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Spectrophotometric investigations of binary and ternary complexes of Cd(II), methylthiourea and halide ions in aqueous solution
Pdyhe&m Vol. 5, No. 8, pp. 1307-1310, 1986 Printed in Great Britain
0277-5387186 S3.00+.00 Pcrgamon Journals Ltd
SPECTROPHOTOMETRIC INVESTIGATIONS OF BINARY AND TERNARY COMPLEXES OF Cd(n), METHYLTHIOUREA AND HALIDE IONS IN AQUEOUS SOLUTION JOANNA lUASkOWSKA* and EL;tBIETA
CHRUSCINSKA
Institute of General Food Chemistry, Technical University of Eodi, 90-924 Lodi, Stefanowskiego 4/10, Poland (Received 20 September 1985 ; accepted 6 November 1985) Abstrict-The results of spectrophotometric investigations of mixed-complex formation equilibria of Cd(II), methylthiourea (MeTU) and halide ions (Cl-, Br- and I-) in aqueous solution have been reported. The formation of binary and ternary complexes has been found. The values of the equilibrium constants (log units) for the reaction are 1.03+0.10, 1.17kO.12 and 1.34kO.08 for [CdMeTUCl]+, MA2 + MB2 ‘2MAB, 7 [CdMeTUBr]+ and [CdMeTUI]+, respectively. It was found that the stability of ternary complexes depends on the values of the normal redox potential of halide ions.
The spectrophotometric method was used to determine the equilibrium constants of the mixed complexes of mercuric halides’ and the mixed complexes of Ct.@) with pyrophosphate and ethylenediamine,2 ethylenediamine and iminodiacetate3 and ethylenediamine and oxalate.4 Because of lack of data for the mixed complexes of Cd(I1) with methylthiourea (MeTU) and halide ions, we undertook the studies in this field; In our previous papers’s6 we proved the formation of the mixed complexes of Cd(I1) with derivatives of thiourea and halide ions using electrochemical methods: potentiometry and polarography. Thus it seemed to be appropriate to use the spectrophotometric method to study the equilibrium reaction of the formation of mixed complexes in similar systems, in order to find new experimental data and extend the information about these little known complexes. EXPERIMENTAL
was prepared by treating cadmium oxide with perchloric acid. Cd(C104)2 solution was standardized by a complexometric method.7 Sodium chloride, bromide and iodide were puri8ed by crystallization from aqueous solution. NaCl, NaBr and NaI solutions were standardized by argentometric methods. All stock solutions were acidified with HC104 to pH = 2.0 to prevent any hydrolysis of the complexes. The concentrations used were selected according to Spiro and Hume’s data.’ The concentration of Cd(H) was kept constant, 5 x 10e5 M ; total concentration of NaCl (NaBr, NaI) and that of MeTU was about 0.01 M. The ionic strength of solutions was p = 0.02. Apparatus The measurements were recorded on a Specord UV-Vis (Carl Zeiss, Jena) spectrophotometer in the range of 12= 200-300 mn with l-cm quartz cells.
Reagents
Measurement method and calculation procedure
Triply distilled water was used. N-methylthiourea was recrystallized from water-ethanol mixtures and dried in vacua over P4010. Cadmium perchlorate
Spiro and Hume’ described a spectrophotometric method for the determination of the equilibrium constant, K, for the reaction of the mixedcomplex formation [(l)] : MA2+MB2=2MAB.
* Author to whom correspondence should be addressed.
(1)
The equilibrium constant K for the reaction (1) is 1307
and E. CHRUSCIfiSKA
J. MASLOWSKA
1308
(7) and (8) can be obtained for these conditions :
given by eqn (2) :
(for low R) jY= (1-a11)2-(1-~R/1+R)2’
(2) A-S~OCMA,
hence :
(7)
=(2~r-~20)cMBZ>
(for high R)
1 ail = 1-4K-’
A-&OZ~MLQ=
( l* J l-
4R(l-4K-1) (1+@2
(2'511-'502hA,*
(8)
, >
The plots of the left-hand side of eqns (7) and (8) cMB, or cMA*, respectively, should be straight lines passing through the origin. From their slopes sll can be calculated. As R approaches unity, the solutions change from two- to three-component systems, and the above plots then deviate from linearity (provided AE ><0). The statement of this deviation is very important because it is the evidence for mixed-complex formation. If no deviation occurs, the solutions always contain two components, and the plots are linear over the entire concentration range. In the present paper, absorptivities of the mixed complexes were calculated from linear plots of A-E~~c~~, VS CMB, at low R and Of A- &02CMA, against cr&&,at high R. The resulting spectra are shown in Figs 1-3. For each system, the absorptivities were determined at maximal values of AE. Values of s20and ,so2were calculated as the slopes of absorbance vs concentration plots for the binary complexes, while E,~ was determined from linear plots at low and high R. The results are shown in Table 1.
(2’) against where
R = CMB,ICMA~, a,, = [MAB]/c, (the mole fraction of metal ion present as the complex MAB), of metal, CM = the total concentration cMA, and cMB,= the total concentrations of binary complexes, MA, or MB*. The equilibrium constant K can be calculated for each value of R, if the value of all is known. The value of all can be determined from the spectrophotometric data. The total absorbance of solution of three species MA2, MB2 and MAB is given by eqn (3) : A = E~~[MA~]+E,I[MAB]+E~~[MBZ],
(3)
where E20, sl, and so2 are the molar absorptivities of MA2, MAB and MB2, respectively. From the stoichiometry of the reaction (1) and eqn (3), the result is :
RESULTS
AND DISCUSSION
&20+&02 &=&II
--
2
For formation of mixed complexes, of type [CdMeTUX] +, where X = Cl, Br or I, has been
’
dA = A-(~~O~MA~+EO~CM~IBJ,
(5)
If the absorptivity of the mixed complex is the same as the absorptivity of the binary complexes, AE = 0, then there is no spectrophotometric evidence of mixed-complex formation. On the other hand, the greater the value of AEwhich can be obtained relative to the total molar absorbance, the greater will be the sensitivity of the measurements. Therefore, the calculations should be carried out at those wavelengths where AE is greatest. In eqn (5), CM.&, and cMr,, are known and values of e20 and so2 can be measured, while values of el, can be determined at low or high values of R. For those values of R one of the binary complexes is present in excess, and the other is converted to the mixed complex. The result is a two-component mixture, for which the single unknown a,, can be easily determined. Equations
200
220
240
260
260
300
X (nm)
Fig. 1. Absorption spectra of binary and ternary complexes of Cd(I1) with MeTU and Cl- at various molar fractions of MeTU and Cl-. (1) l:O; (2) 9: 1; (3) 0: 1. ccd = 5 x 1O-5M, pH = 2.00.
Spectrophotometric
investigations
of Cd(I1) complexes
1309
X (nm)
X hm)
Fig. 2. Absorption spectra of binary and ternary complexes of Cd(II) with MeTU and Br- at various molar fractions of MeTU and Br-. (1) 1 :O; (2) 9: 1; (3) 0: 1. ccd = 5 x 1O-s M, pH = 2.00.
found. Values of a, 1were calculated for each point in the vicinity of R equal to unity and plotted vs log R (Fig. 4). The equilibrium constant was calculated for each point and log K was averaged for all points in each researched system (Table 2).
An analysis of the plots, presented on Fig. 4, indicates a scatter in the points resulting from different wavelengths of the research solutions. This is a reflection of inaccuracies in the spectrophotometric measurements. There are two principal sources of error in the above spectrophotometric method. One is the uncertainty in the determination of all, which depends on the experimental technique used and which is increased in
Fig. 3. Absorption spectra of binary plexes of Cd(I1) with MeTU and Ifractions of MeTU and I-. (1) 1 :O; ccd = 5 x 1O-5 M, pH =
and ternary comat various molar (2) 9: 1; (3) 0: 1. 2.00.
the subsequent calculation of K. Differentiation of eqn (2’) with respect to a,, at R = 1 (where the highest accuracy in determining K is to be expected) provides eqn (9) :
T =(K’/2+2) !$, which means that a small relative error in the determination of alI is magnified by a factor of (K”2 + 2) in calculating K even at the point of greatest accu-
racy. Clearly, the determination of large constants involves a considerable uncertainty on theoretical considerations alone. The other possible and serious source of error is the assumption that the equilibrium reaction of the formation of the mixed complex is mainly tixed and
Table 1. Molar absorptivities E(lo3 mol-’ cm-‘) of the binary and ternary complexes of Cd@) Complex [CdMeTU2]2+ CdCl, [CdMeTU,] 2+ CdBr, [CdMeTU,] ‘+ Cdl2
E20
E20
3.60f0.01 3.65kO.01 2.92f0.02 2.59kO.02 3.41 fO.O1 3.30+0.01
15.02+0.06 13.43 f0.05 3.07f0.01 3.16+0.01 27.06+0.11 26.76kO.09
El1
X29+0.06 5.02kO.05 3.75 * 0.02 3.6OkO.02 8.95_+0.11 8.87kO.09
-4.02+0.06 -3.53f0.05 0.76kO.02 0.72 +0.02 -6.29fO.11 -6.16*0.09
1310
J. MASLOWSKA and E. CHRUSCINSKA I -[CdMeTUCl]’ 2-[CdMeTUBr]’ 3-[CdMeTUIl’
08‘t
06
-
a= 0.4
0
-I
I -08
I -06
I
I -04
-0.2
I
I
I
I
0
0.2
0.4
0.6
I 0.8
I
I
log R
Fig. 4. Plots of the relationshipa, 1 = f(logR): (1) (0) I = 219rq (0) 1= 222nm (for [CdMeTUCl]+); (2) (0) 1= 225 run, (0) L = 227 mn (for [CdMeTUBr]+); (3) (0) 1= 221 nm, (0) I = 224 run (for [CdMeTUT]+).
Table 2. Equilibrium constants of mixed complexes ([CdMeTUX]+): T = 298 K, pH = 2.00 Complex r
@lMeTUCll_+ [CdMe’MJBr]+ [CdMeTUI]+
log K 1.03+0.10 1.17kO.12 1.34kO.08
that both the dissociation and disproportionation of the binary complexes are not being considered. This means that the accepted concentrations of species can differ considerably from those existing in the researched solutions. We conclude that the present method is less pre-
cise than the potentiometric and polarographic methods described in our previous papers.‘s6 In order to explain the relations existing between changes of mixed complex stability and redox properties of halide ions, the plot of log K vs the normal ,~~~~i~~~~~~~~~~~~~~~~~~~~~ 5). The above relation shows the mixed-complex stability increases after the addition of halide ion with the greater reducing action, in the following order : [CdMeTUCl] + < [CdMeTUBr] + < [CdMeTUI] + . This indicates that Cd(I1) forms mixed complexes in which a significant contribution of the covalent bond between Cd(I1) and halide ions occurs with a small Ez. *
REFERENCES 1. T. G. Spiro and D. N. Hume, J. Am. Chem. Sot. 1961, 83,4305. 2. J. I. Watters and E. D. Loughran, J. Am. Chem. Sot. 1953,75,4819. 3. W. E. Bennet, J. Am. Chem. Sot. 1957,79,1291. 4. R. De Wit and J. I. Watters, J. Am. Chem. Sot. 1954, 76, 3810.
5. J. Maslowska and E. ChruSciriska,Pal. J. Chem. 1983, 57, 399.
Fig. 5. The equilibriumconstants (log K) of [CdMeTuXj-, X = Cl, Br or I, as a function of the normal redox Potential of halide ions (Q.
6. J. Maslowska and E. Chrusciiiska, Pal. J. Chem. 1982, 56, 617. 7. F. J. Welcher, The Analytical Uses of Ethylenediamine Tetraacetic Acid. Van Nostrand, Amsterdam (1958). 8. Y. Marcus, Coord. Chem. Rev. 1969,4,273.
0277-5387186 S3.00+.00 Pcrgamon Journals Ltd
SPECTROPHOTOMETRIC INVESTIGATIONS OF BINARY AND TERNARY COMPLEXES OF Cd(n), METHYLTHIOUREA AND HALIDE IONS IN AQUEOUS SOLUTION JOANNA lUASkOWSKA* and EL;tBIETA
CHRUSCINSKA
Institute of General Food Chemistry, Technical University of Eodi, 90-924 Lodi, Stefanowskiego 4/10, Poland (Received 20 September 1985 ; accepted 6 November 1985) Abstrict-The results of spectrophotometric investigations of mixed-complex formation equilibria of Cd(II), methylthiourea (MeTU) and halide ions (Cl-, Br- and I-) in aqueous solution have been reported. The formation of binary and ternary complexes has been found. The values of the equilibrium constants (log units) for the reaction are 1.03+0.10, 1.17kO.12 and 1.34kO.08 for [CdMeTUCl]+, MA2 + MB2 ‘2MAB, 7 [CdMeTUBr]+ and [CdMeTUI]+, respectively. It was found that the stability of ternary complexes depends on the values of the normal redox potential of halide ions.
The spectrophotometric method was used to determine the equilibrium constants of the mixed complexes of mercuric halides’ and the mixed complexes of Ct.@) with pyrophosphate and ethylenediamine,2 ethylenediamine and iminodiacetate3 and ethylenediamine and oxalate.4 Because of lack of data for the mixed complexes of Cd(I1) with methylthiourea (MeTU) and halide ions, we undertook the studies in this field; In our previous papers’s6 we proved the formation of the mixed complexes of Cd(I1) with derivatives of thiourea and halide ions using electrochemical methods: potentiometry and polarography. Thus it seemed to be appropriate to use the spectrophotometric method to study the equilibrium reaction of the formation of mixed complexes in similar systems, in order to find new experimental data and extend the information about these little known complexes. EXPERIMENTAL
was prepared by treating cadmium oxide with perchloric acid. Cd(C104)2 solution was standardized by a complexometric method.7 Sodium chloride, bromide and iodide were puri8ed by crystallization from aqueous solution. NaCl, NaBr and NaI solutions were standardized by argentometric methods. All stock solutions were acidified with HC104 to pH = 2.0 to prevent any hydrolysis of the complexes. The concentrations used were selected according to Spiro and Hume’s data.’ The concentration of Cd(H) was kept constant, 5 x 10e5 M ; total concentration of NaCl (NaBr, NaI) and that of MeTU was about 0.01 M. The ionic strength of solutions was p = 0.02. Apparatus The measurements were recorded on a Specord UV-Vis (Carl Zeiss, Jena) spectrophotometer in the range of 12= 200-300 mn with l-cm quartz cells.
Reagents
Measurement method and calculation procedure
Triply distilled water was used. N-methylthiourea was recrystallized from water-ethanol mixtures and dried in vacua over P4010. Cadmium perchlorate
Spiro and Hume’ described a spectrophotometric method for the determination of the equilibrium constant, K, for the reaction of the mixedcomplex formation [(l)] : MA2+MB2=2MAB.
* Author to whom correspondence should be addressed.
(1)
The equilibrium constant K for the reaction (1) is 1307
and E. CHRUSCIfiSKA
J. MASLOWSKA
1308
(7) and (8) can be obtained for these conditions :
given by eqn (2) :
(for low R) jY= (1-a11)2-(1-~R/1+R)2’
(2) A-S~OCMA,
hence :
(7)
=(2~r-~20)cMBZ>
(for high R)
1 ail = 1-4K-’
A-&OZ~MLQ=
( l* J l-
4R(l-4K-1) (1+@2
(2'511-'502hA,*
(8)
, >
The plots of the left-hand side of eqns (7) and (8) cMB, or cMA*, respectively, should be straight lines passing through the origin. From their slopes sll can be calculated. As R approaches unity, the solutions change from two- to three-component systems, and the above plots then deviate from linearity (provided AE ><0). The statement of this deviation is very important because it is the evidence for mixed-complex formation. If no deviation occurs, the solutions always contain two components, and the plots are linear over the entire concentration range. In the present paper, absorptivities of the mixed complexes were calculated from linear plots of A-E~~c~~, VS CMB, at low R and Of A- &02CMA, against cr&&,at high R. The resulting spectra are shown in Figs 1-3. For each system, the absorptivities were determined at maximal values of AE. Values of s20and ,so2were calculated as the slopes of absorbance vs concentration plots for the binary complexes, while E,~ was determined from linear plots at low and high R. The results are shown in Table 1.
(2’) against where
R = CMB,ICMA~, a,, = [MAB]/c, (the mole fraction of metal ion present as the complex MAB), of metal, CM = the total concentration cMA, and cMB,= the total concentrations of binary complexes, MA, or MB*. The equilibrium constant K can be calculated for each value of R, if the value of all is known. The value of all can be determined from the spectrophotometric data. The total absorbance of solution of three species MA2, MB2 and MAB is given by eqn (3) : A = E~~[MA~]+E,I[MAB]+E~~[MBZ],
(3)
where E20, sl, and so2 are the molar absorptivities of MA2, MAB and MB2, respectively. From the stoichiometry of the reaction (1) and eqn (3), the result is :
RESULTS
AND DISCUSSION
&20+&02 &=&II
--
2
For formation of mixed complexes, of type [CdMeTUX] +, where X = Cl, Br or I, has been
’
dA = A-(~~O~MA~+EO~CM~IBJ,
(5)
If the absorptivity of the mixed complex is the same as the absorptivity of the binary complexes, AE = 0, then there is no spectrophotometric evidence of mixed-complex formation. On the other hand, the greater the value of AEwhich can be obtained relative to the total molar absorbance, the greater will be the sensitivity of the measurements. Therefore, the calculations should be carried out at those wavelengths where AE is greatest. In eqn (5), CM.&, and cMr,, are known and values of e20 and so2 can be measured, while values of el, can be determined at low or high values of R. For those values of R one of the binary complexes is present in excess, and the other is converted to the mixed complex. The result is a two-component mixture, for which the single unknown a,, can be easily determined. Equations
200
220
240
260
260
300
X (nm)
Fig. 1. Absorption spectra of binary and ternary complexes of Cd(I1) with MeTU and Cl- at various molar fractions of MeTU and Cl-. (1) l:O; (2) 9: 1; (3) 0: 1. ccd = 5 x 1O-5M, pH = 2.00.
Spectrophotometric
investigations
of Cd(I1) complexes
1309
X (nm)
X hm)
Fig. 2. Absorption spectra of binary and ternary complexes of Cd(II) with MeTU and Br- at various molar fractions of MeTU and Br-. (1) 1 :O; (2) 9: 1; (3) 0: 1. ccd = 5 x 1O-s M, pH = 2.00.
found. Values of a, 1were calculated for each point in the vicinity of R equal to unity and plotted vs log R (Fig. 4). The equilibrium constant was calculated for each point and log K was averaged for all points in each researched system (Table 2).
An analysis of the plots, presented on Fig. 4, indicates a scatter in the points resulting from different wavelengths of the research solutions. This is a reflection of inaccuracies in the spectrophotometric measurements. There are two principal sources of error in the above spectrophotometric method. One is the uncertainty in the determination of all, which depends on the experimental technique used and which is increased in
Fig. 3. Absorption spectra of binary plexes of Cd(I1) with MeTU and Ifractions of MeTU and I-. (1) 1 :O; ccd = 5 x 1O-5 M, pH =
and ternary comat various molar (2) 9: 1; (3) 0: 1. 2.00.
the subsequent calculation of K. Differentiation of eqn (2’) with respect to a,, at R = 1 (where the highest accuracy in determining K is to be expected) provides eqn (9) :
T =(K’/2+2) !$, which means that a small relative error in the determination of alI is magnified by a factor of (K”2 + 2) in calculating K even at the point of greatest accu-
racy. Clearly, the determination of large constants involves a considerable uncertainty on theoretical considerations alone. The other possible and serious source of error is the assumption that the equilibrium reaction of the formation of the mixed complex is mainly tixed and
Table 1. Molar absorptivities E(lo3 mol-’ cm-‘) of the binary and ternary complexes of Cd@) Complex [CdMeTU2]2+ CdCl, [CdMeTU,] 2+ CdBr, [CdMeTU,] ‘+ Cdl2
E20
E20
3.60f0.01 3.65kO.01 2.92f0.02 2.59kO.02 3.41 fO.O1 3.30+0.01
15.02+0.06 13.43 f0.05 3.07f0.01 3.16+0.01 27.06+0.11 26.76kO.09
El1
X29+0.06 5.02kO.05 3.75 * 0.02 3.6OkO.02 8.95_+0.11 8.87kO.09
-4.02+0.06 -3.53f0.05 0.76kO.02 0.72 +0.02 -6.29fO.11 -6.16*0.09
1310
J. MASLOWSKA and E. CHRUSCINSKA I -[CdMeTUCl]’ 2-[CdMeTUBr]’ 3-[CdMeTUIl’
08‘t
06
-
a= 0.4
0
-I
I -08
I -06
I
I -04
-0.2
I
I
I
I
0
0.2
0.4
0.6
I 0.8
I
I
log R
Fig. 4. Plots of the relationshipa, 1 = f(logR): (1) (0) I = 219rq (0) 1= 222nm (for [CdMeTUCl]+); (2) (0) 1= 225 run, (0) L = 227 mn (for [CdMeTUBr]+); (3) (0) 1= 221 nm, (0) I = 224 run (for [CdMeTUT]+).
Table 2. Equilibrium constants of mixed complexes ([CdMeTUX]+): T = 298 K, pH = 2.00 Complex r
@lMeTUCll_+ [CdMe’MJBr]+ [CdMeTUI]+
log K 1.03+0.10 1.17kO.12 1.34kO.08
that both the dissociation and disproportionation of the binary complexes are not being considered. This means that the accepted concentrations of species can differ considerably from those existing in the researched solutions. We conclude that the present method is less pre-
cise than the potentiometric and polarographic methods described in our previous papers.‘s6 In order to explain the relations existing between changes of mixed complex stability and redox properties of halide ions, the plot of log K vs the normal ,~~~~i~~~~~~~~~~~~~~~~~~~~~ 5). The above relation shows the mixed-complex stability increases after the addition of halide ion with the greater reducing action, in the following order : [CdMeTUCl] + < [CdMeTUBr] + < [CdMeTUI] + . This indicates that Cd(I1) forms mixed complexes in which a significant contribution of the covalent bond between Cd(I1) and halide ions occurs with a small Ez. *
REFERENCES 1. T. G. Spiro and D. N. Hume, J. Am. Chem. Sot. 1961, 83,4305. 2. J. I. Watters and E. D. Loughran, J. Am. Chem. Sot. 1953,75,4819. 3. W. E. Bennet, J. Am. Chem. Sot. 1957,79,1291. 4. R. De Wit and J. I. Watters, J. Am. Chem. Sot. 1954, 76, 3810.
5. J. Maslowska and E. ChruSciriska,Pal. J. Chem. 1983, 57, 399.
Fig. 5. The equilibriumconstants (log K) of [CdMeTuXj-, X = Cl, Br or I, as a function of the normal redox Potential of halide ions (Q.
6. J. Maslowska and E. Chrusciiiska, Pal. J. Chem. 1982, 56, 617. 7. F. J. Welcher, The Analytical Uses of Ethylenediamine Tetraacetic Acid. Van Nostrand, Amsterdam (1958). 8. Y. Marcus, Coord. Chem. Rev. 1969,4,273.