Annali di Chimica, 97, 2007, by Società Chimica Italiana
SPECTROPHOTOMETRIC
STUDY
OF
199
ACIDITY
CONSTANTS
OF
ALIZARINE RED S IN VARIOUS WATER-ORGANIC SOLVENT
Ali NIAZI(°), Amir Ahmad REZAEI, Fatemeh SHAHHOSSEINI Department of Chemistry, Faculty of Sciences, Azad University of Arak, Arak, Iran Summary - The acidity constants of Alizarine Red S were determined spectrophotometrically at 25°C and at constant ionic strength 0.1 M (KNO3) in pure water as well as in aqueous media containing variable mole percentages (5-70 %) of organic solvents. The organic solvents used were methanol, ethanol, N,N-dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile and dioxan. The acidity constants of all related equilibria are estimated using the whole spectral fitting of the collected data to an established factor analysis model. DATAN program was applied for determining of acidity constants and pure spectra of different form of Alizarine Red S. The obtained results indicated that acidity constants decrease as the content of an organic solvent in the medium increases. There are linear relationship between acidity constants and the mole fraction of various organic solvents in the solvent mixtures. Effect of various solvents on acidity constants and pure spectrum of each component are also discussed. INTRODUCTION This work is part of our interest to study the acidity constants of Alizarine Red S in various aqueous mixtures of organic solvents. Recently, the acidity constants of Alizarine Red S in a pure aqueous medium and micellar media solutions were reported.1 Nevertheless, the literature is lacking studies concerning the various water-organic solvent media effects on the acidity constants of Alizarine Red S under investigation. Therefore, the present article is devoted to study the effect of organic solvents medium on the acidity constants of Alizarine Red S. The knowledge of acidity constants is considered to be of interest for organic and inorganic compounds because it has a significant role in many chemical reactions such as acid-base titration, solvent extraction, complex formation, and ion transport. It has been shown that the acid-base properties affect the toxicity, chromatographic retention behavior, and pharmaceutical properties of organic acids and bases. Much of the theoretical foundation of modern organic chemistry is based on the observation of the effects on acid-base equilibrium of changing molecular structure.2,3 Mixed solvents are interesting, because two solvents mixed together produce a solvent with quite different properties, both physically (dielectric, density and viscosity) and chemically (acidbase and donor-acceptor properties). As far as the acid-base properties are concerned, an important feature is that the nature of the solvent is crucial for the strength of acids and bases. Particularly the (°)
Corresponding author. Tel.: +98 8613663041, Fax: +98 8613670017, E-mail address:
[email protected]
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NIAZI and coworkers
proton affinity is important, or in other words, the proton-donating and proton-accepting properties of solvent, as well as its polarity. In addition, the ionization degree of solute depends on the dielectric constant of solvent. Media of high dielectric constants are strongly ionizing, whereas those of low dielectric constants ionize to a lesser extent 4. By mixing solvents of different polarity in proper ratios, dielectric constant of the medium can be varied and, at the same time, the strength of dissolved acids and bases. It should also be emphasized that solvent mixtures can be more convenient than individual solvents owing to enhanced solubilising efficiency, increased sharpness of color change of indicators during titration and more manageable shape of acid-base titration curves.5-9 Spectrophotometric methods are in general highly sensitive and are as such suitable for studying chemical equilibria solution. If the components involved can be obtained in pure form, or if their spectral responses do not overlap, such analysis is, in general, trivial. For many systems, particularly those with similar components, this is not the case, and these have been difficult to analyze 10-11. DATAN program was applied for determination of acidity constants. Output of DATAN program are pKa values, number of principal components, concentration distribution diagrams and pure spectrum of each assumed species. DATAN program was developed by Kubista group.12-16 The application of DATAN was reported in several papers.1,10-13,19-22 In this work, we applied the UV-Visible spectrophotometry to determine the acidity constants of Alizarine Red S in different binary water-organic mixtures at 25°C and at ionic strength of 0.1 M (KNO3). The used organic solvents were the amphiprotic (methanol, ethanol), dipolar aprotic [N,N-dimethyl formamide (DMF), dimethyl sulfoxide (DMSO)], low basic aprotic (acetonitrile) and low basic non-polar (dioxane). The analysis is readily performed with the DATAN program 23. The obtained pKa values are discussed in terms of both the content and nature of the organic solvent. EXPERIMENTAL Chemical reagents Alizarine Red S, methanol, ethanol, DMF, DMSO, acetonitrile, dioxane, hydrochloric acid, sodium hydroxide and potassium nitrate were analytical grade commercial products from Merck. These reagents were used without further purification. Stock solutions (8.0×10-4 M) of the Alizarine Red S were prepared by dissolving a known mass of the solid in the required volume of the waterorganic mixtures. All solutions were prepared in deionized water. Instrumentation and software A Scinco (SUV-2120) spectrophotometer controlled by a computer and equipped with a 1cm path length quartz cell was used for UV-Vis spectra acquisition. Spectra were acquired between 300 and 650 nm. A HORIBA M-12 pH-meter furnished with a combined glass-saturated calomel electrode was calibrated with at least two buffer solutions at pH 3.00 and 9.00. All absorption spectra were digitized at five data points per nanometer in the wavelength 300-650 nm and transferred (in ASCII format) to an AMD 2000 XP (256 Mb RAM) computer for subsequent analysis by MATLAB software, version 6.5 (The MathWork) and for processing by using DATAN program.23 Spectrophotometric titrations For the Alizarine Red S (2.0×10-4 M) in pure water, water-methanol, water-ethanol, waterDMF, water-DMSO, water-acetontrile and water-dioxane mixtures titrations, absorption spectra
201 were measured with a titration set-up consisting of a computer interfaced to a spectrophotometer. After each pH adjustment by hydrochloric acid and hydroxide sodium, solution is transferred into the cuvette and the absorption spectra are recorded. Ionic strength was maintained at 0.1 M by adding appropriate amounts of KNO3. All measurements were carried out at the temperature (25 ± 0.5 °C). The pH values in various solvent-water mixtures were corrected using the equation pH * = pH ( R) − δ , where pH * is the corrected reading and pH (R) is the pH-meter reading obtained in a partially aqueous organic solvent, determined by Douheret.24,25 RESULTS AND DISCUSSION The chemical structure of Alizarine Red S, given in Scheme 1, implies a low acidity of the OH groups. Consequently, the determination of their highly alkaline dissociation constants is a difficult task. The studied Alizarine Red S presents two ionizable OH groups. The previous reported values of acidity constants are mainly in water (pK1 = 5.50 and pK2 = 11.00) 26 and in mixtures of dioxane with water (pK1 = 6.10 and pK2 = 10.80) 27. The observed differences between the pKa values are due to probable experimental errors in different methods. Also, the difference between the results of this study and reported results26-28 can be because of experimental conditions. Also, the acidity constants of Alizarine Red S in a pure aqueous medium and micellar media solutions are reported 1. O
OH OH SO3-
O
SCHEME 1.- Chemical structure of Alizarine Red S. The absorption spectra of Alizarine Red S in pure water and binary solvent mixtures at various pH values in 300-650 nm intervals were recorded. Sample spectra of Alizarine Red S at different pH values in water with pH ranging from 2.20 to 11.74 and 50% or 70% (w/v) different solvents to water with pH ranging from about 2.00 to 12.00 are shown in Figures 1 to 7. Singular value decomposition analysis performed on all absorption data matrices obtained at various pH values for Alizarine Red S gives the number of components that best represent the system. Three significant factors are also supported by the statistical indicators of Elbergali et al.18. These factors could be attributed to the two dissociation equilibria of a diprotic acid such as Alizarine Red S. The pKa values of Alizarine Red S were investigated in seven different water-solvent binary mixtures spectrophotometrically at 25 °C and at ionic strength of 0.1 M. Acidity constants of Alizarine Red S in several mixtures were evaluated with the DATAN program using the corresponding spectral absorption - pH data. The obtained pKa values are listed in Tables 1 and 2. The pKa values cited in Tables 1 and 2 indicate that the acid ionization constants are dependent on both the proportion and the nature of the organic solvent. Generally, the acidity constants decrease by increasing the mole percentage of all organic solvents used. However, some
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pKa values could not be determined in solutions containing high mole fraction of applied organic solvents (Table 1 and 2).
Absorbance
2 1.5 1 0.5 0 300
S11 400
500
Solution No.
S1 600
Wavelength (nm) FIGURE 1. - Absorption spectra of Alizarine Red S (2.0×10-4 M) in pure water at 0.1 M KNO3 at different pH values: (1) 2.02, (2) 2.52, (3) 3.02, (4) 3.53, (5) 4.03, (6) 4.53, (7) 5.06, (8) 5.56, (9) 6.07, (10) 6.60, (11) 7.08, (12) 7.58, (13) 8.08, (14) 8.66, (15) 9.19, (16) 9.71, (17) 10.19, (18) 10.71, (19) 11.22, (20) 11.74.
Absorbance
2 1.5 1 0.5 0 300
S11 400
500
S1
Solution No.
600
Wavelength (nm)
FIGURE 2. - Absorption spectra of Alizarine Red S (2.0×10-4 M) in 70 wt. % methanol to water at 0.1 M KNO3 at different pH values: (1) 2.04, (2) 2.54, (3) 3.05, (4) 3.52, (5) 4.02, (6) 4.34, (7) 4.66, (8) 4.96, (9) 5.29, (10) 5.59, (11) 5.92, (12) 6.23, (13) 6.55, (14) 6.87, (15) 7.20, (16) 7.50, (17) 7.98, (18) 8.51, (19) 9.02, (20) 9.53, (21) 10.02, (22) 10.53, (23) 11.02, (24) 11.51, (25) 12.01.
203
Absorbance
2 1.5 1 0.5 0 300
S11 400
500
Solution No.
S1 600
Wavelength (nm)
Absorbance
FIGURE 3. - Absorption spectra of Alizarine Red S (2.0×10-4 M) in 70 wt. % ethanol to water at 0.1 M KNO3 at different pH values: (1) 2.06, (2) 2.54, (3) 3.05, (4) 3.33, (5) 3.65, (6) 4.02, (7) 4.40, (8) 4.71, (9) 5.05, (10) 5.43, (11) 5.82, (12) 6.23, (13) 6.65, (14) 7.04, (15) 7.49, (16) 8.07, (17) 8.60, (18) 9.06, (19) 9.61, (20) 10.13, (21) 10.64, (22) 11.14, (23) 11.51, (24) 12.13.
2 1.6 1.2 0.8 0.4 0 300
S21 S11 400
500
Solution No.
S1 600
Wavelength (nm) FIGURE 4. - Absorption spectra of Alizarine Red S (2.0×10-4 M) in 70 wt. % DMF to water at 0.1 M KNO3 at different pH values: (1) 2.07, (2) 2.58, (3) 3.04, (4) 3.57, (5) 3.92, (6) 4.25, (7) 4.55, (8) 4.87, (9) 5.19, (10) 5.51, (11) 6.14, (12) 6.44, (13) 6.77, (14) 7.09, (15) 7.39, (16) 7.88, (17) 8.35, (18) 9.87, (19) 10.37, (20) 10.86, (21) 11.38, (22) 11.60, (23) 12.10. Increasing the mole fraction of different solvents in the medium leads to an increase in the pKa for Alizarine Red S. The acid ionization constant in a pure aqueous medium (Ka(w)) is related to that in a partly aqueous medium (Ka(s)) by the relation 29: Ka(w) = Ka(s) (γA- γH+ / γHA) where γ is the activity coefficient of the respective species in a partly aqueous medium relative to that in pure water. The electrostatic effect resulting from the change in the relative permittivity of the medium operates on the activity coefficient of any charged species 29. Generally, by increasing
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the mole percentage of different solvents in the aqueous medium, the relative permittivity of the medium is lowered. This will increase both the γA- and γH+ yielding a decrease in the acid ionization constants (i.e., high pKa values). This is consistent with the results obtained for the acid ionization constants (pKa) of Alizarine Red S in various water – solvents mixtures. However, in the light of the relation: pKa = e2 / (2.303aKT(D)m), which correlates the variation of pKa with the relative permittivity of the medium (D)m, points on the plots of pKa values against {1/(D)m} for different solvent, where linear regression analysis was used to obtain the best fit correlation, are deviated from the linearity (Fig. 8). The relative permittivity of the medium (D)m is obtained from the relation (Table 1 and 2): 6 (D)m = D(w)Xf(w) + D(s)Xf(s) Respectively, D and Xf are the relative permittivity and mole fraction and the subscripts w and s refer to water and organic solvent. The linear equations and also R2 are shown in Fig. 8. Moreover, pure electrostatic effect should lead to increasing the acidity in the order: DMSO > DMF > ≈ acetonitrile > methanol > dioxane > ethanol. However, the measured results, in solutions containing the same mole percentages, exhibit a different order. The above considerations clearly indicate that other solvent effects such as hydrogen bonding and solvent basicity as well as protonsolvent interaction and dispersion forces, in addition to the electrostatic effect, exert a profound influence on the ionization processes of the Alizarine Red S in different aqueous mixtures. The same trend has already been reported for various organic molecules in different solvent mixtures.10,11,13. It has been reasonably assumed that preferential solvation of the charged particles by water is mainly responsible for such a monotonic dependence of acidity constants of Alizarine Red S on the solvent composition.
Absorbance
1.6 1.2 0.8 0.4 0 300
S11 400
500
600
S1
Solution No.
Wavelength (nm) FIGURE 5. - Absorption spectra of Alizarine Red S (2.0×10-4 M) in 50 wt. % DMSO to water at 0.1 M KNO3 at different pH values: (1) 2.53, (2) 3.02, (3) 4.03, (4) 4.33, (5) 4.64, (6) 4.96, (7) 5.33, (8) 5.73, (9) 6.10, (10) 6.84, (11) 7.67, (12) 8.56, (13) 9.04, (14) 9.78, (15) 10.47, (16) 11.22, (17) 12.01.
205
Absorbance
1.6 1.2 0.8 0.4 0 300
S11 400
500
Solution No.
S1 600
Wavelength (nm)
FIGURE 6. - Absorption spectra of Alizarine Red S (2.0×10-4 M) in 50 wt. % acetonitrile to water at 0.1 M KNO3 at different pH values: (1) 2.00, (2) 2.51, (3) 3.02, (4) 3.53, (5) 3.85, (6) 4.15, (7) 4.46, (8) 4.76, (9) 5.06, (10) 5.36, (11) 5.68, (12) 6.01, (13) 6.43, (14) 6.73, (15) 8.00, (16) 8.46, (17) 8.92, (18) 9.43, (19) 9.87, (20) 10.52, (21) 11.05, (22) 11.58, (23) 12.08.
Absorbance
1.2 0.8 0.4 0 300
S11 400
500
Solution No.
S1 600
Wavelength (nm) FIGURE 7. - Absorption spectra of Alizarine Red S (2.0×10-4 M) in 50 wt. % dioxane to water at 0.1 M KNO3 at different pH values: (1) 1.45, (2) 2.02, (3) 2.49, (4) 2.98, (5) 3.49, (6) 4.01, (7) 4.50, (8) 5.01, (9) 5.49, (10) 6.02, (11) 6.50, (12) 7.00, (13) 7.51, (14) 8.00, (15) 8.51, (16) 9.30, (17) 10.01, (18) 10.52, (19) 11.05, (20) 12.05. It was recognized that solvent effects such as hydrogen bonding and solvent basicity as well as dispersion forces and proton-solvent interactions exert a profound influence on the ionization process of weak acids in the presence of organic solvents. The effective density of dispersion centers in the organic solvent used is higher than in pure water. According, by one can expect higher stabilization of the conjugate base A- from each step of ionization by dispersion forces, which are established between the delocalized oscillator dipole of the solvent. Furthermore, the proton is expected to be highly stabilized in aqueous mixtures by its interactions with organic solvent and water molecules (proton-solvent interaction) compared with water molecules alone in pure water. Consequently, both A- and H+ are being highly stabilized upon increasing of the mole fraction of solvent in aqueous medium, i.e., γA- and γH+ decrease. Thus, the acid ionization constants
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of the studied Alizarine Red S increase (pKa decrease) with increasing solvents content in the medium. However, this is not the case, as is evident from the data cited in Table 1 and 2. Therefore, one can conclude that both the dispersion forces and proton-solvent interaction effects do not have significant role in the ionization processes of Alizarine Red S. TABLE 1. - pKa values for Alizarine Red S in different water-organic solvent mixtures at 25 °C and at the constant ionic strength 0.1 M (KNO3). x 0
(D)m 78.40
5 10 20 30 40 50 60 70
76.11 73.82 69.24 64.66 60.08 55.50 50.92 46.34
5 10 20 30 40 50 60 70
75.70 72.99 67.58 62.17 56.76 51.35 45.95 40.53
5 10 20 30 40 50 60 70
76.28 74.23 70.06 65.89 61.72 57.55 52.96 48.72
{1/(D)m}102 pK1 1.28 4.72 ± 0.03 Methanol 1.31 4.85 ± 0.04 1.35 5.00 ± 0.04 1.44 5.18 ± 0.05 1.55 5.42 ± 0.04 1.66 5.63 ± 0.06 1.80 5.82 ± 0.05 1.96 6.15 ± 0.07 2.16 6.48 ± 0.07 Ethanol 1.32 4.83 ± 0.03 1.37 4.89 ± 0.05 1.48 5.03 ± 0.04 1.61 5.18 ± 0.05 1.76 5.23 ± 0.06 1.95 5.46 ± 0.06 2.18 5.56 ± 0.07 2.47 5.89 ± 0.08 DMF 1.31 4.84 ± 0.04 1.35 4.94 ± 0.05 1.43 5.09 ± 0.05 1.52 5.28 ± 0.06 1.62 5.35 ± 0.06 1.74 5.46 ± 0.07 1.89 5.55 ± 0.06 2.05 5.71 ± 0.07
pK2 10.08 ± 0.09 10.17 ± 0.09 10.24 ± 0.10 10.30 ± 0.10 10.39 ± 0.11 10.48 ± 0.12 10.56 ± 0.12 10.63 ± 0.12 10.72 ± 0.14 10.12 ± 0.08 10.17 ± 0.09 10.21 ± 0.11 10.24 ± 0.10 10.28 ± 0.12 10.31 ± 0.13 10.35 ± 0.14 10.39 ± 0.015 10.11 ± 0.09 10.15 ± 0.10 10.21 ± 0.11 10.25 ± 0.12 10.29 ± 0.12 10.35 ± 0.14 10.39 ± 0.14 10.46 ± 0.15
207 TABLE 2. - pKa values for Alizarine Red S in different water-organic solvent mixtures at 25 °C and at the constant ionic strength 0.1 M (KNO3). x 0
(D)m 78.40
5 10 20 30 40 50
77.62 73.82 69.24 64.66 60.08 55.50
5 10 20 30 40 50
76.36 74.16 67.58 62.17 56.76 51.35
{1/(D)m}102 pK1 1.28 4.72 ± 0.03 DMSO 1.29 5.15 ± 0.05 1.35 5.25 ± 0.05 1.44 5.38 ± 0.06 1.55 5.45 ± 0.07 1.66 5.59 ± 0.07 1.80 5.72 ± 0.08 Acetonitrile 1.31 5.32 ± 0.04 1.35 5.43 ± 0.05 1.48 5.55 ± 0.05 1.61 5.64 ± 0.06 1.76 5.72 ± 0.06 1.95 5.78 ± 0.07
5 10 20 30 40 50
74.59 70.77 70.06 65.89 61.72 57.55
1.34 1.41 1.43 1.52 1.62 1.74
Dioxane 4.79 ± 0.03 4.83 ± 0.04 4.89 ± 0.03 4.95 ± 0.05 5.01 ± 0.06 5.08 ± 0.06
pK2 10.08 ± 0.09 10.28 ± 0.07 10.35 ± 0.08 10.47 ± 0.09 10.61 ± 0.09 10.69 ± 0.10 10.80 ± 0.10 10.19 ± 0.08 10.38 ± 0.08 10.55 ± 0.10 10.67 ± 0.09 10.83 ± 0.11 10.95 ± 0.11 10.15 ± 0.07 10.17 ± 0.08 10.19 ± 0.08 10.24 ± 0.09 10.26 ± 0.10 10.25 ± 0.10
On the other hand, water molecules are characterized by a high tendency to act as hydrogen donors compared with other solvent molecules. Therefore, the conjugate base A- is expected to be less stabilized by hydrogen-bonding interaction with solvent molecules as the mole fraction of solvents are increasing (i.e., γA- increase). This will tend to increase the pKa values of all steps in the Alizarine Red S system. Accordingly, the observed increase in the pKa values of Alizarine Red S upon increasing mole fraction of different organic solvents in aqueous mixtures can ascribed, in addition to the electrostatic effect, to the hydrogen-bonding interaction between the conjugate base A- and different solvents. One of the very important outputs of the DATAN program is the calculated spectrum of different forms of Alizarine Red S in each water-organic solvents media. Sample spectrum of the calculated spectra of all species in pure water is shown in Fig. 9. It is interesting to note that the nature of the organic solvents have a fundamental effect on each pure spectrum. As it is clear from Fig. 10, this effect is apparently observed in different species of Alizarine Red S. The solvent effect on this spectrum is very interesting. However, according to Fig. 10, the effect of nature of solvents is seen in the spectra of HL- more than for H2L and L2- species. These results are in accordance with the previous study in which Alizarine Red S is investigated in micellar medium 1.
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12
pK a
8
2
pKa1 y = 1.8716x + 2.4711
4
2
R = 0.9946
2 0 1.4
1.6
1.8
2
{1/(D)m}10
pK a1 y = 0.8783x + 3.7063
2
R = 0.9869
2
1.2
1.4
2
1.6 1.8
2.2
2.4
2
12
c
d
10
pK a2 y = 0.4506x + 9.5492
pKa2 y = 1.0345x + 8.9661
8
2
R = 0.9789
6 4
pKa1 y = 1.1162x + 3.4762
2
R = 0.9475
2
R = 0.984
6
2
4
pKa1 y = 1.0859x + 3.7783
2
R = 0.9879
2
0
0 1.2
1.4
1.6
1.8
1.2
2
1.4
2
12
10
pKa2 y = 1.1087x + 8.8469
8
8
2
pK a
R = 0.9422
6 pKa1 y = 0.6817x + 4.4985
4
f pKa2 y = 0.279x + 9.7887 2
R = 0.8106
6 4
2
R = 0.9237
2
1.8 2
12
e
10
1.6 {1/(D)m}10
{1/(D)m}10
pK a
2
{1/(D)m}10
pK a
pK a
4
2.2
12
8
R = 0.944
6
0
1.2
10
pKa2
+ 9.8736 8 y = 0.2179x 2
R = 0.9749
6
b
10
pKa2 y = 0.6366x + 9.3834
pK a
10
12
a
pKa1 y = 0.7292x + 3.824
2
2
R = 0.9726
0
0 1.2
1.4
1.6
1.8 2
{1/(D)m}10
2
1.3
1.4
1.5
1.6
{1/(D)m}10
1.7
1.8
2
FIGURE 8. - The pure spectra of different forms of Alizarine Red S in (a) 70% (v/v) methanol to water, (b) 70% (v/v) ethanol to water, (c) 70% (v/v) DMF to water, (d) 50% (v/v) DMSO to water, (e) 50% (v/v) acetonitrile to water and (f) 50% (v/v) dioxane to water.
209
2 2-
L
Absorbance
1.6 1.2
-
HL
H2L
0.8 0.4 0 300
400
500
600
Wavelength (nm)
FIGURE 9. - The pure spectra of different forms of Alizarine Red S in pure water.
1.6 1.2 0.8
-
H 2L
e 2-
1.2
2-
L
Absorbance
Absorbance
1.6
d
HL
0.4
L H2L
0.8
-
HL
0.4
0
0
300
400
500
600
300
400
Wavelength (nm)
500
600
Wavelength (nm)
f
1.2 2-
Absorbance
L
0.8 H2L -
HL
0.4
0 300
400
500
600
Wavelength (nm)
FIGURE 10. - The pure spectra of different forms of Alizarine Red S in (a) 70% (v/v) methanol to water, (b) 70% (v/v) ethanol to water, (c) 70% (v/v) DMF to water, (d) 50% (v/v) DMSO to water, (e) 50% (v/v) acetonitrile to water and (f) 50% (v/v) dioxane to water.
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NIAZI and coworkers CONCLUSIONS
In this work, the behavior of acidity constants of Alizarine Red S in solvent-water mixtures at 25 °C and at ionic strength of 0.1 M is studied by a multiwavelength spectrophotometric method. The used organic solvents were methanol, ethanol, N,N-dimethyl formamide (DMF), dimethylsulfoxide (DMSO), acetonitrile and dioxan. The shifts in acidity constants of Alizarine Red S in the mixtures were derived. There are linear relationship between acidity constants and the mole fraction of different solvents in the solvent mixtures. The effect of solvent properties on acid-base behavior is discussed. This indicates that the acid ionization constants of Alizarine Red S obtained in various percentages of different solvents–water mixtures are governed by electrostatic effects. Received June 21st, 2006 REFERENCES 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23) 24) 25) 26) 27)
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