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The chemistry of iron in biosystems—III. Complex formation between FeIII and malonic acid in aqueous solutions
Po/yhedron Vol. 8, No. 6, pp. 81~818, Printed in Great Britain
1989 0
0277-5387189 t3.00+.00 1989 Pergamon Press plc
T-HE CHEMISTRY OF IRON IN BIOSYSTEMS-III. COMPLEX FORMATION BETWEEN Fern AND MALONIC ACID IN AQUEOUS SOLUTIONS V. SALVADO
and X. RIBAS
Departament de Quimica, Col.legi Universitari de Girona (U.A.B.), Pea. Hospital 6, E 17071 Girona, Catalonia, Spain V. ZELANO
Dipartimento
and G. OSTACOLI
di Chimica Analitica, Universita di Torino, Via Bidone 36, I 10125 Torino, Italy and M. VALIENTE*
Departament
de Quimica, Quimica Analitica, Universitat Autonoma de Barcelona, E 08 193 Bellaterra, Catalonia, Spain (Received 16 August 1988 ; accepted 20 October 1988)
Abstract-Complex formation between Fe3+ ions and malonic acid (H,L), in acidic aqueous solutions containing 0.5 M NaNO, as inert ionic medium at 25”C, has been studied potentiometrically. The data were consistent with the following reactions and related stability constants :
Fe3+ +L2-
= FeL+
log p0,1,1= 7.52
Fe3++2L2-
= FeL;
log/&,,,, = 13.29
Fe3+ +3L2-
= FeLi-
log fi0,1,3= 16.93
which agree partially with previous models. No mixed complexes were detected, i.e. hydrolytic complex species. A suggestion of possible structures for the complex species formed is briefly discussed. In separate experiments and in the absence of Fe”‘, protolytic reactions of malonic acid were studied under the same experimental conditions. A numerical comparison of these results with those already published was carried out by means of the Specific Interaction Theory (SIT).
The biological activity of metal ions depends to a great extent on the interactions with those metal complexing agents present in the environment’ and the formation of strong soluble complexes between Fe”’ and ligands bearing carboxylic groups which are present in nature, is widely known. A systematic study including chemical characterization of these agents is currently being carried out in our laboratories. Results on the complexation of
*Author to whom correspondence should be addressed. 813
Fe”’ with tartrate have been published.’ We have also investigated iron(III)-malonate complexes in acidic solutions. The presence of malonic acid retards to lower pH the precipitation of ferric hydroxide in aqueous solutions. These differences with respect to tartaric acid excited our curiosity to ascertain the role of side hydroxyl groups in similar ligands. Iron(III)-malonate complexes have been studied by several authors using different techniques and thermodynamic conditionspg (see Table 1). Thus, a kinetic study carried out by Cavasino4 found
V. SALVADb
814
et al.
Table 1. Summary of literature data on Fe”*-malonate complexes in aqueous solutions Model
Ionic media
FeL, FeL, FeL FeL, HFeL FeL
0.5 (NaClO,) 1.O (NaClO,) 0.5 (LiClOJ
FeL, FeL, FeL, FeL, FeL,, FeL,
0.5 (NaClO,) 0.1 (NaClO,) 1.O (NaClO,)
FeL, FeL,, FeL,
0.5 (NaNO,)
References
Method
3 4 5 6
Spectrophotometric Kinetic Arnperometric Potentiometric and spectrophotometric Polarographic Potentiometric Potentiometric and calorimetric Potentiometric
7 8 9 This work
-
the species FeL. Gordienko,’ using amperometric measurements, postulates the species FeL and FeHL. In a polarographic study, Schaap et a1.6 suggested the species FeL,. Alternatively, Ramamoorthy and Manning,’ from potentiometric data, give a model with the species FeL and FeL, to exlain the behaviour of the iron(III)--malonate system. Finally Dellien,’ working in very acidic media and employing both potentiometric and calorimetric techniques, found the complex species FeL, FeL, and FeL,. All these studies were performed at 25°C and different ionic media were employed. A careful and systematic study of the chemical interactions between Fe3+ and malonic acid in aqueous solutions has been carried out in the present work. Potentiometric data obtained in a wide concentration range of the reacting species were analysed to quantitatively characterize the complex species and related stability constants.
EXPERIMENTAL Reagents, solutions and apparatus Malonic acid (Scharlau, AR) was used with no further purification, its purity was verified by potentiometric titration. Ferric nitrate (Merck, AR) was used as supplied to prepare stock solutions in 0.5 M NO;. The Fe”’ content was determined as described in ref. 10. The free acidity, h, in iron solutions was obtained following ref. 11. NaNO, (Probus, AR), purified as described in ref. 12, was used as the ionic media. Solutions of carbonate-free NaOH were prepared and determined as described of malonic acid, elsewhere. ’ 2 Stock solutions Fe(N0,)3, HN03 and NaOH were conditioned in NaN03 to achieve an ionic strength of 0.5 M. A digital voltimeter (Crison Digilab 517) of 0.1 mV precision interfaced to an Epson HX 20 computer was used to control the potentiometric experi-
ments. A glass electrode (Metrohm 1028) connected against a double junction reference electrode (Orion 9020) were employed to generate the EMF data.
Method of investigation Continuous EMF measurements have been performed in order to analyse the equilibrium state of the system. The hydrogen ion concentration, h, has been experimentally determined in a number of solutions prepared from stock solutions of Fe(N03)3, malonic acid, HN03 and NaN03. Measurements of EMF were performed by titrations using the galvanic cell I formulated by : RE//TS/GE,
(I)
where GE indicates a glass electrode and RE represents a double junction reference electrode. Stability criteria of the EMF measurements considered the potential to be stable when the variation was less than 0.1 mV within 10 min. For each experiment, the analytical concentrations of Fe”’ (B) and malonate (L) were varied in the range 2.0-6.0 mM and 5.0-20.0 mM, respectively. Values of pH varied from 1.5 to 5.5, where precipitation of ferric hydroxide was observed. The experimental data are collected in Fig. 1 in the form 2 vs -log h at the different B and L levels. Z represents the number of H+ bound to the total malonate and is defined by : Z = (H - h + Kw/h)/L
(1)
where H represents the analytical concentration of hydrogen ion. In order to verify our equilibrium data, experimental points obtained in back titrations are plotted in Fig. 1 by means of full symbols. The good agreement between the forward and back titration data shows that only reversible processes take part in the
Fe”’ and malonic acid complex formation [Fe3 1.959mM 2.939mM
LLI
815
components) requires a well established knowledge of the binary interactions, i.e. the protolysis of malonic acid and hydrolysis of Fe”‘. In our case, data for Fe”’ hydrolysis has been taken from a previous work, ’ 6 which was performed under the same experimental conditions. With respect to malonic acid, the following study was carried out.
8.918mM
10.255mM I.959 mM 6.699mM I.959 mM 4.459mM
Protolytic equilibria of malonic acid
2
-log h
Fig. 1. 2 plotted vs -log h for the different sets of experiments. Full points represent back titrations. Solid lines have been calculated by the model proposed in the present work. complexation reactions. All the experiments performed at 25 + 0.2”C.
were
B: = B?YHYLIYHL
RESULTS Analysis of the experimental information is based on the complex formation between ferric ions, Fe3+, and malonic acid, expressed by the general equation : pH +qFe+rL
= J--&Fe,&,; &g,,
Experiments in the absence of Fe”’ were carried out in order to quantitatively characterize malonic acid protolysis in the desired thermodynamic conditions. In this case, the EMF data have been treated both by the Rossotti’s graphical method” and numerically by the programs LETAGROP and SUPERQUAD. Table 2 gives the results for these treatments, which are found to be in good agreement. On the other hand and for the sake of comparison with other results reported in the literature,4*5~‘8-20 the different values of stability constants have been correlated using the Specific Interaction Theory, SIT.2’*22 Such a treatment of the data can be summarized by the expression :
where /?{ represents the protolytic constant at the ionic strength Z, /I: is the value of this constant at Z = 0 and yn, yL, yHL the activity coefficients of H, L and HL species. The activity coefficients follow the expression : log
yi
=
-
z?D + c &ikmk k
(2)
constants of where Bp,q,rrepresents the formation the complex species with different p, q and r values. Charges are omitted for simplicity. The stoichiometry of the complexes, and the by means constant &,4,r have been determined of a numerical analysis of the experimental data. For this purpose, the computer programs LETAGROPi3 and SUPERQUAD14 were used. In both cases, the treatment is based on the minimization of an error square sum U, defined by :
where EeXprepresents the experimental EMF values, Ecalcis a corresponding magnitude calculated by the program, assuming a determined model of species formed and related stability constants, and Np is the number of experimental points. As has been discussed previously,‘*’ ’ a systematic analysis of such ternary systems (three reacting
(4)
where D is the Debye-Huckel term, k represents the species with opposed charge to i in the ionic media, mk represents their molality and .sikare the specific interaction coefficients between the corresponding species present in the aqueous solution. Applying SIT and rearranging eq. (4) the following expressions can be written for each one of the protoTable 2. Results obtained for the behaviour of malonic acid in aqueous 0.5 M NaNO,, by different graphical and numerical methods. K,, = /lJ?; ‘, 8, = K,;’ where K,, and K,, are the corresponding acidic constants Method
log BI
log P2
log K2
Rossotti” LETAGROPb SUPERQUADb
5.05 5.06&0.008 5.05+0.004
7.62 7.62kO.016 7.61+0.010
2.57 2.56 2.56
BGraphical (ref. 17). bNumerical.
816
V. SALVADb
lytic constants (X = NO;) :
Table 3. Results obtained on the numerical treatment of the experimental data for the system Fe”‘-malonic acid
Y1 = log j?{ + 40 - EH,XmX = logby+ Y, = log/?‘,+6D-2eH,XmX
et al.
@L,M -EHL,M)~M
(6)
= log/I!j+EL,,mM.
Model
(P, 4, 4
u
Q (mv)
1
(0, 1, 1) (0, L2)
0.44x lo4
6.89
6.46f0.14 11.75+0.06
2
(0, 1, 1) (0, 1, 2) (1, 1, 1)
0.44x lo4
6.85
6.52f0.13 11.73f0.07 Rejected
3
(0, 1, 1) (0, 192) CO,1, 3) (-2,2,2)
0.43x 103
2.12
7.65f0.15 13.35f0.14 16.83kO.15 Rejected
0.77 x 10
0.29
7.52kO.01 13.29f0.01 16.93kO.01
(7)
In Fig. 2, Y, and Y, vs Z are represented for the different data considered. As seen, our results show a good fit in such a correlation.
Numerical treatment of the ternary system
With respect to the Fe”‘-malonate system, a numerical analysis of data has been carried out, starting from previous results3-9 to fit to our experimental data. The results of the calculation are collected in Table 3. As seen in this table, the best fit to the experimental data corresponds to the model where the species FeL, FeL, and FeL, are formed. This result is expressed by the following reaction equations, and the corresponding /Ip,q,rvalues are given : Fe3++L2-
= FeL+
log Bo,l,l = 7.52
(8)
Fe3++2L2-
= FeL;
log B0.1,~= 13.29
(9)
Fe3++3L2-
= FeI$
logp,,,,, = 16.93 (10)
Further analysis of the results, including polynuclear and mixed species (i.e. protolytic species) has shown that such species do not fit the experimental data. Figure 3 shows the individual values of (&,,CEexp) mV in the acidic range studied. As seen, the proposed model shows no systematic deviations with respect to the experimental data.
4”
(0, 1, 1) (0, 1,2) (0, 1, 3)
loisBP.%
“After correction of possible analytical errors.
DISCUSSION
The results obtained in the present work correspond to the formation of a complex between Fe”’ and malonic acid in aqueous solutions of 0.5 M NaNO, as the ionic medium. The experimental data have verified the equilibrium state of the system as well as that no irreversible or sideways process is taking part in the reactions studied. The complex species formed are represented by eqs (8)-(10). From the numerical treatment of the data, no polynuclear or protonated species were detected. This fact indicates a different behaviour of this system with respect to the Fe”‘-tartrate system studied previously* in which the formation of polynuclear complexes has been postulated. On the other hand, the stability of the Fe”‘-tartrate com-
[Fe1 IL1 e,O.O01959M mca9bM oO.O02939# O.OIO255M
I
Fig. 2. Correlation of the protolytic constants of malonic acid by means of the Specific Interaction Theory (SIT).
I 2
I 3
I 4
Fig. 3. (E,,,, - EeXP)mV vs -log h for the different experimental values analysed. Ecalc correspond to the EMF values calculated by the program on the basis of the model. _proposed _
817
Fe”’ and malonic acid complex formation
PH
Fig. 4. Fraction diagram of the Fe”‘-malonate species as a function of -log h.
complex
plexes is higher than the Fe”‘-malonate, thus precipitation of ferric hydroxide is observed at a pH > 5.5, while in the presence of tartaric acid this does not occur. These differences may be explained by the contribution of the hydroxyl groups of the tartaric acid to form more stable complexes. In Fig. 1 the solid lines represent calculated values of Z vs -log h using the model given by eqs (8k (10). The agreement with experimental values shows a high confidence level on this model. Figure 4 shows the distribution diagram of the species, calculated by the program HALTAFALL,23 in the acidic range studied. As seen, a step-wise formation of complexes is observed. The proposed model in the present work corresponds to species of stoichiometry 1 : 1, 1 : 2 and 1 : 3 respectively. For these stoichiometries, and taking into account that Fe”’ complexes usually have an octahedral co-ordination geometry,24 it is possible to speculate on probable structures of the proposed species. In Fig. 5, possible structures for the species FeL, FeL, and FeL, are suggested. These speculations are consistent with the absence of both polynuclear and protolytic complex species in the Fe”‘-malonate system. At any rate, further experimental information would be needed to properly determine such structures, i.e. direct spectroscopic measurements of the related solutions. Taking into account the information summarized in Table 1, a correlation between our results and those obtained by Dellieng is observed. It is found that the differences in the respective constant values are small in spite of the different media utilized (see Table 4). The work of Dellien shows, in the case of FeL, species, that the uncertainty in p3 is rather large [(4f 2) x 1016]. This may be a consequence of the experimental information, which has been obtained in a very acidic range (the acid content, h, ranges from 30 to 150 mM). In this sense, it is interesting to observe the concentrations of the
Fig. 5. Suggested structures for the Fe”‘--malonate soluble complexes assuming an octahedral co-ordination of Fe”‘.
complex species as a function of pH. Thus, in Fig. 4 it is observed that the presence of FeL, is less than 1% at pH -C 1, while this species is the most abundant at pH 3.5. It is difficult to see how under such experimental conditions one can differentiate between species of such low concentration. In our case the work was extended ‘up to pH < 5 where the different species, and especially FeL,, can be easily distinguished. Table 4. Differences between the values of stability constants for the Fe3+ -malonate equilibria obtained by Dellien’ and the respective values proposed in this work
Species
log B (ref. 9)
log B (this work)
FeL FeL, FeL,
7.50 13.04 16.60
7.52 13.29 16.93
V. SALVAD6 et al.
818
On the other hand, the slight differences observed between the constants derived in both investigations may explain that Fe3+-NO; interactions” are negligible under our experimental conditions. Acknowledgements-Thanks are due to Professor P. G. Daniele from the Universita di Torino for help and advice. The “Patronat de1 Col. legi Universitari de Girona” and the Spanish-Italian Co-operation Programme provided financial support for this joint work.
REFERENCES 1. I. M. Harrison and R. J. Hoare, Metals in Biochemistry. Chapman and Hall, London (1980). 2. V. Salvado, X. Ribas and M. Valiente, Znorg. Chim. Acta 1987, 137, 155. 3. F. F. Grigorenko and L. M. Sidler, Vest. Kiev Univ. 1969, 10,11. 4. F. P. Cavasino and E. Di Dio, J. Chem. Sot. A. 1971, 3176. 5. V. I. Gordienko, I. Yu. Mikhailyuk and V. I. Sidorenko, Zh. Obshch. Khim. 1971,41, 3, 507. 6. W. B. Schaap, H. A. Laitinien and J. C. Bailar, J. Am. Chem. Sot. 1954,76,5868. 7. M. Deneux, R. Meiller and R. L. Benoit, Can. J. Chem. 1968,46, 1383. 8. S. Ramamoorthy and P. G. Manning, J. Inorg. Nucl. Chem. 1973,35, 1571. 9. I. Dellien, Acta Chem. Stand. A 1977, 31, 473. 10. A. I. Vogel, Quantitative Inorganic Analysis. Longman, London (1963).
11. G. Charlot, Chimie Analytique Generule. Masson & Cie, Paris (1969). 12. Some Laboratory Methods. Mimeograph, Dept. of Inorganic Chemistry, The Royal Institute of Technology, Stockholm (1956). 13. L. G. Sill& and B. Warnquist, Ark. Kemi 1969, 31, 377. 14. A. Vacca and A. Sabatini, Proceedings of The Znternational School of Metal Complexes in Solution. , z Palermo, Italy 1983. X. Ribas, V. Salvado and M. Valiente, submitted. 1.7. 16. X. Ribas, Doctoral thesis. Universitat Autonoma de Barcelona (1987). 17. F. J. C. Rossotti and H. S. Rossotti, Acta Chem. Stand. 1955,9, 1166. 18. I. Grenthe and E. Hansson, Acta Chem. Stand. 1969, 23, 611. 19. S. Ito, H. Tomiyasu and H. Ohtari, Bull. Chem. Sot. Japan 1973,46,2238. 20. I. Dellien and I. Grenthe, Acta Chem. Stand. 1971, 25, 1387. 21. L. Ciavatta, Annali Chim. 1980,70, 551. 22. C. F. Baes and R. E. Mesner, The Hydrolysis of Cations, John Wiley, New York (1976). 23. N. Ingri, W. Kakolowicz, L. G. Sillin and B. Warnquist, Tulanta 1967, 14, 1261. 24. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry : A Comprehensive Text. John Wiley, New York (1980). 25. IUPAC, Stability Constants of Metal Zon Complexes (1973).
1989 0
0277-5387189 t3.00+.00 1989 Pergamon Press plc
T-HE CHEMISTRY OF IRON IN BIOSYSTEMS-III. COMPLEX FORMATION BETWEEN Fern AND MALONIC ACID IN AQUEOUS SOLUTIONS V. SALVADO
and X. RIBAS
Departament de Quimica, Col.legi Universitari de Girona (U.A.B.), Pea. Hospital 6, E 17071 Girona, Catalonia, Spain V. ZELANO
Dipartimento
and G. OSTACOLI
di Chimica Analitica, Universita di Torino, Via Bidone 36, I 10125 Torino, Italy and M. VALIENTE*
Departament
de Quimica, Quimica Analitica, Universitat Autonoma de Barcelona, E 08 193 Bellaterra, Catalonia, Spain (Received 16 August 1988 ; accepted 20 October 1988)
Abstract-Complex formation between Fe3+ ions and malonic acid (H,L), in acidic aqueous solutions containing 0.5 M NaNO, as inert ionic medium at 25”C, has been studied potentiometrically. The data were consistent with the following reactions and related stability constants :
Fe3+ +L2-
= FeL+
log p0,1,1= 7.52
Fe3++2L2-
= FeL;
log/&,,,, = 13.29
Fe3+ +3L2-
= FeLi-
log fi0,1,3= 16.93
which agree partially with previous models. No mixed complexes were detected, i.e. hydrolytic complex species. A suggestion of possible structures for the complex species formed is briefly discussed. In separate experiments and in the absence of Fe”‘, protolytic reactions of malonic acid were studied under the same experimental conditions. A numerical comparison of these results with those already published was carried out by means of the Specific Interaction Theory (SIT).
The biological activity of metal ions depends to a great extent on the interactions with those metal complexing agents present in the environment’ and the formation of strong soluble complexes between Fe”’ and ligands bearing carboxylic groups which are present in nature, is widely known. A systematic study including chemical characterization of these agents is currently being carried out in our laboratories. Results on the complexation of
*Author to whom correspondence should be addressed. 813
Fe”’ with tartrate have been published.’ We have also investigated iron(III)-malonate complexes in acidic solutions. The presence of malonic acid retards to lower pH the precipitation of ferric hydroxide in aqueous solutions. These differences with respect to tartaric acid excited our curiosity to ascertain the role of side hydroxyl groups in similar ligands. Iron(III)-malonate complexes have been studied by several authors using different techniques and thermodynamic conditionspg (see Table 1). Thus, a kinetic study carried out by Cavasino4 found
V. SALVADb
814
et al.
Table 1. Summary of literature data on Fe”*-malonate complexes in aqueous solutions Model
Ionic media
FeL, FeL, FeL FeL, HFeL FeL
0.5 (NaClO,) 1.O (NaClO,) 0.5 (LiClOJ
FeL, FeL, FeL, FeL, FeL,, FeL,
0.5 (NaClO,) 0.1 (NaClO,) 1.O (NaClO,)
FeL, FeL,, FeL,
0.5 (NaNO,)
References
Method
3 4 5 6
Spectrophotometric Kinetic Arnperometric Potentiometric and spectrophotometric Polarographic Potentiometric Potentiometric and calorimetric Potentiometric
7 8 9 This work
-
the species FeL. Gordienko,’ using amperometric measurements, postulates the species FeL and FeHL. In a polarographic study, Schaap et a1.6 suggested the species FeL,. Alternatively, Ramamoorthy and Manning,’ from potentiometric data, give a model with the species FeL and FeL, to exlain the behaviour of the iron(III)--malonate system. Finally Dellien,’ working in very acidic media and employing both potentiometric and calorimetric techniques, found the complex species FeL, FeL, and FeL,. All these studies were performed at 25°C and different ionic media were employed. A careful and systematic study of the chemical interactions between Fe3+ and malonic acid in aqueous solutions has been carried out in the present work. Potentiometric data obtained in a wide concentration range of the reacting species were analysed to quantitatively characterize the complex species and related stability constants.
EXPERIMENTAL Reagents, solutions and apparatus Malonic acid (Scharlau, AR) was used with no further purification, its purity was verified by potentiometric titration. Ferric nitrate (Merck, AR) was used as supplied to prepare stock solutions in 0.5 M NO;. The Fe”’ content was determined as described in ref. 10. The free acidity, h, in iron solutions was obtained following ref. 11. NaNO, (Probus, AR), purified as described in ref. 12, was used as the ionic media. Solutions of carbonate-free NaOH were prepared and determined as described of malonic acid, elsewhere. ’ 2 Stock solutions Fe(N0,)3, HN03 and NaOH were conditioned in NaN03 to achieve an ionic strength of 0.5 M. A digital voltimeter (Crison Digilab 517) of 0.1 mV precision interfaced to an Epson HX 20 computer was used to control the potentiometric experi-
ments. A glass electrode (Metrohm 1028) connected against a double junction reference electrode (Orion 9020) were employed to generate the EMF data.
Method of investigation Continuous EMF measurements have been performed in order to analyse the equilibrium state of the system. The hydrogen ion concentration, h, has been experimentally determined in a number of solutions prepared from stock solutions of Fe(N03)3, malonic acid, HN03 and NaN03. Measurements of EMF were performed by titrations using the galvanic cell I formulated by : RE//TS/GE,
(I)
where GE indicates a glass electrode and RE represents a double junction reference electrode. Stability criteria of the EMF measurements considered the potential to be stable when the variation was less than 0.1 mV within 10 min. For each experiment, the analytical concentrations of Fe”’ (B) and malonate (L) were varied in the range 2.0-6.0 mM and 5.0-20.0 mM, respectively. Values of pH varied from 1.5 to 5.5, where precipitation of ferric hydroxide was observed. The experimental data are collected in Fig. 1 in the form 2 vs -log h at the different B and L levels. Z represents the number of H+ bound to the total malonate and is defined by : Z = (H - h + Kw/h)/L
(1)
where H represents the analytical concentration of hydrogen ion. In order to verify our equilibrium data, experimental points obtained in back titrations are plotted in Fig. 1 by means of full symbols. The good agreement between the forward and back titration data shows that only reversible processes take part in the
Fe”’ and malonic acid complex formation [Fe3 1.959mM 2.939mM
LLI
815
components) requires a well established knowledge of the binary interactions, i.e. the protolysis of malonic acid and hydrolysis of Fe”‘. In our case, data for Fe”’ hydrolysis has been taken from a previous work, ’ 6 which was performed under the same experimental conditions. With respect to malonic acid, the following study was carried out.
8.918mM
10.255mM I.959 mM 6.699mM I.959 mM 4.459mM
Protolytic equilibria of malonic acid
2
-log h
Fig. 1. 2 plotted vs -log h for the different sets of experiments. Full points represent back titrations. Solid lines have been calculated by the model proposed in the present work. complexation reactions. All the experiments performed at 25 + 0.2”C.
were
B: = B?YHYLIYHL
RESULTS Analysis of the experimental information is based on the complex formation between ferric ions, Fe3+, and malonic acid, expressed by the general equation : pH +qFe+rL
= J--&Fe,&,; &g,,
Experiments in the absence of Fe”’ were carried out in order to quantitatively characterize malonic acid protolysis in the desired thermodynamic conditions. In this case, the EMF data have been treated both by the Rossotti’s graphical method” and numerically by the programs LETAGROP and SUPERQUAD. Table 2 gives the results for these treatments, which are found to be in good agreement. On the other hand and for the sake of comparison with other results reported in the literature,4*5~‘8-20 the different values of stability constants have been correlated using the Specific Interaction Theory, SIT.2’*22 Such a treatment of the data can be summarized by the expression :
where /?{ represents the protolytic constant at the ionic strength Z, /I: is the value of this constant at Z = 0 and yn, yL, yHL the activity coefficients of H, L and HL species. The activity coefficients follow the expression : log
yi
=
-
z?D + c &ikmk k
(2)
constants of where Bp,q,rrepresents the formation the complex species with different p, q and r values. Charges are omitted for simplicity. The stoichiometry of the complexes, and the by means constant &,4,r have been determined of a numerical analysis of the experimental data. For this purpose, the computer programs LETAGROPi3 and SUPERQUAD14 were used. In both cases, the treatment is based on the minimization of an error square sum U, defined by :
where EeXprepresents the experimental EMF values, Ecalcis a corresponding magnitude calculated by the program, assuming a determined model of species formed and related stability constants, and Np is the number of experimental points. As has been discussed previously,‘*’ ’ a systematic analysis of such ternary systems (three reacting
(4)
where D is the Debye-Huckel term, k represents the species with opposed charge to i in the ionic media, mk represents their molality and .sikare the specific interaction coefficients between the corresponding species present in the aqueous solution. Applying SIT and rearranging eq. (4) the following expressions can be written for each one of the protoTable 2. Results obtained for the behaviour of malonic acid in aqueous 0.5 M NaNO,, by different graphical and numerical methods. K,, = /lJ?; ‘, 8, = K,;’ where K,, and K,, are the corresponding acidic constants Method
log BI
log P2
log K2
Rossotti” LETAGROPb SUPERQUADb
5.05 5.06&0.008 5.05+0.004
7.62 7.62kO.016 7.61+0.010
2.57 2.56 2.56
BGraphical (ref. 17). bNumerical.
816
V. SALVADb
lytic constants (X = NO;) :
Table 3. Results obtained on the numerical treatment of the experimental data for the system Fe”‘-malonic acid
Y1 = log j?{ + 40 - EH,XmX = logby+ Y, = log/?‘,+6D-2eH,XmX
et al.
@L,M -EHL,M)~M
(6)
= log/I!j+EL,,mM.
Model
(P, 4, 4
u
Q (mv)
1
(0, 1, 1) (0, L2)
0.44x lo4
6.89
6.46f0.14 11.75+0.06
2
(0, 1, 1) (0, 1, 2) (1, 1, 1)
0.44x lo4
6.85
6.52f0.13 11.73f0.07 Rejected
3
(0, 1, 1) (0, 192) CO,1, 3) (-2,2,2)
0.43x 103
2.12
7.65f0.15 13.35f0.14 16.83kO.15 Rejected
0.77 x 10
0.29
7.52kO.01 13.29f0.01 16.93kO.01
(7)
In Fig. 2, Y, and Y, vs Z are represented for the different data considered. As seen, our results show a good fit in such a correlation.
Numerical treatment of the ternary system
With respect to the Fe”‘-malonate system, a numerical analysis of data has been carried out, starting from previous results3-9 to fit to our experimental data. The results of the calculation are collected in Table 3. As seen in this table, the best fit to the experimental data corresponds to the model where the species FeL, FeL, and FeL, are formed. This result is expressed by the following reaction equations, and the corresponding /Ip,q,rvalues are given : Fe3++L2-
= FeL+
log Bo,l,l = 7.52
(8)
Fe3++2L2-
= FeL;
log B0.1,~= 13.29
(9)
Fe3++3L2-
= FeI$
logp,,,,, = 16.93 (10)
Further analysis of the results, including polynuclear and mixed species (i.e. protolytic species) has shown that such species do not fit the experimental data. Figure 3 shows the individual values of (&,,CEexp) mV in the acidic range studied. As seen, the proposed model shows no systematic deviations with respect to the experimental data.
4”
(0, 1, 1) (0, 1,2) (0, 1, 3)
loisBP.%
“After correction of possible analytical errors.
DISCUSSION
The results obtained in the present work correspond to the formation of a complex between Fe”’ and malonic acid in aqueous solutions of 0.5 M NaNO, as the ionic medium. The experimental data have verified the equilibrium state of the system as well as that no irreversible or sideways process is taking part in the reactions studied. The complex species formed are represented by eqs (8)-(10). From the numerical treatment of the data, no polynuclear or protonated species were detected. This fact indicates a different behaviour of this system with respect to the Fe”‘-tartrate system studied previously* in which the formation of polynuclear complexes has been postulated. On the other hand, the stability of the Fe”‘-tartrate com-
[Fe1 IL1 e,O.O01959M mca9bM oO.O02939# O.OIO255M
I
Fig. 2. Correlation of the protolytic constants of malonic acid by means of the Specific Interaction Theory (SIT).
I 2
I 3
I 4
Fig. 3. (E,,,, - EeXP)mV vs -log h for the different experimental values analysed. Ecalc correspond to the EMF values calculated by the program on the basis of the model. _proposed _
817
Fe”’ and malonic acid complex formation
PH
Fig. 4. Fraction diagram of the Fe”‘-malonate species as a function of -log h.
complex
plexes is higher than the Fe”‘-malonate, thus precipitation of ferric hydroxide is observed at a pH > 5.5, while in the presence of tartaric acid this does not occur. These differences may be explained by the contribution of the hydroxyl groups of the tartaric acid to form more stable complexes. In Fig. 1 the solid lines represent calculated values of Z vs -log h using the model given by eqs (8k (10). The agreement with experimental values shows a high confidence level on this model. Figure 4 shows the distribution diagram of the species, calculated by the program HALTAFALL,23 in the acidic range studied. As seen, a step-wise formation of complexes is observed. The proposed model in the present work corresponds to species of stoichiometry 1 : 1, 1 : 2 and 1 : 3 respectively. For these stoichiometries, and taking into account that Fe”’ complexes usually have an octahedral co-ordination geometry,24 it is possible to speculate on probable structures of the proposed species. In Fig. 5, possible structures for the species FeL, FeL, and FeL, are suggested. These speculations are consistent with the absence of both polynuclear and protolytic complex species in the Fe”‘-malonate system. At any rate, further experimental information would be needed to properly determine such structures, i.e. direct spectroscopic measurements of the related solutions. Taking into account the information summarized in Table 1, a correlation between our results and those obtained by Dellieng is observed. It is found that the differences in the respective constant values are small in spite of the different media utilized (see Table 4). The work of Dellien shows, in the case of FeL, species, that the uncertainty in p3 is rather large [(4f 2) x 1016]. This may be a consequence of the experimental information, which has been obtained in a very acidic range (the acid content, h, ranges from 30 to 150 mM). In this sense, it is interesting to observe the concentrations of the
Fig. 5. Suggested structures for the Fe”‘--malonate soluble complexes assuming an octahedral co-ordination of Fe”‘.
complex species as a function of pH. Thus, in Fig. 4 it is observed that the presence of FeL, is less than 1% at pH -C 1, while this species is the most abundant at pH 3.5. It is difficult to see how under such experimental conditions one can differentiate between species of such low concentration. In our case the work was extended ‘up to pH < 5 where the different species, and especially FeL,, can be easily distinguished. Table 4. Differences between the values of stability constants for the Fe3+ -malonate equilibria obtained by Dellien’ and the respective values proposed in this work
Species
log B (ref. 9)
log B (this work)
FeL FeL, FeL,
7.50 13.04 16.60
7.52 13.29 16.93
V. SALVAD6 et al.
818
On the other hand, the slight differences observed between the constants derived in both investigations may explain that Fe3+-NO; interactions” are negligible under our experimental conditions. Acknowledgements-Thanks are due to Professor P. G. Daniele from the Universita di Torino for help and advice. The “Patronat de1 Col. legi Universitari de Girona” and the Spanish-Italian Co-operation Programme provided financial support for this joint work.
REFERENCES 1. I. M. Harrison and R. J. Hoare, Metals in Biochemistry. Chapman and Hall, London (1980). 2. V. Salvado, X. Ribas and M. Valiente, Znorg. Chim. Acta 1987, 137, 155. 3. F. F. Grigorenko and L. M. Sidler, Vest. Kiev Univ. 1969, 10,11. 4. F. P. Cavasino and E. Di Dio, J. Chem. Sot. A. 1971, 3176. 5. V. I. Gordienko, I. Yu. Mikhailyuk and V. I. Sidorenko, Zh. Obshch. Khim. 1971,41, 3, 507. 6. W. B. Schaap, H. A. Laitinien and J. C. Bailar, J. Am. Chem. Sot. 1954,76,5868. 7. M. Deneux, R. Meiller and R. L. Benoit, Can. J. Chem. 1968,46, 1383. 8. S. Ramamoorthy and P. G. Manning, J. Inorg. Nucl. Chem. 1973,35, 1571. 9. I. Dellien, Acta Chem. Stand. A 1977, 31, 473. 10. A. I. Vogel, Quantitative Inorganic Analysis. Longman, London (1963).
11. G. Charlot, Chimie Analytique Generule. Masson & Cie, Paris (1969). 12. Some Laboratory Methods. Mimeograph, Dept. of Inorganic Chemistry, The Royal Institute of Technology, Stockholm (1956). 13. L. G. Sill& and B. Warnquist, Ark. Kemi 1969, 31, 377. 14. A. Vacca and A. Sabatini, Proceedings of The Znternational School of Metal Complexes in Solution. , z Palermo, Italy 1983. X. Ribas, V. Salvado and M. Valiente, submitted. 1.7. 16. X. Ribas, Doctoral thesis. Universitat Autonoma de Barcelona (1987). 17. F. J. C. Rossotti and H. S. Rossotti, Acta Chem. Stand. 1955,9, 1166. 18. I. Grenthe and E. Hansson, Acta Chem. Stand. 1969, 23, 611. 19. S. Ito, H. Tomiyasu and H. Ohtari, Bull. Chem. Sot. Japan 1973,46,2238. 20. I. Dellien and I. Grenthe, Acta Chem. Stand. 1971, 25, 1387. 21. L. Ciavatta, Annali Chim. 1980,70, 551. 22. C. F. Baes and R. E. Mesner, The Hydrolysis of Cations, John Wiley, New York (1976). 23. N. Ingri, W. Kakolowicz, L. G. Sillin and B. Warnquist, Tulanta 1967, 14, 1261. 24. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry : A Comprehensive Text. John Wiley, New York (1980). 25. IUPAC, Stability Constants of Metal Zon Complexes (1973).