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Studies of FeIII behaviour in the presence of bis(2-hydroxyethyl)dithiocarbamate
Po(vhedron Vol. 15. No. 5 6, pp. 839 843, 1996
~)
Pergamon 0277-5387(95)00330-4
Copyright ~', 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0277 5387,'96 $15.00+0.00
STUDIES OF Fe m BEHAVIOUR IN THE PRESENCE OF BIS(2-HYDROXYETHYL)DITHIOCARBAMATE S. T. BREVIGLIERI, E. T. G. CAVALHEIRO* and G. O. CHIERICEt Instituto de Quimica de Silo Carlos, USP, CP 780-CEP, 13560-970-S~o Carlos, SP, Brasil
(Received 17 November 1994; accepted 12 July 1995) Abstract--To explain the abnormal stoichiometry found in Fem/bis(2-hydroxyethyl)dithiocarbamate potentiometric titrations, i.e. 2 : 1 instead of 3 : 1, conductometric studies, determination of the conditional standard potential for the bis(2-hydroxyethyl)dithiocarbamate oxidation, stability constants and decomposition kinetics studies have been made. A mechanism is proposed to elucidate the phenomenon.
The dithiocarbamates have been studied since the beginning of this century and these studies had a great impulse after their utilization as reagents for heavy metals. L2 The complexing properties of dithiocarbamates are related to the presence of two-electron donor sulphur atoms, which determine the nature of binding to metals and the stability of the resulting complex. The complexing capacity with many metal ions 3 5 resulted in dithiocarbamates being applicable in several areas such as industry, agriculture, medicine and in analytical chemistry. In this work, we wish initially to establish the conditions for potentiometric titrations of Fe IHwith ammonium bis(2-hidroxyethyl)dithiocarbamate (DEDC) in aqueous medium. It has been noted that the equivalence point does not occur at the 1 : 3 metal:ligand ratio, but was found at 1 : 2. A similar phenomenon has been observed by Babko and Tselinskii, 6 in Bi m amperometric titrations with diethyldithiocarbamate (DDC), in aqueous/organic medium. These authors attributed the phenomenon to the precipitation of Bi(OH) (DDC)2. It is important to note that these authors were working in a completely different situation in relation to the present work, where precipitation
* Present address: Depto de Quimica, UFSCar, CP 676, CEP 13560-970, S~o Carlos, SP, Brasil. t Author to whom correspondence should be addressed.
was not verified (in the concentration range studied) and an aqueous medium was employed. Changes in the spin state for tris(dimethyldithiocarbamate), tris(pyrrolidyldithiocarbamate), tris(di-n-butyldithiocarbamate) and tris(di-isobutyldithiocarbamate) have been discussed by Ewald et al., 7 who related the strength of the ligand field with the spin state, using results from magnetic moment measurements as a function of temperature. Ligand field theory predicts that truly octahedral complexes, ML6, could exist in which both true types are in thermal equilibrium at ordinary temperatures. For the Fe(DEDC)3 complexes these studies were made by Pandeya and Singh, 8 who used the EPR technique to measure the magnetic moment between 15.3 and 298 K, concluding that at low temperatures (from 21.4 K), the low-spin system predominates. At 298 K the equilibrium of 88% high-spin to 12% low-spin forms is established. The N M R measurements have been applied to study anomalous paramagnetism in some FeH~/ DTC systems, by Hughes and Lawson, 9 using Evans' method. In all these studies the third complex species was the subject of the investigation in its solid form. Several studies have now been carried out including conditional standard potential determination for the DEDC, conductometric titrations, kinetic measurements and stability constant determination for the Fem/DEDC system. Finally a redox/complexing mixed mechanism is proposed to elucidate the observed phenomena. 839
840
S. T. BREVIGLIERI et al. EXPERIMENTAL
Reayents and solutions All the reagents were of analytical grade and the solutions were prepared with distilled and deionized water. The N H 4 D E D C was synthesized according to ref. 10. The compound presents the following structural formula :
HO--C--C
/ HO--C--C
$
\
//
N--C
\ 8 "
NH
4-
4
thermostated cell, with temperature controlled at (25.0_+0.1)~'C. The ligand solution concentration was 10 times more concentrated than the metallic solution ([Fe 3+] = 1.0 × 10 -3 M and [NH4DEDC] = 1.0 x 10 -2 M), to minimize the dilution effect. The decomposition measurements were done measuring the absorbance at 512 nm as a function of time. A 1.0x 10 -3 M solution of Fe(DEDC)3 salt, pH = 3.9, was used. The stability constants were measured using potentiometric titration as described above, with [Fe 3+]= 1.025x10 5 M a n d [ N H 4 D E D C ] = 1.0x 10 -4 M. The Fe'/dipyridine complex was 5.0x 10 -6 M. The ionic strength was adjusted at 1.00 M, with NaC104, and the temperature was kept constant at (25.0 + 0.1)°C.
The Fe(DEDC)3 complex was obtained by reaction of N H 4 D E D C and FeC13" 6H20 solutions. The resulting precipitate was recrystallized from ethanol/water, and characterized by I R and microanalysis.
Considering the potentiometric titration results some experiments have been developed to explain the phenomena, which are described below.
Apparatus
Potentiometric titrations
A Corning 130 p H meter, equipped with a platinum wire and home-made Ag/AgC1 reference electrodes, was used in the potentiometric measurements. The kinetic measurements were carried out using an HP 8451A spectrophotometer, associated with an HP 9121 floppy diskette unit and an HP7470A plotter. A thermostatic bath was applied and 1.0 cm Beckmann quartz cells were used. In the conductometric experiments a Micronal B331 conductivimeter has been used, with a conductometric cell Metrohm C H 9100.
A typical titration curve for the system is given in Fig. 1. F r o m the beginning of the titration to the equivalence point, the solution showed a reddish-brown colour, which disappeared completely at that point. This fact suggests the possibility of using this phenomenon as a visual indicator for the end point detection.
R E S U L T S AND D I S C U S S I O N
600
Procedure 5OO
The potentiometric titrations were made in a 25 cm 3 thermostated cell. The ionic strength was controlled at 1.0 M with NaC104 and the temperature was kept constant at (25.0_+0.1)°C. The concentrations used were 1.0x 10 3 M for Fe Iu, 1.0x 10 -2 M for N H 4 D E D C and varied from 1.0 x 10 -4 to 1.0 x 10 -3 M for the Fe"/2,2'-dipyridine complex. The ligand solutions were prepared just before use, to prevent ligand decomposition. The titrations were made in the presence of the Fe"/2,2'-dipyridine complex. This red complex is thermodynamically stable and does not liberate free Fe". The objective of its use is to obtain the Fern/ Fe" redox pair which gives the platinum electrode potentiometric response. The conductometric titrations were made in a
'~
40O
300
200
lOG
I
I
I
I
I
2,0
2,9
3,0
4,0
5,0
VOLUME
(m~)
Fig. 1. Typical titration curve for the Fe'~/DEDC system : [Fe 3+] = 1.0 × 10 -3 M; [Fe 2+] = 5.0 × 10-4 M; [NH4DEDC] = 1.0 × 10 2 M.
841
Studies of Fe ~Hbehaviour In the presence of the FdI/2,2'-dipyridine complex, the electrode showed a quick and stable response to each ligand addition. A potential jump of almost 400 mV (in the present conditions) is noted in a 2L : 1Fe m stoichiometry. Other titrations were carried out, varying the Fe"/2,2'-dipyridine complex without influencing the final result. The fact which calls for comment is the stoichiometric proportion being different from the 3 : 1 expected for a complexometric system and the complete colour disappearance at the equivalence point which suggests the complex decomposition. The qualitative tests with o-phenanthroline on the resulting solution showed the presence of free Fe", yet the Fe"/2,2'-dipyridine complex was not used in the initial procedure. To elucidate this fact that the conditional standard potential for DEDC/DEDC-thiuramdissulphide has been determined. Conductometric titrations, Fe(DEDC)3 salt decomposition kinetics, and stability constants for the Fe"I/DEDC system determination have been obtained and the results are described below.
corresponding to Fe "~, FeL 2+, FeE2 ~, FeL3 and ligand excess. This evidences that the process is not a pure complexation case, indicating Fe n at the titration end ; the curve has been interpreted as follows. The first step should be the Fe "~ complex formation in 2 : 1 (nL : riM), as verified by the solution discolouring. The second step is proposed to be the redox process beginning, with conductance diminution, caused by Fem ---,Fe u and D E D C --* DEDC-thiuramdissulphide redox processes. The solution again assumes the light yellow colour in the ratio 3 : 1 and the colour intensity is increased to 5:1. The colour reappearance is evidence for FeU/DEDC complex formation, with the ligand excess.
In step III, the presence of excess free ligand increases the conductance. In this way the conductance studies suggest the occurrence of a complexation process followed by a redox process, evidenced by a diminution of the solution's conductance.
Decomposition kinetics Conditional standard potential, E°', Jor the ligand In this case iodimetric titration was used, because the reagent is widely applied in dithiocarbamate studies, since it shows an appropriate potential t~ t3 with small pH dependence. The semi-reactions involved on the process are :
¢o-~-~ 2
t.o-c-o'
,~
'~'I--C"I
',]
12+2e
~__o_~ '~n~ ¢
+ 2e ~ HO--C
.
--C ,
2 % ~_~. ,o_,
¢--n
. C --C
~H
(1)
In this case a colour decay curve of pseudo-firstorder for the Fe(DEDC)3 complex was obtained. The graph of In ( A ~ - A t ) against time showed a straight line, represented in Fig. 3. The angular coefficient gave the apparent rate constant K,,b~ = 0.13 min-~, with t,..2 = In 2 / K o b s = 5.5 rain.
5. Stability constant determination Finally, the stability constant has been determined by the potentiometric method for the Fern/
"2I ,E °'=0.536V
(2)
The final obtained potential is E °' = 0.144V. This result confirms that Fe l" has sufficient potential to promote the ligand oxidation, since E°'ve"IFc" = 0.771 V. This fact evidences the fact that a redox process between Fe l" and the ligand could occur and that the system has not necessarily a single complexation behaviour.
3,0
2,5
,k 8 z,o lzr °.xi
Conductometric titrations
J
1'50
The system's conductometric titration curve has the general form shown in Fig. 2. In Fig. 2 it is worth noting that there are three well defined steps. A pure complexing system must show five steps,
i
1
i
i
i
r
.
i
i
3
4
5
n(DEDC)
~riFe5+
2
i
°
~ i
6
Fig. 2. Conductometric titration curve for the Fern/ DEDC system: [Fe 3+]= 1.0x!0 3 M;[NHaDEDC]= 1.0x 10 -2 M; V, = 20.0 cm3 ; L..... = L[(Vo+v)/Vo].
842
S. T. BREVIGLIERI et 5,0
al. lO0
%o 801
8
L" 6"s
~_
40
/
g,o
40-
\./
Lo
.o 0,0
I
i
lO
I
20
,"
I
;7 ,&2 \,"\.
27"
TEMPO (min )
Fig. 3. Velocity curve for the Fe(DEDC)3 complex.
o
-7.5
i
-6.5 i
i
-5.5 i
r
-4.5
LOGIC] Fig. 4. Distribution diagram for the Fem/DEDC system. D E D C system, applying Leden's method. TM The data obtained were refined by the Matrix Method. 15 The proposed values are shown in Table 1. The deviation of 12% can be justified by the fact that the system has not a single complexation process, but it has a redox step too. It is important to note that the first and the third species shows the largest stability constant values, while the second species has the minor value, being the weakest one. Figure 4 shows the distribution diagram for this system. Based upon all these experiments it is supposed that in the Fem/DEDC titration, initially complexation occurs. When the second species predominates in solution, which has the weakest stability constant, it starts a redox process in which Fe l" is reduced to Fe II, while the ligand is oxidized to thiuramdisulphide, discolouring the solution. This occurs at 2 : 1 stoichiometry. The conductometric studies showed that after total Fem and ligand consumption, the addition of excess ligand could form complexes with Fe n, thus returning to the original solution colour. The following steps describe what should occur with the system : Fe 3+ + L
,
" [FeL] 2+
(3)
[FeL] =+L- .
" [FeL2] +
(4)
Table 1. Overall (/}i) and stepwise (Ki) stability constants for the Fem/DEDC system Species [Fe(DEDC)] 2+ [Fe(DEDC)2] + [Fe(DEDC)3]
//,.
Ki
5.07 x 105 5.07x 105 3.20x 10~° 6.31 x 104 2.08 x 10~6 6.50x 105
2[FeL2] + + L -
fast
,2Fe 2+ + (R2NCSS)2 + L -
(5) [FeL2] + + L
slow
, [FeL3].
(6)
The redox reaction should occur, supposing only the potentials of the semi-reactions (1) and (2). However, the first complex species having a relatively high stability constant (fl~ = 5.07×105) retains the ligand in a complex form. In a similar way the third species makes it possible to isolate the salt Fe(DEDC)3 in the solid state. However, when a solution is prepared with the salt, it discolours slowly, showing that the above equilibria are established and the reagents are consumed by the redox process. The phenomenon occurs when the second species is in greater concentration, because it has the weakest stability constant (10 times lower than the other two species) and generates the largest free ligand concentration and supports the process represented by eq. (3). It is possible that the phenomenon observed by Ewald e t al. 7 and Pandeya and Singh 8 described in the introduction, influence the behaviour of the Feln/DEDC system during the titration and in the proposed mechanism. We have tried to obtain N M R and EPR measurements with the system, varying the ligand and fixing the Fe nl concentration, with the objective of studying the behaviour of FeL, FeLx and FeE3 species in solution under liquid nitrogen temperature. These experiments do not lead us to a significant conclusion, because the position of solvent signals in N M R and the EPR spectral shape do not show important variations. Perhaps the use of temperature approximately that of liquid helium should give more conclusive results.
Studies of Fe HI behaviour
REFERENCES 1. 2. 3. 4.
H. Debus, Anal. Chem. Liebgs 1850, 73, 26. M. Del6phine, Bull. Soc. Chem. Fr. 1908, 3, 643. D. Coucouvanis, Prog. lnor9. Chem. 1970, 11,233. R. J. Magee and J. O. Hill, Rev. Anal. Chem. 1985, 8,5. 5. G. D. Thorn and R. A. Ludwig, The Dithiocarbamates and Related Compounds. Elsevier, New York (1962). 6. A . K . Babko and K. Tselinskii, Zh. Anal. Khim. 1968, 23, 547. 7. A. H. Ewald, R. L. Martin, I. G. Ross and A. H. White, Proc. R. Soc. 1964, 208A, 235.
843
8. K.B. Pandeya and R. Singh, lnor9. Chim. Aeta 1988, 147, 5. 9. J. G. Hughes and P. J. Lawson, J. Chem. Educ. 1987, 64, 973. 10. W. Hass and T. Schwarz, Mikrochim. Acta 1963, 254. 11. I. M. Kolthoff and R. Belcher, Volumetric Analysis, Vol. II, p. 169. Interscience, New York (1969). 12. B. C. Verma and S. Kumar, Mikrochim. Acta 1976, 21,209. 13. B. C. Verma and S. Kumar, Mikrochim. Acta 1976, 21,237. 14. I. Z. Leden, Phys. Chem. 1941, 188, 160. 15. N. C. Milcken, Thesis, Univ. Silo Paulo, Sho Paulo (1977).
~)
Pergamon 0277-5387(95)00330-4
Copyright ~', 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0277 5387,'96 $15.00+0.00
STUDIES OF Fe m BEHAVIOUR IN THE PRESENCE OF BIS(2-HYDROXYETHYL)DITHIOCARBAMATE S. T. BREVIGLIERI, E. T. G. CAVALHEIRO* and G. O. CHIERICEt Instituto de Quimica de Silo Carlos, USP, CP 780-CEP, 13560-970-S~o Carlos, SP, Brasil
(Received 17 November 1994; accepted 12 July 1995) Abstract--To explain the abnormal stoichiometry found in Fem/bis(2-hydroxyethyl)dithiocarbamate potentiometric titrations, i.e. 2 : 1 instead of 3 : 1, conductometric studies, determination of the conditional standard potential for the bis(2-hydroxyethyl)dithiocarbamate oxidation, stability constants and decomposition kinetics studies have been made. A mechanism is proposed to elucidate the phenomenon.
The dithiocarbamates have been studied since the beginning of this century and these studies had a great impulse after their utilization as reagents for heavy metals. L2 The complexing properties of dithiocarbamates are related to the presence of two-electron donor sulphur atoms, which determine the nature of binding to metals and the stability of the resulting complex. The complexing capacity with many metal ions 3 5 resulted in dithiocarbamates being applicable in several areas such as industry, agriculture, medicine and in analytical chemistry. In this work, we wish initially to establish the conditions for potentiometric titrations of Fe IHwith ammonium bis(2-hidroxyethyl)dithiocarbamate (DEDC) in aqueous medium. It has been noted that the equivalence point does not occur at the 1 : 3 metal:ligand ratio, but was found at 1 : 2. A similar phenomenon has been observed by Babko and Tselinskii, 6 in Bi m amperometric titrations with diethyldithiocarbamate (DDC), in aqueous/organic medium. These authors attributed the phenomenon to the precipitation of Bi(OH) (DDC)2. It is important to note that these authors were working in a completely different situation in relation to the present work, where precipitation
* Present address: Depto de Quimica, UFSCar, CP 676, CEP 13560-970, S~o Carlos, SP, Brasil. t Author to whom correspondence should be addressed.
was not verified (in the concentration range studied) and an aqueous medium was employed. Changes in the spin state for tris(dimethyldithiocarbamate), tris(pyrrolidyldithiocarbamate), tris(di-n-butyldithiocarbamate) and tris(di-isobutyldithiocarbamate) have been discussed by Ewald et al., 7 who related the strength of the ligand field with the spin state, using results from magnetic moment measurements as a function of temperature. Ligand field theory predicts that truly octahedral complexes, ML6, could exist in which both true types are in thermal equilibrium at ordinary temperatures. For the Fe(DEDC)3 complexes these studies were made by Pandeya and Singh, 8 who used the EPR technique to measure the magnetic moment between 15.3 and 298 K, concluding that at low temperatures (from 21.4 K), the low-spin system predominates. At 298 K the equilibrium of 88% high-spin to 12% low-spin forms is established. The N M R measurements have been applied to study anomalous paramagnetism in some FeH~/ DTC systems, by Hughes and Lawson, 9 using Evans' method. In all these studies the third complex species was the subject of the investigation in its solid form. Several studies have now been carried out including conditional standard potential determination for the DEDC, conductometric titrations, kinetic measurements and stability constant determination for the Fem/DEDC system. Finally a redox/complexing mixed mechanism is proposed to elucidate the observed phenomena. 839
840
S. T. BREVIGLIERI et al. EXPERIMENTAL
Reayents and solutions All the reagents were of analytical grade and the solutions were prepared with distilled and deionized water. The N H 4 D E D C was synthesized according to ref. 10. The compound presents the following structural formula :
HO--C--C
/ HO--C--C
$
\
//
N--C
\ 8 "
NH
4-
4
thermostated cell, with temperature controlled at (25.0_+0.1)~'C. The ligand solution concentration was 10 times more concentrated than the metallic solution ([Fe 3+] = 1.0 × 10 -3 M and [NH4DEDC] = 1.0 x 10 -2 M), to minimize the dilution effect. The decomposition measurements were done measuring the absorbance at 512 nm as a function of time. A 1.0x 10 -3 M solution of Fe(DEDC)3 salt, pH = 3.9, was used. The stability constants were measured using potentiometric titration as described above, with [Fe 3+]= 1.025x10 5 M a n d [ N H 4 D E D C ] = 1.0x 10 -4 M. The Fe'/dipyridine complex was 5.0x 10 -6 M. The ionic strength was adjusted at 1.00 M, with NaC104, and the temperature was kept constant at (25.0 + 0.1)°C.
The Fe(DEDC)3 complex was obtained by reaction of N H 4 D E D C and FeC13" 6H20 solutions. The resulting precipitate was recrystallized from ethanol/water, and characterized by I R and microanalysis.
Considering the potentiometric titration results some experiments have been developed to explain the phenomena, which are described below.
Apparatus
Potentiometric titrations
A Corning 130 p H meter, equipped with a platinum wire and home-made Ag/AgC1 reference electrodes, was used in the potentiometric measurements. The kinetic measurements were carried out using an HP 8451A spectrophotometer, associated with an HP 9121 floppy diskette unit and an HP7470A plotter. A thermostatic bath was applied and 1.0 cm Beckmann quartz cells were used. In the conductometric experiments a Micronal B331 conductivimeter has been used, with a conductometric cell Metrohm C H 9100.
A typical titration curve for the system is given in Fig. 1. F r o m the beginning of the titration to the equivalence point, the solution showed a reddish-brown colour, which disappeared completely at that point. This fact suggests the possibility of using this phenomenon as a visual indicator for the end point detection.
R E S U L T S AND D I S C U S S I O N
600
Procedure 5OO
The potentiometric titrations were made in a 25 cm 3 thermostated cell. The ionic strength was controlled at 1.0 M with NaC104 and the temperature was kept constant at (25.0_+0.1)°C. The concentrations used were 1.0x 10 3 M for Fe Iu, 1.0x 10 -2 M for N H 4 D E D C and varied from 1.0 x 10 -4 to 1.0 x 10 -3 M for the Fe"/2,2'-dipyridine complex. The ligand solutions were prepared just before use, to prevent ligand decomposition. The titrations were made in the presence of the Fe"/2,2'-dipyridine complex. This red complex is thermodynamically stable and does not liberate free Fe". The objective of its use is to obtain the Fern/ Fe" redox pair which gives the platinum electrode potentiometric response. The conductometric titrations were made in a
'~
40O
300
200
lOG
I
I
I
I
I
2,0
2,9
3,0
4,0
5,0
VOLUME
(m~)
Fig. 1. Typical titration curve for the Fe'~/DEDC system : [Fe 3+] = 1.0 × 10 -3 M; [Fe 2+] = 5.0 × 10-4 M; [NH4DEDC] = 1.0 × 10 2 M.
841
Studies of Fe ~Hbehaviour In the presence of the FdI/2,2'-dipyridine complex, the electrode showed a quick and stable response to each ligand addition. A potential jump of almost 400 mV (in the present conditions) is noted in a 2L : 1Fe m stoichiometry. Other titrations were carried out, varying the Fe"/2,2'-dipyridine complex without influencing the final result. The fact which calls for comment is the stoichiometric proportion being different from the 3 : 1 expected for a complexometric system and the complete colour disappearance at the equivalence point which suggests the complex decomposition. The qualitative tests with o-phenanthroline on the resulting solution showed the presence of free Fe", yet the Fe"/2,2'-dipyridine complex was not used in the initial procedure. To elucidate this fact that the conditional standard potential for DEDC/DEDC-thiuramdissulphide has been determined. Conductometric titrations, Fe(DEDC)3 salt decomposition kinetics, and stability constants for the Fe"I/DEDC system determination have been obtained and the results are described below.
corresponding to Fe "~, FeL 2+, FeE2 ~, FeL3 and ligand excess. This evidences that the process is not a pure complexation case, indicating Fe n at the titration end ; the curve has been interpreted as follows. The first step should be the Fe "~ complex formation in 2 : 1 (nL : riM), as verified by the solution discolouring. The second step is proposed to be the redox process beginning, with conductance diminution, caused by Fem ---,Fe u and D E D C --* DEDC-thiuramdissulphide redox processes. The solution again assumes the light yellow colour in the ratio 3 : 1 and the colour intensity is increased to 5:1. The colour reappearance is evidence for FeU/DEDC complex formation, with the ligand excess.
In step III, the presence of excess free ligand increases the conductance. In this way the conductance studies suggest the occurrence of a complexation process followed by a redox process, evidenced by a diminution of the solution's conductance.
Decomposition kinetics Conditional standard potential, E°', Jor the ligand In this case iodimetric titration was used, because the reagent is widely applied in dithiocarbamate studies, since it shows an appropriate potential t~ t3 with small pH dependence. The semi-reactions involved on the process are :
¢o-~-~ 2
t.o-c-o'
,~
'~'I--C"I
',]
12+2e
~__o_~ '~n~ ¢
+ 2e ~ HO--C
.
--C ,
2 % ~_~. ,o_,
¢--n
. C --C
~H
(1)
In this case a colour decay curve of pseudo-firstorder for the Fe(DEDC)3 complex was obtained. The graph of In ( A ~ - A t ) against time showed a straight line, represented in Fig. 3. The angular coefficient gave the apparent rate constant K,,b~ = 0.13 min-~, with t,..2 = In 2 / K o b s = 5.5 rain.
5. Stability constant determination Finally, the stability constant has been determined by the potentiometric method for the Fern/
"2I ,E °'=0.536V
(2)
The final obtained potential is E °' = 0.144V. This result confirms that Fe l" has sufficient potential to promote the ligand oxidation, since E°'ve"IFc" = 0.771 V. This fact evidences the fact that a redox process between Fe l" and the ligand could occur and that the system has not necessarily a single complexation behaviour.
3,0
2,5
,k 8 z,o lzr °.xi
Conductometric titrations
J
1'50
The system's conductometric titration curve has the general form shown in Fig. 2. In Fig. 2 it is worth noting that there are three well defined steps. A pure complexing system must show five steps,
i
1
i
i
i
r
.
i
i
3
4
5
n(DEDC)
~riFe5+
2
i
°
~ i
6
Fig. 2. Conductometric titration curve for the Fern/ DEDC system: [Fe 3+]= 1.0x!0 3 M;[NHaDEDC]= 1.0x 10 -2 M; V, = 20.0 cm3 ; L..... = L[(Vo+v)/Vo].
842
S. T. BREVIGLIERI et 5,0
al. lO0
%o 801
8
L" 6"s
~_
40
/
g,o
40-
\./
Lo
.o 0,0
I
i
lO
I
20
,"
I
;7 ,&2 \,"\.
27"
TEMPO (min )
Fig. 3. Velocity curve for the Fe(DEDC)3 complex.
o
-7.5
i
-6.5 i
i
-5.5 i
r
-4.5
LOGIC] Fig. 4. Distribution diagram for the Fem/DEDC system. D E D C system, applying Leden's method. TM The data obtained were refined by the Matrix Method. 15 The proposed values are shown in Table 1. The deviation of 12% can be justified by the fact that the system has not a single complexation process, but it has a redox step too. It is important to note that the first and the third species shows the largest stability constant values, while the second species has the minor value, being the weakest one. Figure 4 shows the distribution diagram for this system. Based upon all these experiments it is supposed that in the Fem/DEDC titration, initially complexation occurs. When the second species predominates in solution, which has the weakest stability constant, it starts a redox process in which Fe l" is reduced to Fe II, while the ligand is oxidized to thiuramdisulphide, discolouring the solution. This occurs at 2 : 1 stoichiometry. The conductometric studies showed that after total Fem and ligand consumption, the addition of excess ligand could form complexes with Fe n, thus returning to the original solution colour. The following steps describe what should occur with the system : Fe 3+ + L
,
" [FeL] 2+
(3)
[FeL] =+L- .
" [FeL2] +
(4)
Table 1. Overall (/}i) and stepwise (Ki) stability constants for the Fem/DEDC system Species [Fe(DEDC)] 2+ [Fe(DEDC)2] + [Fe(DEDC)3]
//,.
Ki
5.07 x 105 5.07x 105 3.20x 10~° 6.31 x 104 2.08 x 10~6 6.50x 105
2[FeL2] + + L -
fast
,2Fe 2+ + (R2NCSS)2 + L -
(5) [FeL2] + + L
slow
, [FeL3].
(6)
The redox reaction should occur, supposing only the potentials of the semi-reactions (1) and (2). However, the first complex species having a relatively high stability constant (fl~ = 5.07×105) retains the ligand in a complex form. In a similar way the third species makes it possible to isolate the salt Fe(DEDC)3 in the solid state. However, when a solution is prepared with the salt, it discolours slowly, showing that the above equilibria are established and the reagents are consumed by the redox process. The phenomenon occurs when the second species is in greater concentration, because it has the weakest stability constant (10 times lower than the other two species) and generates the largest free ligand concentration and supports the process represented by eq. (3). It is possible that the phenomenon observed by Ewald e t al. 7 and Pandeya and Singh 8 described in the introduction, influence the behaviour of the Feln/DEDC system during the titration and in the proposed mechanism. We have tried to obtain N M R and EPR measurements with the system, varying the ligand and fixing the Fe nl concentration, with the objective of studying the behaviour of FeL, FeLx and FeE3 species in solution under liquid nitrogen temperature. These experiments do not lead us to a significant conclusion, because the position of solvent signals in N M R and the EPR spectral shape do not show important variations. Perhaps the use of temperature approximately that of liquid helium should give more conclusive results.
Studies of Fe HI behaviour
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