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Kinetics of the reaction of malachite green and crystal violet with formally aprotic solvents
Analyticc Chimica Acta, 117 (1980) 353-357 0 Elsevier Scientific Publishing Company, Amsterdam
-
Printed
in The
Netherlands
Short Communication
KINETICS OF THE REACTION OF MALACHITE GREEN AND CRYSTAL VIOLET WITH FORMALLY AF’ROTIC SOLVENTS
MAREK
K. KALINOWSKI*,
Department (Poland) (Received
of Chemistry. 19th November
JAROSJkAW University
STACHURSKI
of Warsaw.
I Pasteur
and KONRAD Street
02-093
R. JANOWSKI IVarszawa
1979)
Summary. The stability of malachite green and crystal violet has heen studied spectrophotometrically in 17 formally aprotic media. The cations of these compounds react with solvents that are Lewis bases; the pseudo-first order rate constant values (k’) are dependent on Cutmann’s donor numbers. The sensitivity of the two dyes to the solvent effect correlates with the net charge on their central carbon atoms.
The basic triphenylmethane dyes, malachite green (;\lG) and crystal violet (CV) are very useful in analytical chemistry. They are valuable as acid-base [ 1, 21 and redox [3 J indicators in both aqueous and non-aqueous media. Their cations form effective ion-exchange sites in the membranes of ionmalachite green, crystal violet and selective electrodes [4-71 . Moreover, other ttiphenylmethane reagents are useful in sensitive determinations of elements [S] . Since all these applications are important in formally aprotic media, the effects of such solvents on the stabilities of malachite green and crystal violet have been studied in detai1, as described below. Kemula and Axt [9] have demonstrated that these two dyes can react with many organic solvents_ By applying electrochemical and spectroscopic techniques, they found that the fading of triphenylmethane reagents is probably connected with an addition of a molecule of solvent to the central carbon atom of the carbonium cation, that the rates of these reactions can be described by pseudo-first order kinetic equations [ ILO], and that the rate constants are strongly influenced by the nature of the solvent. The last finding was only qualitative [ 101 and it remained unclear which properties of the solvents decisively influenced the observed processes_ The purpose of this work is to formulate an empirical but more quantitative description of solvent effects on the stability of MG and CV. A series of formally aprotic solvents differing markedly in their polarities and electrondonating properties was chosen.
Experimental A Specord u.v.-visible spectrophotometer was used, and solutions were maintained at 25°C. CV and MG in their chloride forms (high-purity reagents, Merck) were used as received_ All solvents were dried and purified by estab-
354
iished procedures [ 111 immediately before use. Numerical calculations were made with an ODRA 1204 computer_
To determine the kinetics of the reactions of MC and CV with the solvents, the changes of the absorbance were examined at the long-wavelength absorption maxima of the dyes. The wave-numbers (v) and molar absorptivities (E) determined are summarized in Table 1, Beer’s law was obeyed at these v values in most of the solvents. in a few solvents, however, Beer’s law was not obeyed; in such cases the kinetics of the dye fading were not investigated_ The reaction of a dye with a large escess of solvent was pseudo-first order, as reported earlier [ 101 for dimethylformamide solutions_ Plots of log (absorbance) vs. time gave straight lines, and the half-lives of the reactions were independent of the initial concentration of RIG or CV, Rate constants [k’) were therefore determined from the slopes of these plots. Measurements -.vere performed for three different. initial concentrations of dyes. The mean values of I:’ and some physicochemicaI parameters of the solvents are collected T.kBLE
1
Characteristics of the long-wavetcngth bands in the absorption spectra of MC and CV Solvent
hlc? c (X lo41 mol-r cm-r )
1,2-Diethoxyethane (DEE) Dioxane (DK) Tributylphosphate (TBP) Tetrahydrofuran (THF) Nitromethane (NM) Dimethylsulfoxide (DI\ISO) Pyridine (PY) Hexamethylphosphotriamide (HNPA) Djmethylace~amide (DMA) Acetonitrite (ACN) PropionitriIe (PN) Dimethylformamide (DMF) Acetone (AC) Diethylformamide (DEF) Propylene carbonate (PC) rv-bIethylpyrroIidinohe-2 jhWP) Ethylenediamine (EDA)
b
b b b ‘7.1
5.5 4.1 3.8 6.7 6.3 7.1 4.6 7.2 6.0 6.7 5.8 c
CV= r’
(X 10’
cm-‘) 15.9 15.8 15.6 15.8 15.9 15.6 15.7 15.6 15.7 16.0 16.0 15.8 15.9 15.8 15.9 15.9 C
f (X IO41 mol-’ cm-’ )
I’ (X 10’ cm-‘)
b
16.6
b
16.5
7.7 8.5 7.4 8.9 9.5 9.4 8.7 9.8 8.6 8.3 8.5 c
16.3 16.4 16.6 16.4 16.6 16.6 16.5 16.7 16.4 16.5 16.4 C
JThe applicability of Beer’s law was examined in the concentration range 5.0 x 1O6-1.0 X 10S4 W. In solutions in which the dye-solvent reaction occurs(cf_ Table 3) corresponding molar ahsorptivities were estimated after extrapolation of the plot log (absorbance) vs. I to t = 0.
bBeer’s faw not obeyed. Wet
determined because MC and CV decotorise very rapidly.
355 TABLE
2
Pseudo-first
order
rate constants
of the reaction
of MC
and CV
with
organic
solvents
at
25°C Solventa
It’ (X lo4 s-‘)
Db mol-‘)
N-M ACN DX PC PN AC THF TBP DEE
2.7 14.1 14.8 15.1 16.1 17.0 20.0 23.7 “4
35.9 38.0 2.2 69.0 27.2 20.7 7.6 6.8 7.0
MG
Cv
d
d
d
d
e d
d
1.1 0.7 e
d d
c
d
Solventa
DMF NMP DMA DMSO DEF PY HMPA EDA
DN=
Ii?’(X 1o-us-‘)
(kcal mole’)
MG
Cv
36.1 32 38.9 45.0
26.6 27.3 27.5 29-s 30.9
29.0 17.2 19.9 53.s
d 0.17 0.16 0.35 0.37
12.3 30.0 14.2
33.1 38.S 55.0
40.0 79.4
0.40 1.30
Db
f
f
e
[13]. CGutmann’sdonor number [13]. =For abbreviations, see Table 1. bDielectricconstant dPeaction was not observed_ eReaction proceeds but Beer’- ) law is not obeyed. ‘Decolorizxtion is instantaneous after addition of the dye to the solvent.
in Table 2, and a typical set of spectrophotometric data used for kinetic analysis is presented in Fig. 1. As can be seen from Table 2, the solvents can be divided into two groups. The first group contains those with which reaction occurs; the second comprises the solvents which are inert with respect to the dyes. This division is
not connected
with
the polarity
of solvents;
Table 2 indicates
clearly
that
there is no simple correlation between 1:’ and the dielectric constant (relative permittivity). The value of the dielectric constant does, however, play an important role in the applicability of Beer’s law to the solutions. This law is not obeyed in THF, TBP, DX and DEE, i.e. in the solvents with dielectric constants below 10. In al1 the remaining solvents (D > 10) Beer’s law is obeyed. This phenomenon is presumably connected with the relatively
ir
(nIO’cn.‘l
Fig. 1. Variation of the absorbance of 1.0 X lo-’ M malachite green in dimethylacetamide solutions with time: (a) 1 min;(b) 2 min;(c) 3 mini(d) 12 min:(e) 18 min:(f) 24 h after addition of the dye to the solvent (pathlength 0.198 cm).
strong interactions between substituted triphenylmethyl and chloride ions in weakly polar solvents. as was previously observed in the case of Victoria blue f 12]_ It was also found that Gutmann’s donor numbers, which espress the nudeophilic properties of solvents [ 131, accurately predict the kinetics of the fading of AIG and CV. Fading of these compounds takes place only in solvents with donor numbers above 15.5 and 27.3 for MG and CV, respectively. ln addition. I:’ values increase significantly as the donor number increases. Analysis of the dependence between Iog /z’ and the donor number by the least-squares method showed that the simple relation tog h’ = ABN -f- B satisfactorily describes the experimental results. The values of ihe coefficients A and B are given in Table 3. It can therefore be assumed that the reactions between MG and CV and the various organic solvents may be considered in terms of the donor-acceptor concept for solvent--solute interactions [ 13 1, i-e_ the molecules of a given solvent should be treated as electron-pair donors. This means that tile stability of triphenylmethane dyes in solution is reduced by an increase in the Lewis basicity of the solvent. This is confirmed by the very fast decolorization in EDA, a strongly basic solvent. Qwantum calculations show that most of the positive charge of the unsubstituted tri~~len~7lrnet~l~~lcation is localized at tlte central carbon atom. This result was obtained by simple [ 141 and extended [ 151 HMO methods, the w-technique [ 141, and by SCF [ 16 1 and CNDO [ 17 1 methods, and is confirmed by carbon-13 n.m.r. studies [IT]. It may thus be espectcd that a solvent molecule reacts principally with the central carbon atoms of hlG and CV. although the dimethylarnino groups also bear some of the charge deloonhzed from this atom. This conclusion was also reached by Kemuta and Ast [9] who investigated the i-r. and U.V. spectra of the MG--D&IF adduct. It may further be assumed that in the course of the reaction between the dye and a solvent, the hybridization of the central carbon atom is changed to sp3. The geometrical structures of the solvent molecules will therefore also affect the kinetics of the reaction and may be responsible for deviations from the dependence between log Iz’ and donor number. Comparison of the kinetic data (Table 2) and the A values (Table 3) indicates clearly that the hlG cation is more sensitive to reaction with solvents than the CV cation. Such a phenomenon can be also explained on the basis TABLE 3 Kinetics
of the reactions
of blG
Compound
A
MG cv
0.095 t 0.016d 0.0'76r_0.009
aNumber deviations.
and CV
of value pairs. bCorrelation
with organic
soivents
B
-5.43 2 0.456 -6.84 -'0.30 coefficient.
CSnedecor’s
na
.b
FC
8 6
0.941
0.979
18.7 64.0
F-test.
dErrors
-
are standard
357
of the charge distributions in the two cations. The positive charge density at the central carbon atom of MC is greater than that in CV [ 15, 17]_ This means that the MG cation is a stronger Lewis acid and is therefore more sensitive to solvent effects, as is demonstrated by the present results. REFERENCES in E. Bishop (Ed.), Indicators, Pergamon Press, Oxford, 1972, Ch. S B. in E. Bishop (Ed.), Indicators, Pergamon Press. Osford, 1972. Ch. 3. J. S. Fritz in E. Bishop (Ed.), Indicators, Pergamon Press, Oxford, 1972, Ch. 4. N. Ishibashi and H. Kohara, Jpn. Anal., 21 (1972) 100. K. Kina, N. Mackawa and N. fshibashi, Bull. Chem. Sot. Jpn., 16 (1973) 2’778. N. Ishibashi, H. Kohara and K. Horinouchi, Talanta, 20 (1973) S67. N. lshibashi and A. Jyo, Microchem. J., 18 (1973) 220. See, e.g., Z. Marczenko, Mikrochim. Acta, (197’i II) 65’7. W. Kemula and A. Axt, Bull. Acad. Polon. Sci., Ser. Sci. Chim., 15 (196i) 43. W. Kemula and A. Axt, Rocz. Chem., 36 (1961) 663. C. K. Mann. in A. J. Bard (Ed.), Electroanalytical Chemistry, Vol. 3.31. Dekker, New York, 1969, Ch. 9. F. Fichtmayr and J. Schiag, Ber. Bunsenges. Phys. Chem.. 6S (1964) 95. V. Gutmann, The Donor-Acceptor Approach to Molecular Interactions, Plenum Press. New York, 1976. A. Streitwieser, Jr., Rlolecular Orbital Theory, J. \Viley, New York, 1963, Ch. 12. 1. J. A. Pople, J. Phys. Chem., 61 (196’7) 6. A. C. Hopkinson, K. Yates and G. Csizmndia, Tetrahedron. 3-6 (19iO) 1815. G. J. Ray, R. J. Kurland and A. K. Colter, Tetrahedron, 27 (197 1) i35.
1 E. Bishop, 2 E. Banyai,
3 4 5 6 7 8 9 10 11 12 13 l-i 15 16 17
-
Printed
in The
Netherlands
Short Communication
KINETICS OF THE REACTION OF MALACHITE GREEN AND CRYSTAL VIOLET WITH FORMALLY AF’ROTIC SOLVENTS
MAREK
K. KALINOWSKI*,
Department (Poland) (Received
of Chemistry. 19th November
JAROSJkAW University
STACHURSKI
of Warsaw.
I Pasteur
and KONRAD Street
02-093
R. JANOWSKI IVarszawa
1979)
Summary. The stability of malachite green and crystal violet has heen studied spectrophotometrically in 17 formally aprotic media. The cations of these compounds react with solvents that are Lewis bases; the pseudo-first order rate constant values (k’) are dependent on Cutmann’s donor numbers. The sensitivity of the two dyes to the solvent effect correlates with the net charge on their central carbon atoms.
The basic triphenylmethane dyes, malachite green (;\lG) and crystal violet (CV) are very useful in analytical chemistry. They are valuable as acid-base [ 1, 21 and redox [3 J indicators in both aqueous and non-aqueous media. Their cations form effective ion-exchange sites in the membranes of ionmalachite green, crystal violet and selective electrodes [4-71 . Moreover, other ttiphenylmethane reagents are useful in sensitive determinations of elements [S] . Since all these applications are important in formally aprotic media, the effects of such solvents on the stabilities of malachite green and crystal violet have been studied in detai1, as described below. Kemula and Axt [9] have demonstrated that these two dyes can react with many organic solvents_ By applying electrochemical and spectroscopic techniques, they found that the fading of triphenylmethane reagents is probably connected with an addition of a molecule of solvent to the central carbon atom of the carbonium cation, that the rates of these reactions can be described by pseudo-first order kinetic equations [ ILO], and that the rate constants are strongly influenced by the nature of the solvent. The last finding was only qualitative [ 101 and it remained unclear which properties of the solvents decisively influenced the observed processes_ The purpose of this work is to formulate an empirical but more quantitative description of solvent effects on the stability of MG and CV. A series of formally aprotic solvents differing markedly in their polarities and electrondonating properties was chosen.
Experimental A Specord u.v.-visible spectrophotometer was used, and solutions were maintained at 25°C. CV and MG in their chloride forms (high-purity reagents, Merck) were used as received_ All solvents were dried and purified by estab-
354
iished procedures [ 111 immediately before use. Numerical calculations were made with an ODRA 1204 computer_
To determine the kinetics of the reactions of MC and CV with the solvents, the changes of the absorbance were examined at the long-wavelength absorption maxima of the dyes. The wave-numbers (v) and molar absorptivities (E) determined are summarized in Table 1, Beer’s law was obeyed at these v values in most of the solvents. in a few solvents, however, Beer’s law was not obeyed; in such cases the kinetics of the dye fading were not investigated_ The reaction of a dye with a large escess of solvent was pseudo-first order, as reported earlier [ 101 for dimethylformamide solutions_ Plots of log (absorbance) vs. time gave straight lines, and the half-lives of the reactions were independent of the initial concentration of RIG or CV, Rate constants [k’) were therefore determined from the slopes of these plots. Measurements -.vere performed for three different. initial concentrations of dyes. The mean values of I:’ and some physicochemicaI parameters of the solvents are collected T.kBLE
1
Characteristics of the long-wavetcngth bands in the absorption spectra of MC and CV Solvent
hlc? c (X lo41 mol-r cm-r )
1,2-Diethoxyethane (DEE) Dioxane (DK) Tributylphosphate (TBP) Tetrahydrofuran (THF) Nitromethane (NM) Dimethylsulfoxide (DI\ISO) Pyridine (PY) Hexamethylphosphotriamide (HNPA) Djmethylace~amide (DMA) Acetonitrite (ACN) PropionitriIe (PN) Dimethylformamide (DMF) Acetone (AC) Diethylformamide (DEF) Propylene carbonate (PC) rv-bIethylpyrroIidinohe-2 jhWP) Ethylenediamine (EDA)
b
b b b ‘7.1
5.5 4.1 3.8 6.7 6.3 7.1 4.6 7.2 6.0 6.7 5.8 c
CV= r’
(X 10’
cm-‘) 15.9 15.8 15.6 15.8 15.9 15.6 15.7 15.6 15.7 16.0 16.0 15.8 15.9 15.8 15.9 15.9 C
f (X IO41 mol-’ cm-’ )
I’ (X 10’ cm-‘)
b
16.6
b
16.5
7.7 8.5 7.4 8.9 9.5 9.4 8.7 9.8 8.6 8.3 8.5 c
16.3 16.4 16.6 16.4 16.6 16.6 16.5 16.7 16.4 16.5 16.4 C
JThe applicability of Beer’s law was examined in the concentration range 5.0 x 1O6-1.0 X 10S4 W. In solutions in which the dye-solvent reaction occurs(cf_ Table 3) corresponding molar ahsorptivities were estimated after extrapolation of the plot log (absorbance) vs. I to t = 0.
bBeer’s faw not obeyed. Wet
determined because MC and CV decotorise very rapidly.
355 TABLE
2
Pseudo-first
order
rate constants
of the reaction
of MC
and CV
with
organic
solvents
at
25°C Solventa
It’ (X lo4 s-‘)
Db mol-‘)
N-M ACN DX PC PN AC THF TBP DEE
2.7 14.1 14.8 15.1 16.1 17.0 20.0 23.7 “4
35.9 38.0 2.2 69.0 27.2 20.7 7.6 6.8 7.0
MG
Cv
d
d
d
d
e d
d
1.1 0.7 e
d d
c
d
Solventa
DMF NMP DMA DMSO DEF PY HMPA EDA
DN=
Ii?’(X 1o-us-‘)
(kcal mole’)
MG
Cv
36.1 32 38.9 45.0
26.6 27.3 27.5 29-s 30.9
29.0 17.2 19.9 53.s
d 0.17 0.16 0.35 0.37
12.3 30.0 14.2
33.1 38.S 55.0
40.0 79.4
0.40 1.30
Db
f
f
e
[13]. CGutmann’sdonor number [13]. =For abbreviations, see Table 1. bDielectricconstant dPeaction was not observed_ eReaction proceeds but Beer’- ) law is not obeyed. ‘Decolorizxtion is instantaneous after addition of the dye to the solvent.
in Table 2, and a typical set of spectrophotometric data used for kinetic analysis is presented in Fig. 1. As can be seen from Table 2, the solvents can be divided into two groups. The first group contains those with which reaction occurs; the second comprises the solvents which are inert with respect to the dyes. This division is
not connected
with
the polarity
of solvents;
Table 2 indicates
clearly
that
there is no simple correlation between 1:’ and the dielectric constant (relative permittivity). The value of the dielectric constant does, however, play an important role in the applicability of Beer’s law to the solutions. This law is not obeyed in THF, TBP, DX and DEE, i.e. in the solvents with dielectric constants below 10. In al1 the remaining solvents (D > 10) Beer’s law is obeyed. This phenomenon is presumably connected with the relatively
ir
(nIO’cn.‘l
Fig. 1. Variation of the absorbance of 1.0 X lo-’ M malachite green in dimethylacetamide solutions with time: (a) 1 min;(b) 2 min;(c) 3 mini(d) 12 min:(e) 18 min:(f) 24 h after addition of the dye to the solvent (pathlength 0.198 cm).
strong interactions between substituted triphenylmethyl and chloride ions in weakly polar solvents. as was previously observed in the case of Victoria blue f 12]_ It was also found that Gutmann’s donor numbers, which espress the nudeophilic properties of solvents [ 131, accurately predict the kinetics of the fading of AIG and CV. Fading of these compounds takes place only in solvents with donor numbers above 15.5 and 27.3 for MG and CV, respectively. ln addition. I:’ values increase significantly as the donor number increases. Analysis of the dependence between Iog /z’ and the donor number by the least-squares method showed that the simple relation tog h’ = ABN -f- B satisfactorily describes the experimental results. The values of ihe coefficients A and B are given in Table 3. It can therefore be assumed that the reactions between MG and CV and the various organic solvents may be considered in terms of the donor-acceptor concept for solvent--solute interactions [ 13 1, i-e_ the molecules of a given solvent should be treated as electron-pair donors. This means that tile stability of triphenylmethane dyes in solution is reduced by an increase in the Lewis basicity of the solvent. This is confirmed by the very fast decolorization in EDA, a strongly basic solvent. Qwantum calculations show that most of the positive charge of the unsubstituted tri~~len~7lrnet~l~~lcation is localized at tlte central carbon atom. This result was obtained by simple [ 141 and extended [ 151 HMO methods, the w-technique [ 141, and by SCF [ 16 1 and CNDO [ 17 1 methods, and is confirmed by carbon-13 n.m.r. studies [IT]. It may thus be espectcd that a solvent molecule reacts principally with the central carbon atoms of hlG and CV. although the dimethylarnino groups also bear some of the charge deloonhzed from this atom. This conclusion was also reached by Kemuta and Ast [9] who investigated the i-r. and U.V. spectra of the MG--D&IF adduct. It may further be assumed that in the course of the reaction between the dye and a solvent, the hybridization of the central carbon atom is changed to sp3. The geometrical structures of the solvent molecules will therefore also affect the kinetics of the reaction and may be responsible for deviations from the dependence between log Iz’ and donor number. Comparison of the kinetic data (Table 2) and the A values (Table 3) indicates clearly that the hlG cation is more sensitive to reaction with solvents than the CV cation. Such a phenomenon can be also explained on the basis TABLE 3 Kinetics
of the reactions
of blG
Compound
A
MG cv
0.095 t 0.016d 0.0'76r_0.009
aNumber deviations.
and CV
of value pairs. bCorrelation
with organic
soivents
B
-5.43 2 0.456 -6.84 -'0.30 coefficient.
CSnedecor’s
na
.b
FC
8 6
0.941
0.979
18.7 64.0
F-test.
dErrors
-
are standard
357
of the charge distributions in the two cations. The positive charge density at the central carbon atom of MC is greater than that in CV [ 15, 17]_ This means that the MG cation is a stronger Lewis acid and is therefore more sensitive to solvent effects, as is demonstrated by the present results. REFERENCES in E. Bishop (Ed.), Indicators, Pergamon Press, Oxford, 1972, Ch. S B. in E. Bishop (Ed.), Indicators, Pergamon Press. Osford, 1972. Ch. 3. J. S. Fritz in E. Bishop (Ed.), Indicators, Pergamon Press, Oxford, 1972, Ch. 4. N. Ishibashi and H. Kohara, Jpn. Anal., 21 (1972) 100. K. Kina, N. Mackawa and N. fshibashi, Bull. Chem. Sot. Jpn., 16 (1973) 2’778. N. Ishibashi, H. Kohara and K. Horinouchi, Talanta, 20 (1973) S67. N. lshibashi and A. Jyo, Microchem. J., 18 (1973) 220. See, e.g., Z. Marczenko, Mikrochim. Acta, (197’i II) 65’7. W. Kemula and A. Axt, Bull. Acad. Polon. Sci., Ser. Sci. Chim., 15 (196i) 43. W. Kemula and A. Axt, Rocz. Chem., 36 (1961) 663. C. K. Mann. in A. J. Bard (Ed.), Electroanalytical Chemistry, Vol. 3.31. Dekker, New York, 1969, Ch. 9. F. Fichtmayr and J. Schiag, Ber. Bunsenges. Phys. Chem.. 6S (1964) 95. V. Gutmann, The Donor-Acceptor Approach to Molecular Interactions, Plenum Press. New York, 1976. A. Streitwieser, Jr., Rlolecular Orbital Theory, J. \Viley, New York, 1963, Ch. 12. 1. J. A. Pople, J. Phys. Chem., 61 (196’7) 6. A. C. Hopkinson, K. Yates and G. Csizmndia, Tetrahedron. 3-6 (19iO) 1815. G. J. Ray, R. J. Kurland and A. K. Colter, Tetrahedron, 27 (197 1) i35.
1 E. Bishop, 2 E. Banyai,
3 4 5 6 7 8 9 10 11 12 13 l-i 15 16 17