“…Thus, the fully protonated form of MTB is displayed by LH9 . The previous potentiometric studies on the acid-base behavior of MTB revealed that MTB undergoes a six-step acidic dissociation and, thus can be considered as a six-protic acid (LH 6 )[56]. This conclusion is consistent with values of the reported pK a for the functional groups of sulfonic, ketone and one of carboxylics that are very low[57].…”
“…Thus, the fully protonated form of MTB is displayed by LH9 . The previous potentiometric studies on the acid-base behavior of MTB revealed that MTB undergoes a six-step acidic dissociation and, thus can be considered as a six-protic acid (LH 6 )[56]. This conclusion is consistent with values of the reported pK a for the functional groups of sulfonic, ketone and one of carboxylics that are very low[57].…”
“…Depending on pH these coordinating groups can be protonated in the free state as well as, to a certain extend, in the bound state. The dissociation of the phenolic OH group contributes mainly to K 12 and that of the protonated amine nitrogen atoms to K 23 and K 34 , while the carboxylic groups are likely to be deprotonated within the investigated pH range. , According to this, one of the coordinating atoms in the states LMgH 2 2- and LMg 2 H - (only one site concerned) is protonated, which is likely to be the amine nitrogen. Thus, the binding reactions 2→5 and 6→8 (cf.…”
Section: Discussionmentioning
confidence: 95%
“…Figure ), suggests furthermore that two cations can be bound per molecule of MTB. Potentiometric and spectrophotometric titrations provided experimental evidence for this stoichiometry, , as well as for the existence of several protolytic states of free MTB and its cation complexes. − 1 Structure of the completely deprotonated state of Methylthymol Blue (MTB; Kekulé notation).…”
The kinetics of complex formation between Methylthymol
Blue (MTB) and Mg2+ has been investigated
between
pH 6.15 and 8.0 by laser temperature-jump relaxation spectrometry.
In addition to a fast, not time-resolved
amplitude change, which is attributed to protolytic reactions, two
resolvable relaxation processes have been
found in the millisecond time range and are assigned to the binding of
Mg2+ to different protolytic states of
MTB. The ratio of the corresponding relaxation times is around 3.
From the Mg2+ concentration and the pH
dependence of the slower of the two relaxation times the kinetic
parameters related to 1:1 and 1:2 complex
formation (MTB:Mg2+) have been calculated on the basis of
a detailed binding model considering different
protolytic states of bound and unbound ligand. The rate constants
of complex formation between the ligand's
fully deprotonated coordination sites and Mg2+ are around
107 M-1
s-1 and are in good agreement with
the
Eigen−Wilkins mechanism, whereas those between its monoprotonated
sites and the cation are of the order
of 5 × 103 M-1
s-1 and are thus by a factor of about 200
lower than those expected by the Eigen−Wilkins
mechanism. This difference is interpreted in molecular terms by
assuming that the monoprotonated state of
the coordination sites of the ligand is stabilized by intramolecular
hydrogen bond formation. Opening of this
hydrogen bond is thought to be a rapidly equilibrating step,
characterized by an equilibrium constant around
200, preceding the cation coordination. The rate-limiting step of
Mg2+ binding is, in accordance to the
Eigen−Wilkins mechanism, still attributed to inner-sphere solvate substitution
of the cation.
“…The molar absorptivity at 435 nm for aqueous MTB solutions of pH 1.8 to 5.4 has been measured by Yoshino etal. (4). The reported value is 1.891. mol-1 cm-1.…”
mentioning
confidence: 92%
“…The authors interpreted this as an indication of two types of carbonaceous materials on the surface. Thereafter, a bifurcated peak in the DTA curve has been quite commonly related to the notion of the existence of two kinds of carbonaceous materials (3,4). Furthermore, it has been suggested that, by matching with a synthesized bifurcated DTA peak of a standard reference mixture consisting of two known carbonaceous materials, an unknown carbonaceous material could be analyzed quantitatively (5).…”
The linearization of the callbratlon curve of the automated methylthymol blue (MTB) procedure for the determination of sulfates is described. Thls is accomplished by altering the barium-to-MTB molar ratio of the barium-MTB reagent used. Because of the low purlty of the commerclal dye, bariumto-MTB molar ratios of 0.9:l or lower are usually needed for obtaining linear callbratlon curves. A llnear relatlon between absorbance and sulfate concentration with a correlatlon coefficient of 0.9995 or better is obtained. The relative standard deviation among repllcates, in the range of 0 to 100 pg/ml, is 1.4 to 0.4%. The sensitivity for this range is 4 pg/ml SO4*-.Routine determination of sulfate in a large number of water and air samples is usually performed by a variety of automated wet chemical methods. The automated methylthymol blue method developed by Lazrus et al. (I) is gradually displacing all the other methods, and currently most of the Federal and State water and air pollution agencies have adopted this method for the determination of sulfates.The method involves displacement of methylthymol blue (MTB) from a barium-MTB complex by sulfate and colorimetric measurement of the freed MTB in highly basic solution.The absorption maximum of the barium-MTB complex is at 610 nm and that of the free MTB is a t 460 nm. Therefore, the free MTB can be measured a t 460 nm without major interference from the barium-MTB complex. As Equation 2 shows, the concentration of MTB is directly related to the concentration of sulfate, which permits determination of the sulfate by measuring the concentration of the displaced MTB.One of the most unwanted characteristics of this method has been the nonlinear relationship between the output of the instrument (absorbance) and the sulfate concentration. The plot of absorbance vs. concentration appears to be a parabolic function which becomes linear above a certain concentration of sulfate. Thus, in most cases, a calibration curve may be assumed as consisting of two linear portions of different slopes. The first linear portion corresponds to low sulfate concentrations (about one fifth of the total range), and the second to higher sulfate concentrations. The slope of the first linear portion of the calibration curve is lower by at least a factor of two from the slope of the second portion. Currently, two different approaches are used for the utilization of the nonlinear calibration curves for determination of sulfates. In the first approach, the output signals corresponding to each point of the calibration curve are linearized electronically with respect to the standard sulfate concentration, and thus the entire range can be utilized. In the second approach, only the linear portion of the calibration curve corresponding to higher sulfate concentrations is used for the analysis of samples. Both approaches have several disadvantages. For the electronic linearization, additional equipment and some extra effort to set the calibration are required. In the other approach, the sensitivity of the metho...
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