y L T R I F L U O R O A C E T O N E H. K A W A M O T O and H. A K A l W A Department of Chemistry, College of Technology, Gunma University, Kiriu, Gunma, Japan
(Received 20 November 1967; in revised form 29January 1968) -Synergistic effects of pyridine bases(B) on the extraction of Mn(ll)-thenoyltrifluoroaeetonate(MnT2) into benzene hav,e been studied. The results show that the synergism is attributable to the formation and preferential extraction of the 1 : 2 adduct, MnT2B2 and that stabilities of the resulting adducts decrease with the following order: y-picofine > pyridine > a-picoline > quinoline. The main factors affecting the stability of the adduct were considered to be base strength of the synergist and steric effect of its substituent. The above order does not agree with the observed trend in low pH region, in which destruction of synergism caused by protonation of a base might occur.
INTRODUCTION
TnE SYNERCISTIC extraction of a metal chelate with a neutral base has been extensively investigated in recent years and the so-called synergistic effect has been generally interpreted through formation of an adduct[1-10]. Some of these systems have been applied to the separation and determination of metal ions [11-13]. Recently, the authors have carded out the solvent extraction of manganese(II) with a mixture of 2-thenoyltrifluoroacetone (HT) and pyridine for analytical purpose[13]. The present paper intends to clarify the role of pyridine and effects of substituent in a-picohne, y-picoline and quinoline on the synergistic extraction of MnT2. 1. H. Irving and D. N. Edgington, J. inorg, nucl. Chem. 27, 419, 1359 (1964); H. M. N. H. Irving and N. S. AI-Niaimi, ibid. 27, 717, 2231 (1965). 2. T. Sekine, M. SakaJri, F. Shimada and Y. Hasegawa, Bull. chem. Soc. Japan 38, 847 (1965); 38, 2087 (1965). 3. N. C. Li, S. M. Wang and W. R. Walker, J. inorg, nuci. Chem. 27, 2263 (1965); R. J. Casey, J. J. M. Fardy and W. R. Walker, ibid. 29, 1139 (1967). 4. T. Shigematsu, M. Tabushi, M. Matsui and T. Hongo, Bull. chem. Soc. Japan 39, 165 (1966). 5. L. Newman and P. KIotz, Inorg. Chem. 5, 461 (1966). 6. M. Zangen, J. inorg, nucl. Chem. 38, 1693 (1966). 7. S. Takei,J. chem. Soc. Pure Chem. Sect.Japan 87,949 (1966). 8. J. Hala, J. inorg, nucl. Chem. 29, 1777 (1967). 9. M. Tanaka and I. Kojima, J. inorg, nucl. Chem. 29, ! 769 (1967). I0. H. Akaiwa and H. Kawamoto, J. chem. Soc. Japan, Pure Chem. Sect. 88, 56 (1967); J. inorg. nuci. Chem. 29, 1345 (1967). I I. T. Taketatsu and C. V. B anks,A nalyt. Chem. 38, 1524 (1966). 12. T. Sekine and D. Dyrssen,A nalytica, chim. A cta 37, 217 (1967). 13. H. Akaiwa and H. Kawamoto,Japan Analyst 16, 359 (1967). 3065
3066
H. K A W A M O T O and H. A K A I W A EXPERIMENTAL
Materials 2-thenoyltrifluoroacetone was obtained from Wako Pure Chemical Inc. and used without further purification, a-picoline and y-picoline (extra pure grade) were obtained from Tokyo Chemical Inc. Pyridine, quinoline and all other materials used in this work were of guaranteed grade. Solutions Stock solution of pyridine base was prepared by dissolving a weighed amount of the reagent in benzene. The base solution was mixed with benzene solution of 0.02 M 2-thenoyltrifluoroacetone before the extraction experiments. Standard solution of manganese was prepared by dissolving manganous sulfate in 0-I N sulfuric acid. The accurate concentration of manganous ion was determined spectrophotometrical4y by flag permanganate method. Aqueous solutions containing 0.1 M sodium perchlorate and 5.67 × 10-4 M manganous ion were prepared by dissolving weighed amounts of sodium perchlorate in water and mixed with the standard solution of manganese at various pH values, which were roughly preadjusted by adding acetic acid or ammonia. Extraction procedure Equal volumes (10 ml) of aqueous and organic phases were taken in a separatory funnel and the mixture was vigorously stirred at room temperature (about 20°C). The rate of extraction of manganese was so rapid that the equilibria were reached within 5 rain. In the present work, however, the mixture was allowed to equilibrate for 20 min to ensure the completion of the reaction. After having been separated from aqueous phase the organic phase was discarded. Determination of distribution ratio Five ml of the aqueous phase was taken in a flask and manganous ion was oxidized to permanganate ion by adding potassium periodate and boiling gently for 60 rain in sulfuric acid medium. Absorbance of the resulting solution was measured at 530 rap, with a Hitachi FPW-4 type photoelectricphotometer using 5 cm-glass cells. The presence of small amount of pyridine base did not affect the determination of manganese. The distribution ratio of manganese obtained by the above mentione~ procedt~e was given by D =
Total conc. of Mn(1 I) in the organic phase Total conc. of Mn(I I) in the aqueous phase"
(1)
pH measurements The pH value of the aqueous phase was obtained by measuring the pH of the residual aliquot. The pH measurement was carried out with a Hitachi-Horiba F-5 type pH meter. RESULTS AND DISCUSSION
The authors have already suggested that the main complex of manganese in the organic phase might be MnT~B~, where T- and B represent 2-thenoyltrifluoroacetonate ion and pyridine base, respectively[13]. When the extracted complex is shown by MnT2 B., total concentration of manganese(l I) in the organic phase is given by [Mn]org. = •
[MnT~B.lorg. = [MnT2Jorg.+ [MnT2B~]or~.
(2)
n~0
where subscript org refers to the organic phase and the formula MnT2B~-represents the average composition of the synergistic complexes in the organic phase. Manganese(II) present in the aqueous phase may exist in the form of complexes
Synergistic effect of pyridine bases
3067
with the components of the solution, namely Mn(NHa),(OH-)v(Ac-) ,. The species MnT~B~2-~ may also be formed by adding HT and a pyridine base. Among above complexes, the relative concentration of Mn(NHa)~(OH-)u to the free manganous ion can be estimated by using the published stability constants for the species Mn(NHa)~ and Mn(OH-)u[14]. For example, in the case of 0.1 M ammonia, both values of ~ [Mn(NHa)x]/[Mn 2+] and ~ [Mn(OH-)u]/ x = 1
y=l
[Mn 2+] are calculated to be less than 0.01 at pH 6. Since the concentration of ammonia used in the present experiments was less than 0.1 M, the above estimation indicates that the concentration of the Mn(NH3)~(OH-)u in the pH region below 6 is negligible compared with that of manganous ion. Therefore, total concentration of manganese(I I) in the aqueous phase is given by
[Mn]aq. = E [Mn(Ac-)z]+ E E [MnT,Bj 2-'] z=O
(3)
i=1 J=l
and the extraction constant is defined by the following equation k~.~7= [MnTzB~]°rg'[H+]2 [Mn]aq.[HT]~[B]en
(4)
where subscript f refers to the concentration of an extractant HT or B, which is not combined with manganese(II). It should be noted that the ke,~ value is significant only when ~ [Mn(Ac-)z] >> ~ ~ [MnTiBj2-~]. Assuming that log D z =0
i=lj=l
approaches to infinity and considering maximum manganese concentration used in the present experiment, the value of log [HT]total/[HT] I is less than 0.03. Therefore we could say that [HT]total is nearly equal to [HT]s within experimental errors. Since concentration of manganese(II) extracted in the organic phase was much less than that of [B]total, [B]total is approximately equal to [B] sIntroducing D -----([MnT2]org.+ [MnT2Bn]org.)/[Mn]aq. to Equation (4), the following equation is obtained. log (D -- Do) = log ke, ; + 2 log [HT]total + h log [B]total+ 2 pH
(5)
where Do = [MnTz]org./[Mn]aq., which corresponds to the distribution ratio in the absence of synergist. Although the value of Do could not be obtained due to low extractability of manganese(II) in the pH region below 7, the Do value could be neglected compared with the D value because all manganese(II) added was left quantitatively in the aqueous phase in the absence of synergist throughout the present experiment. Therefore log D is employed in the following discussion instead of log (D -- Do). Using the above approximation, in the case in which the k~,~in Equation (5) is constant, a log D/O log [HT]total and 0 log D/O log [B]total a r e obtained to be 2 and h, respectively. However, if a considerable amount of the MnT~Bj2-t is present in the aqueous phase, the experimentally found values of 0 log D/O 14. A. Ringbom, Complexation in Analytical Chemistry (Translated by N. Tanaka and H. Sugi), Sangyo Tosho, Tokyo (I 965).
3068
H. KAWAMOTO and H. A K A I W A
log[HT]tot~ and O log D/O log [B]total should differ from those theoretically obtained. The predicted value of a log D/a log [HT]tota~ agrees with the slope obtained from Fig. 1. And from the results shown in Fig. 2, the value of 0 log D/0 log [B]tota~ is obtained to be 2, which corresponds to the ~ value because the existence of MnB~2+ can be neglected as is described below. Therefore, the main complex extracted into the organic phase should be MnT2B2 as far as the present experiments are concerned. i
!
05
0
_o
-05
I
-2-0
I
-I-5 I0 g [H 1~ l
0
0
-I'0
Fig. 1. Distribution ratio D as a function of the concentration of l-IT in the presence of 0.025 M pyridine. I
I
I
-
4
2
a
i
0
y
-I
I
-3
-2
I
-I
I
0
I o g [ BI ,oto, Fig. 2. Distribution ratio D as a function of the concentration of base in the presence of 0.018M HT. 1; pyridine p H = 5 . 4 - 0 . 0 5 ; 2: pyridine pH = 5.2-_.0.05; 3: y-picofine pH = 4.7±0.1; 4: quinofine pH -- 5-3±0.1; 5: .-picolin¢ pH = 5.0----.0-1.
Synergistic effect of pyridine bases
3069
Assuming that the concentration of pyddine in the aqueous phase is 0.1 M, the value of Y~ [Mn(py)j2+]/[Mn 2+] is calculated to be about 10 *°e by using the Jffi0
value of log [Mn(py)2+]/[Mn 2+][py] = 0.14 [ 15]. Since the true concentration of pyridine base in the aqueous phase must be less than 0.1 M, considering large solubility of pyddine in benzene and possible protonation of pyridine in the aqueous phase, antagonism due to the formation of the MnBj 2+ can be neglected as far as the concentration of pyridine base is not too high. However, as is seen in Fig. 2, the slope of plots for quinoline tends to decrease in the region of high concentration of the base. This antagonism may be due to the formation of complex cation MnBj 2+, where Equation (5) is not valid. Slope of the plot of log D against pH is expected to be given by the following equation 0 log D = 0 log ke,2 - q- 2. 0 pH 0 pH
(6)
When the log ke.2 value is constant against pH, the slope in Fig. 3 is expected to be 2 because each value of [HT]totat and [B]total is kept to be constant. Only quinoline shows a linear relation having a slope of 2. When pyridine is used as an additive, approximately linear dependency of log D on pH is obtained. But the linearity is broken by changing pH drastically. Each slope of the plots in Fig. 4 exceeds 2 and an increase in pH leads the slopes of the plots to 2. The observed variation in log D value with pH can be explained by considering the change in log ke.2 which is given in Fig. 5. If the MnTtBj 2-i plays an important role in changing the log ke.2 value, the concentration of MnTtBj 2-t should increase and the value of log ke.2 may decrease along with the increase in pH. Therefore, we can assumethat Y~ [Mn(Ac-)z] ~ Y~ ~ [MnT~B~2-~], as Z=O
i=l
jffil
is described in the above. I
I
I
1.0
o
0
_o
-I.0
I
I
I
4-5
5.0
5.5
pH
6.0
Fig. 3. Distribution ratio D as a function of pH in the presence of 0.018 M HT. 1: v-picoline 0.16 M; 2: a-picoline 0.28 M; 3: quinofine 0.042 M. 15. J. Bjerrum, Chem. Rev. 46, 381 (1950).
3070
H. K A W A M O T O and H. A K A I W A 0'5
a
l
J
I
I
I
I
I
4.5
5"0
5.5
6'0
0
I
I
2 - 0.5
-FO
pH
Fig. 4. Distribution ratio D as a function of p H in the presence of 0-02 M H T . T h e concentration o f pyridine, 1:2.5 × 10-z M; 2 : 1 . 3 x 10-2 M; 3 : 3 . 2 x 10-3 M; 4 : 1 . 6 x 10-a M. I
I
I
O
-3
N
i
O
o
07'
o
O
~o o
0
-o
-5
I
I
!
I
4.5
5-0
55
6.0
pH Fig. 5. Extraction c o n s t a n t ke,2 as a function of pH. 1 :pyridine, 2:7-picoline, 3 :quinoline, 4:a-picoline
In addition to the above discussion, change in the relative concentration of the Mn(Ac-)z to manganous ion, ( o--~-o[Mn(Ac-)z]/[Mn2+] ) by changing pH may be small. For example, in the case of y-picoline, the value of log ~ [Mn(Ac-)z]/ z=0
[Mn ~+] is calculated as a function of the observed pH by using the stability constant for Mn(Ac-)z[14] (Table 1). These considerations show that change of the log ke,2 value with pH may not be attributable to the change of Mn(Ac-)~ and MnT~B~2- j concentrations. One of the possible side reactions caused by adding pyridine base is the
Synergistic effect of pyridine bases
3071
Table 1. Relative concentration of acetate complexes pH
4-49
4.53
4.66
4.87
5.21
0.17
0"18
0-17
0-16
0"13
Y. [Mn(Ac-),] logz=°
[Mn 2+]
protonation of the base in the aqueous phase. When [Mn]total "~ [B]totaL the following equation is obtained by mass balance, [Bltotal = [B]or~. + [ B ] + [BH + ]
to which the distribution coefficient Pa and dissociation constant KBH for the protonated form of a base are introduced, and the resulting situation is expressed by [a]tota~ - [B]org.(1 + an -I + PB-~Kffd[n+]) = I l l org.OtB(H) (7) where aa(m is a variable defined only by the concentration of hydrogen ion and the kind of base. When [Mn]tot~l < [HT]total, mass balance for H T is given by [HT]tota~ = [HT]org. + [HT] + IT-] = [HT] o,g.( 1 + P ~ + P~H~KHT[H +]- ~) = [HT]org.aT(H)
(8)
where P H T - - - - - [ H T l o r g / [ H T ] and KHT = [H+][T-]/[HT], which are reported to be l0 ve and 10-~'23, respectively[16]. Combining Equations (5), (7) and (8), the ke.~ can be rewritten as follows: log k~,2= log K e . 2 - - 2 log aBtn)-- 2 log aT(H)
(9)
where Ke,z is defined by
Ke,2 =
[MnT~B2]or~ [H÷] ~ [Mn]aq.[HT]grg[B]2rg"
(10)
The log OtT(mvalue at the pH of 6.0 is less than 0.03. Consequently, in the pH region below 6.0, log aTtH) is considered to be constant. In contrast, the log aB~H) value may be seriously varied according to the Equation (7) particularly in low pH region and the value approaches to constant with the increase in pH. Using Equation (9) and the constant Ke,2 slopes of the plots in Fig. 5 are given by 0 log**~/O pH = --20 log OgBtH)/C~pH. As is seen in Fig. 5, the slope for pyridine approaches to zero along with the increase of pH. The increase of the slope in 16. E. L. King and W. H. Reas, J. Am. chem. Soc. 73, 1804 (1951); J. C. Reid and M. Calvin, J. Am. chem. Soc. 72, 2948 (1950).
3072
H. K A W A M O T O and H. A K A I W A
the low pH region may be due to the formation of BH +. The log Ke,2 value obtained by using Equation (9) must be constant for each base. In fact, this assumption is available, and the resulting values of log Ke, z are arranged in the following order: ~-picoline (log Ke.2 = -1.75) > pyridine (-1.99) > oL-picoline (-3.83) > quinoline (--4.33). The main factors affecting the synergistic extraction of a metal chelate are considered to be the stability and the organophilic property of the resulting adduct. A competing reaction between the formation of the adduct in question and that of BH ÷ also affects the extent of synersism. But the log ge,z value requires no consideration for the protonation to a base because it was corrected in Equation (9). The organophilic property of the adduct may be represented by the magnitude of distribution coefficient, [adduct]om./[adduct]. If the distribution coefficient of the adduct is appreciably high, the dominating factor in the synergistic extraction would be the stability of an adduct, MnT2B2. For further discussion on the formation of an adduct, the log Ke.z value is divided into two parts: log
ge,2 -----log Ke + log K2
where K~ = [MnT2] org.[H +] 2/[Mn] aq. [HT]~g. and K2 = [MnT2B~]org./[MnT~]o~,. 2 Although both values could not be obtained in the present case because [B]or~. of low extractability of manganese(II) in the absence of synergist, the following treatment enables us to discuss the stability of the adduct. Since extraction constant K, must be equal for different bases, the Ke,2 value is proportional to the formation constant Kz for the particular adduct. Accordingly, a difference in log Ke.2 values between one base and pyridine, which is equal to A log K2, reflects relative ability of the base in regards to pyridine to combine with residual coordinating sites of MnT2. No doubt one of the major factors affecting the stability of a metal complex is the basicity of a ligand, which is essentially the same as the affinity of ligand toward proton. A log K2 values for various bases, relative to pyridine adduct are shown in Table 2. A logK2 values for the adducts of (diacetyl bisbenzoylhydrazone) nickel(l I), Ni(DBH), in benzene[17] and bis(acetylacetonate) copper(II), Cu(acac)2, in chloroform[18] and benzene[19] were obtained by recalculation using the reported formation constants[17-19]. As Table 2. Relative formation constants, A logK2, for adducts of MnT2, Ni(DBH) and A log K1 for adducts of Cu(acac)2 Base
MnT~
Ni(DBH)
Cu(acac)z
y-picoline pyridine a-picoline quinoline
0"24 0 -- 1"84 -- 2"34
0"67 0 -- 3.30 -- 1-70
0" 13 0 --0.21 -- 0.29
PKBH+ 6"02 5.23 5.97 4.85
17. L. Sacconi, G. Lombardo and P. Paoletti, J. chem. Soc. 848 (1958). 18. D. P. Graddon and E. C. Watton, J. inorg, nucl. Chem. 21,49 (1961). 19. H. M. N. H. Irving and N. S. AI-Niaimi, J. inorg, nucl. Chem. 27, 1671 (1965).
Synergistic effect of pyridine bases
3073
is seen in Table 2, the series of A log K2 for adducts of MnT2 show the same trend as that of PKBn+ values, excepting the case of a-picoline. Therefore, stabilities of the adducts are considered to be determined mainly by the availability of electrons on the nitrogen atom of pyridine ring. The stability constants of metal complexes having structurally similar ligands often show a roughly linear dependence on the basic strengths of the ligands[20, 21]. Major deviation from the expected trend are attributed to the steric effects[21]. Other type of deviations from normal values of the stability constants may probably be caused by the presence of ¢r-bonds[22]. Since the ¢r-acceptor capacity of pyridine is greater than that of y-picoline[23, 24], the pyridine adduct may be expected to show the greater 7r-contribution. The observed value of A log K2 for MnT2 (y-picoline)2 is seemed to be smaller than that expected from the difference in PKBH÷value between y-picoline and pyridine. Thus possible ¢r-bonding between MnT2 grid pyridine bases may be expected in the same manner as in the cases of pyridine adducts of Ni(DBH) [ 17] and Cu(acac)2 [22]. Smaller values of A log K2 for a-picoline and quinoline than those expected from proton affinity may be attributable to the steric effects of these bases. In fact, a-picoline adduct of bis(acetylacetonato) manganese(II) has been reported to be unstable compared with its pyridine adduct[25]. The value of A log K1 for a-picoline adduct of Cu(acac)2 is considerably higher than the adducts of MnT2 and Ni(DBH). This indicates that the interaction between the ortho-substituent of the additive and Cu(acac)2 may be relatively small. In contrast, in the case of MnT2, the steric hidrance has considerable effect on the stability of the resulting adduct which probably determines the rigidity of the octahedral adduct of MnT2 as in the case of Ni(DBH)[17]. 20. 21. 22. 23.
D. D. Perrin, Organic Complexing Reagents, p. 47. Interscience Publishers, New York (1964). H. Irving and H. Rossotti, A cta chem. Scand. 10, 72 (1956). J . J . R . Frafsto Da Silva andJ. Goncalves Calado,J. inorg, nucl. Chem. 28, 125 (1966). J. de O. Cabral, H. C. A. King, S. M. Nelson, T. M. Shepherd and E. Koros,J. chem. Soc. (A), 1348 (1966). 24. A. B. P. Lever, S. M. Nelson andT. M. Shepherd, lnorg. Chem. 4, 810(1965). 25. D. P. Creaddon and G. M. Mockler, Aust. J. Chem. 17, 1119 (1964).
SYNERGISTIC EFFECT OF PYRIDINE BASES ON THE SOLVENT EXTRACTION OF MANGANESE(II) WITH 2 - T H E N O Y L T R I F L U O R O A C E T O N E H. K A W A M O T O and H. A K A l W A Department of Chemistry, College of Technology, Gunma University, Kiriu, Gunma, Japan
(Received 20 November 1967; in revised form 29January 1968) -Synergistic effects of pyridine bases(B) on the extraction of Mn(ll)-thenoyltrifluoroaeetonate(MnT2) into benzene hav,e been studied. The results show that the synergism is attributable to the formation and preferential extraction of the 1 : 2 adduct, MnT2B2 and that stabilities of the resulting adducts decrease with the following order: y-picofine > pyridine > a-picoline > quinoline. The main factors affecting the stability of the adduct were considered to be base strength of the synergist and steric effect of its substituent. The above order does not agree with the observed trend in low pH region, in which destruction of synergism caused by protonation of a base might occur.
INTRODUCTION
TnE SYNERCISTIC extraction of a metal chelate with a neutral base has been extensively investigated in recent years and the so-called synergistic effect has been generally interpreted through formation of an adduct[1-10]. Some of these systems have been applied to the separation and determination of metal ions [11-13]. Recently, the authors have carded out the solvent extraction of manganese(II) with a mixture of 2-thenoyltrifluoroacetone (HT) and pyridine for analytical purpose[13]. The present paper intends to clarify the role of pyridine and effects of substituent in a-picohne, y-picoline and quinoline on the synergistic extraction of MnT2. 1. H. Irving and D. N. Edgington, J. inorg, nucl. Chem. 27, 419, 1359 (1964); H. M. N. H. Irving and N. S. AI-Niaimi, ibid. 27, 717, 2231 (1965). 2. T. Sekine, M. SakaJri, F. Shimada and Y. Hasegawa, Bull. chem. Soc. Japan 38, 847 (1965); 38, 2087 (1965). 3. N. C. Li, S. M. Wang and W. R. Walker, J. inorg, nuci. Chem. 27, 2263 (1965); R. J. Casey, J. J. M. Fardy and W. R. Walker, ibid. 29, 1139 (1967). 4. T. Shigematsu, M. Tabushi, M. Matsui and T. Hongo, Bull. chem. Soc. Japan 39, 165 (1966). 5. L. Newman and P. KIotz, Inorg. Chem. 5, 461 (1966). 6. M. Zangen, J. inorg, nucl. Chem. 38, 1693 (1966). 7. S. Takei,J. chem. Soc. Pure Chem. Sect.Japan 87,949 (1966). 8. J. Hala, J. inorg, nucl. Chem. 29, 1777 (1967). 9. M. Tanaka and I. Kojima, J. inorg, nucl. Chem. 29, ! 769 (1967). I0. H. Akaiwa and H. Kawamoto, J. chem. Soc. Japan, Pure Chem. Sect. 88, 56 (1967); J. inorg. nuci. Chem. 29, 1345 (1967). I I. T. Taketatsu and C. V. B anks,A nalyt. Chem. 38, 1524 (1966). 12. T. Sekine and D. Dyrssen,A nalytica, chim. A cta 37, 217 (1967). 13. H. Akaiwa and H. Kawamoto,Japan Analyst 16, 359 (1967). 3065
3066
H. K A W A M O T O and H. A K A I W A EXPERIMENTAL
Materials 2-thenoyltrifluoroacetone was obtained from Wako Pure Chemical Inc. and used without further purification, a-picoline and y-picoline (extra pure grade) were obtained from Tokyo Chemical Inc. Pyridine, quinoline and all other materials used in this work were of guaranteed grade. Solutions Stock solution of pyridine base was prepared by dissolving a weighed amount of the reagent in benzene. The base solution was mixed with benzene solution of 0.02 M 2-thenoyltrifluoroacetone before the extraction experiments. Standard solution of manganese was prepared by dissolving manganous sulfate in 0-I N sulfuric acid. The accurate concentration of manganous ion was determined spectrophotometrical4y by flag permanganate method. Aqueous solutions containing 0.1 M sodium perchlorate and 5.67 × 10-4 M manganous ion were prepared by dissolving weighed amounts of sodium perchlorate in water and mixed with the standard solution of manganese at various pH values, which were roughly preadjusted by adding acetic acid or ammonia. Extraction procedure Equal volumes (10 ml) of aqueous and organic phases were taken in a separatory funnel and the mixture was vigorously stirred at room temperature (about 20°C). The rate of extraction of manganese was so rapid that the equilibria were reached within 5 rain. In the present work, however, the mixture was allowed to equilibrate for 20 min to ensure the completion of the reaction. After having been separated from aqueous phase the organic phase was discarded. Determination of distribution ratio Five ml of the aqueous phase was taken in a flask and manganous ion was oxidized to permanganate ion by adding potassium periodate and boiling gently for 60 rain in sulfuric acid medium. Absorbance of the resulting solution was measured at 530 rap, with a Hitachi FPW-4 type photoelectricphotometer using 5 cm-glass cells. The presence of small amount of pyridine base did not affect the determination of manganese. The distribution ratio of manganese obtained by the above mentione~ procedt~e was given by D =
Total conc. of Mn(1 I) in the organic phase Total conc. of Mn(I I) in the aqueous phase"
(1)
pH measurements The pH value of the aqueous phase was obtained by measuring the pH of the residual aliquot. The pH measurement was carried out with a Hitachi-Horiba F-5 type pH meter. RESULTS AND DISCUSSION
The authors have already suggested that the main complex of manganese in the organic phase might be MnT~B~, where T- and B represent 2-thenoyltrifluoroacetonate ion and pyridine base, respectively[13]. When the extracted complex is shown by MnT2 B., total concentration of manganese(l I) in the organic phase is given by [Mn]org. = •
[MnT~B.lorg. = [MnT2Jorg.+ [MnT2B~]or~.
(2)
n~0
where subscript org refers to the organic phase and the formula MnT2B~-represents the average composition of the synergistic complexes in the organic phase. Manganese(II) present in the aqueous phase may exist in the form of complexes
Synergistic effect of pyridine bases
3067
with the components of the solution, namely Mn(NHa),(OH-)v(Ac-) ,. The species MnT~B~2-~ may also be formed by adding HT and a pyridine base. Among above complexes, the relative concentration of Mn(NHa)~(OH-)u to the free manganous ion can be estimated by using the published stability constants for the species Mn(NHa)~ and Mn(OH-)u[14]. For example, in the case of 0.1 M ammonia, both values of ~ [Mn(NHa)x]/[Mn 2+] and ~ [Mn(OH-)u]/ x = 1
y=l
[Mn 2+] are calculated to be less than 0.01 at pH 6. Since the concentration of ammonia used in the present experiments was less than 0.1 M, the above estimation indicates that the concentration of the Mn(NH3)~(OH-)u in the pH region below 6 is negligible compared with that of manganous ion. Therefore, total concentration of manganese(I I) in the aqueous phase is given by
[Mn]aq. = E [Mn(Ac-)z]+ E E [MnT,Bj 2-'] z=O
(3)
i=1 J=l
and the extraction constant is defined by the following equation k~.~7= [MnTzB~]°rg'[H+]2 [Mn]aq.[HT]~[B]en
(4)
where subscript f refers to the concentration of an extractant HT or B, which is not combined with manganese(II). It should be noted that the ke,~ value is significant only when ~ [Mn(Ac-)z] >> ~ ~ [MnTiBj2-~]. Assuming that log D z =0
i=lj=l
approaches to infinity and considering maximum manganese concentration used in the present experiment, the value of log [HT]total/[HT] I is less than 0.03. Therefore we could say that [HT]total is nearly equal to [HT]s within experimental errors. Since concentration of manganese(II) extracted in the organic phase was much less than that of [B]total, [B]total is approximately equal to [B] sIntroducing D -----([MnT2]org.+ [MnT2Bn]org.)/[Mn]aq. to Equation (4), the following equation is obtained. log (D -- Do) = log ke, ; + 2 log [HT]total + h log [B]total+ 2 pH
(5)
where Do = [MnTz]org./[Mn]aq., which corresponds to the distribution ratio in the absence of synergist. Although the value of Do could not be obtained due to low extractability of manganese(II) in the pH region below 7, the Do value could be neglected compared with the D value because all manganese(II) added was left quantitatively in the aqueous phase in the absence of synergist throughout the present experiment. Therefore log D is employed in the following discussion instead of log (D -- Do). Using the above approximation, in the case in which the k~,~in Equation (5) is constant, a log D/O log [HT]total and 0 log D/O log [B]total a r e obtained to be 2 and h, respectively. However, if a considerable amount of the MnT~Bj2-t is present in the aqueous phase, the experimentally found values of 0 log D/O 14. A. Ringbom, Complexation in Analytical Chemistry (Translated by N. Tanaka and H. Sugi), Sangyo Tosho, Tokyo (I 965).
3068
H. KAWAMOTO and H. A K A I W A
log[HT]tot~ and O log D/O log [B]total should differ from those theoretically obtained. The predicted value of a log D/a log [HT]tota~ agrees with the slope obtained from Fig. 1. And from the results shown in Fig. 2, the value of 0 log D/0 log [B]tota~ is obtained to be 2, which corresponds to the ~ value because the existence of MnB~2+ can be neglected as is described below. Therefore, the main complex extracted into the organic phase should be MnT2B2 as far as the present experiments are concerned. i
!
05
0
_o
-05
I
-2-0
I
-I-5 I0 g [H 1~ l
0
0
-I'0
Fig. 1. Distribution ratio D as a function of the concentration of l-IT in the presence of 0.025 M pyridine. I
I
I
-
4
2
a
i
0
y
-I
I
-3
-2
I
-I
I
0
I o g [ BI ,oto, Fig. 2. Distribution ratio D as a function of the concentration of base in the presence of 0.018M HT. 1; pyridine p H = 5 . 4 - 0 . 0 5 ; 2: pyridine pH = 5.2-_.0.05; 3: y-picofine pH = 4.7±0.1; 4: quinofine pH -- 5-3±0.1; 5: .-picolin¢ pH = 5.0----.0-1.
Synergistic effect of pyridine bases
3069
Assuming that the concentration of pyddine in the aqueous phase is 0.1 M, the value of Y~ [Mn(py)j2+]/[Mn 2+] is calculated to be about 10 *°e by using the Jffi0
value of log [Mn(py)2+]/[Mn 2+][py] = 0.14 [ 15]. Since the true concentration of pyridine base in the aqueous phase must be less than 0.1 M, considering large solubility of pyddine in benzene and possible protonation of pyridine in the aqueous phase, antagonism due to the formation of the MnBj 2+ can be neglected as far as the concentration of pyridine base is not too high. However, as is seen in Fig. 2, the slope of plots for quinoline tends to decrease in the region of high concentration of the base. This antagonism may be due to the formation of complex cation MnBj 2+, where Equation (5) is not valid. Slope of the plot of log D against pH is expected to be given by the following equation 0 log D = 0 log ke,2 - q- 2. 0 pH 0 pH
(6)
When the log ke.2 value is constant against pH, the slope in Fig. 3 is expected to be 2 because each value of [HT]totat and [B]total is kept to be constant. Only quinoline shows a linear relation having a slope of 2. When pyridine is used as an additive, approximately linear dependency of log D on pH is obtained. But the linearity is broken by changing pH drastically. Each slope of the plots in Fig. 4 exceeds 2 and an increase in pH leads the slopes of the plots to 2. The observed variation in log D value with pH can be explained by considering the change in log ke.2 which is given in Fig. 5. If the MnTtBj 2-i plays an important role in changing the log ke.2 value, the concentration of MnTtBj 2-t should increase and the value of log ke.2 may decrease along with the increase in pH. Therefore, we can assumethat Y~ [Mn(Ac-)z] ~ Y~ ~ [MnT~B~2-~], as Z=O
i=l
jffil
is described in the above. I
I
I
1.0
o
0
_o
-I.0
I
I
I
4-5
5.0
5.5
pH
6.0
Fig. 3. Distribution ratio D as a function of pH in the presence of 0.018 M HT. 1: v-picoline 0.16 M; 2: a-picoline 0.28 M; 3: quinofine 0.042 M. 15. J. Bjerrum, Chem. Rev. 46, 381 (1950).
3070
H. K A W A M O T O and H. A K A I W A 0'5
a
l
J
I
I
I
I
I
4.5
5"0
5.5
6'0
0
I
I
2 - 0.5
-FO
pH
Fig. 4. Distribution ratio D as a function of p H in the presence of 0-02 M H T . T h e concentration o f pyridine, 1:2.5 × 10-z M; 2 : 1 . 3 x 10-2 M; 3 : 3 . 2 x 10-3 M; 4 : 1 . 6 x 10-a M. I
I
I
O
-3
N
i
O
o
07'
o
O
~o o
0
-o
-5
I
I
!
I
4.5
5-0
55
6.0
pH Fig. 5. Extraction c o n s t a n t ke,2 as a function of pH. 1 :pyridine, 2:7-picoline, 3 :quinoline, 4:a-picoline
In addition to the above discussion, change in the relative concentration of the Mn(Ac-)z to manganous ion, ( o--~-o[Mn(Ac-)z]/[Mn2+] ) by changing pH may be small. For example, in the case of y-picoline, the value of log ~ [Mn(Ac-)z]/ z=0
[Mn ~+] is calculated as a function of the observed pH by using the stability constant for Mn(Ac-)z[14] (Table 1). These considerations show that change of the log ke,2 value with pH may not be attributable to the change of Mn(Ac-)~ and MnT~B~2- j concentrations. One of the possible side reactions caused by adding pyridine base is the
Synergistic effect of pyridine bases
3071
Table 1. Relative concentration of acetate complexes pH
4-49
4.53
4.66
4.87
5.21
0.17
0"18
0-17
0-16
0"13
Y. [Mn(Ac-),] logz=°
[Mn 2+]
protonation of the base in the aqueous phase. When [Mn]total "~ [B]totaL the following equation is obtained by mass balance, [Bltotal = [B]or~. + [ B ] + [BH + ]
to which the distribution coefficient Pa and dissociation constant KBH for the protonated form of a base are introduced, and the resulting situation is expressed by [a]tota~ - [B]org.(1 + an -I + PB-~Kffd[n+]) = I l l org.OtB(H) (7) where aa(m is a variable defined only by the concentration of hydrogen ion and the kind of base. When [Mn]tot~l < [HT]total, mass balance for H T is given by [HT]tota~ = [HT]org. + [HT] + IT-] = [HT] o,g.( 1 + P ~ + P~H~KHT[H +]- ~) = [HT]org.aT(H)
(8)
where P H T - - - - - [ H T l o r g / [ H T ] and KHT = [H+][T-]/[HT], which are reported to be l0 ve and 10-~'23, respectively[16]. Combining Equations (5), (7) and (8), the ke.~ can be rewritten as follows: log k~,2= log K e . 2 - - 2 log aBtn)-- 2 log aT(H)
(9)
where Ke,z is defined by
Ke,2 =
[MnT~B2]or~ [H÷] ~ [Mn]aq.[HT]grg[B]2rg"
(10)
The log OtT(mvalue at the pH of 6.0 is less than 0.03. Consequently, in the pH region below 6.0, log aTtH) is considered to be constant. In contrast, the log aB~H) value may be seriously varied according to the Equation (7) particularly in low pH region and the value approaches to constant with the increase in pH. Using Equation (9) and the constant Ke,2 slopes of the plots in Fig. 5 are given by 0 log**~/O pH = --20 log OgBtH)/C~pH. As is seen in Fig. 5, the slope for pyridine approaches to zero along with the increase of pH. The increase of the slope in 16. E. L. King and W. H. Reas, J. Am. chem. Soc. 73, 1804 (1951); J. C. Reid and M. Calvin, J. Am. chem. Soc. 72, 2948 (1950).
3072
H. K A W A M O T O and H. A K A I W A
the low pH region may be due to the formation of BH +. The log Ke,2 value obtained by using Equation (9) must be constant for each base. In fact, this assumption is available, and the resulting values of log Ke, z are arranged in the following order: ~-picoline (log Ke.2 = -1.75) > pyridine (-1.99) > oL-picoline (-3.83) > quinoline (--4.33). The main factors affecting the synergistic extraction of a metal chelate are considered to be the stability and the organophilic property of the resulting adduct. A competing reaction between the formation of the adduct in question and that of BH ÷ also affects the extent of synersism. But the log ge,z value requires no consideration for the protonation to a base because it was corrected in Equation (9). The organophilic property of the adduct may be represented by the magnitude of distribution coefficient, [adduct]om./[adduct]. If the distribution coefficient of the adduct is appreciably high, the dominating factor in the synergistic extraction would be the stability of an adduct, MnT2B2. For further discussion on the formation of an adduct, the log Ke.z value is divided into two parts: log
ge,2 -----log Ke + log K2
where K~ = [MnT2] org.[H +] 2/[Mn] aq. [HT]~g. and K2 = [MnT2B~]org./[MnT~]o~,. 2 Although both values could not be obtained in the present case because [B]or~. of low extractability of manganese(II) in the absence of synergist, the following treatment enables us to discuss the stability of the adduct. Since extraction constant K, must be equal for different bases, the Ke,2 value is proportional to the formation constant Kz for the particular adduct. Accordingly, a difference in log Ke.2 values between one base and pyridine, which is equal to A log K2, reflects relative ability of the base in regards to pyridine to combine with residual coordinating sites of MnT2. No doubt one of the major factors affecting the stability of a metal complex is the basicity of a ligand, which is essentially the same as the affinity of ligand toward proton. A log K2 values for various bases, relative to pyridine adduct are shown in Table 2. A logK2 values for the adducts of (diacetyl bisbenzoylhydrazone) nickel(l I), Ni(DBH), in benzene[17] and bis(acetylacetonate) copper(II), Cu(acac)2, in chloroform[18] and benzene[19] were obtained by recalculation using the reported formation constants[17-19]. As Table 2. Relative formation constants, A logK2, for adducts of MnT2, Ni(DBH) and A log K1 for adducts of Cu(acac)2 Base
MnT~
Ni(DBH)
Cu(acac)z
y-picoline pyridine a-picoline quinoline
0"24 0 -- 1"84 -- 2"34
0"67 0 -- 3.30 -- 1-70
0" 13 0 --0.21 -- 0.29
PKBH+ 6"02 5.23 5.97 4.85
17. L. Sacconi, G. Lombardo and P. Paoletti, J. chem. Soc. 848 (1958). 18. D. P. Graddon and E. C. Watton, J. inorg, nucl. Chem. 21,49 (1961). 19. H. M. N. H. Irving and N. S. AI-Niaimi, J. inorg, nucl. Chem. 27, 1671 (1965).
Synergistic effect of pyridine bases
3073
is seen in Table 2, the series of A log K2 for adducts of MnT2 show the same trend as that of PKBn+ values, excepting the case of a-picoline. Therefore, stabilities of the adducts are considered to be determined mainly by the availability of electrons on the nitrogen atom of pyridine ring. The stability constants of metal complexes having structurally similar ligands often show a roughly linear dependence on the basic strengths of the ligands[20, 21]. Major deviation from the expected trend are attributed to the steric effects[21]. Other type of deviations from normal values of the stability constants may probably be caused by the presence of ¢r-bonds[22]. Since the ¢r-acceptor capacity of pyridine is greater than that of y-picoline[23, 24], the pyridine adduct may be expected to show the greater 7r-contribution. The observed value of A log K2 for MnT2 (y-picoline)2 is seemed to be smaller than that expected from the difference in PKBH÷value between y-picoline and pyridine. Thus possible ¢r-bonding between MnT2 grid pyridine bases may be expected in the same manner as in the cases of pyridine adducts of Ni(DBH) [ 17] and Cu(acac)2 [22]. Smaller values of A log K2 for a-picoline and quinoline than those expected from proton affinity may be attributable to the steric effects of these bases. In fact, a-picoline adduct of bis(acetylacetonato) manganese(II) has been reported to be unstable compared with its pyridine adduct[25]. The value of A log K1 for a-picoline adduct of Cu(acac)2 is considerably higher than the adducts of MnT2 and Ni(DBH). This indicates that the interaction between the ortho-substituent of the additive and Cu(acac)2 may be relatively small. In contrast, in the case of MnT2, the steric hidrance has considerable effect on the stability of the resulting adduct which probably determines the rigidity of the octahedral adduct of MnT2 as in the case of Ni(DBH)[17]. 20. 21. 22. 23.
D. D. Perrin, Organic Complexing Reagents, p. 47. Interscience Publishers, New York (1964). H. Irving and H. Rossotti, A cta chem. Scand. 10, 72 (1956). J . J . R . Frafsto Da Silva andJ. Goncalves Calado,J. inorg, nucl. Chem. 28, 125 (1966). J. de O. Cabral, H. C. A. King, S. M. Nelson, T. M. Shepherd and E. Koros,J. chem. Soc. (A), 1348 (1966). 24. A. B. P. Lever, S. M. Nelson andT. M. Shepherd, lnorg. Chem. 4, 810(1965). 25. D. P. Creaddon and G. M. Mockler, Aust. J. Chem. 17, 1119 (1964).