Catechol titration

Figure 13. Distribution curve, as a function of pH, of the various species formed in the ferric-catechol titration experiment.

Figure 11. The influence of residual H2O2 on the reaction testing of phenol hydroxylation to catechol (CAT), hydroquinone (HQ), and para-benzoquinone (BQ) [61], Reaction conditions 4 g phenol 50 mL water solvent 0.2 g a-Fe203 catalyst inner standard ethanol reaction temperature 70°C. Aliquots were sampled at different times and analyzed by (a) HPLC and (b) GC to determine the conversions of PHE (A) and yields of CAT + HQ + BQ ( ), CAT ( ), and BQ (T). Aliquots were also analyzed by (c) iodometric titration to determine the conversion of H2O2 (o). [Reproduced by permission of Elsevier from Ma, N. Ma, Z. Yue, Y. H. Gao, Z. J. Mol. Catal. A 2002, 184, 361-370.]...

The equilibrium constants involved in the reaction Fe3+ + 3 cat2" Fe(cat)33- were determined as follows. An aqueous solution of Fe3+ (5.5 X 10-3M) and catechol (1.48 X 10-2M), initially made basic with the addition of KOH, was titrated with 1.24M HC1 under an oxygen-free atmosphere at 22° and ionic strength (KC1) 0.16-0.22M (Figure 12). The acid dissociation constants for catechol were determined independently (under similar experimental conditions) to be pKai = 9.38 and... 

Humic substances, humic and fulvic acids, are essentially a mixture of compounds of different molecular weights. The total number of base-titratable groups is in the range of 10-20 meq per gram of carbon. Chelation by neighboring carboxyl and phenolic groups is the major mode of metal complexation. Compounds such as malonic acid, phthalic acid, salicylic acid, and catechol serve as convenient monomeric model compounds for estimating the coordi-native properties of humic substances. 

Substantial efforts have been devoted to the development of molecular sensors for dopamine. Raymo et al.70 reported a two-step procedure to coat silica particles with fluorescent 2,7-diazapyrenium dications sensing toward dopamine. The analysis of the fluorescence decay with multiple-equilibria binding model revealed that the electron deficient dications and the electron-rich analytes form 1 1 and 1 2 complexes at the particle/water interface. The interfacial dissociation constants of the 1 1 complexes were 5.6mM and 3.6mM for dopamine and catechol, respectively. Dopamine was dominated by the interaction of its electron-rich dioxyarene fragment with the electron-deficient fluorophore in neutral aqueous environments. Ahn et al.71 reported tripodal oxazoline-based artificial receptors, capable of providing a preorganized hydrophobic environment by rational design, which mimics a hydrophobic pocket predicted for a human D2 receptor. A moderate binding affinity, a dissociation constant of 8.2 mM was obtained by NMR titrations of tripodal oxazoline-based artificial receptor with dopamine in a phosphate buffer solution (pH 7.0). Structurally related ammonium ions, norepinephrine, 2-phenylethylamine,... 

Copper complexation was evaluated by ligand exchange with catechol and cathodic stripping voltammetry of the copper-catechol complexes (28, 29). Titration curves with Cu were obtained by spiking aliquots of lake-water samples with different Cu concentrations. The free [Cu2 ] concentration was calculated from the concentration of copper-catechol complexes formed in equilibrium with free catechol. 

Ternary complex formation is inferred from pH-titration data on the system VO picolinic acid-dihydroxy-compound (dihydroxy-compound = catechol, tiron, or chromotropic acid). ... 

Tert-butyl catechol is a weakly acidic molecule with a pKj value of 11.4(6) and hence basic alumina should favour its adsorption by assuming simple acid-base interactions. Hence, we measured the basicity of alumina. The change in surface chemistry due modification of alumina surface with alkali and acid can be measured by two methods, namely (1) Adsorption /Desorption of Gaseous acid molecule (CO2) (2) Benzoic acid titration method... 

The CSV titrations were performed using a BAS 100 electrochemical system and an EG G Princeton Applied Research 303A static mercury drop electrode. The procedure was essentially that of Van den Berg (18) except that the catechol concentration was 1 x 10 M. 

When pyrocatechase was titrated with the substrate catechol under anaerobic conditions, about 1.6 moles of catechol per mole of enzyme were required to bring about maximum increase in absorbance at 710 m/x, suggesting that 2 moles of catechol can combine with the enzyme (Figure 4 A). Similarly, when protocatechuate 3,4-dioxygenase was titrated with protocatechualdehyde, a competitive inhibitor, under aerobic conditions, approximately 8 moles of the compound per mole of enzyme were required to cause a maximal increase in absorption, suggesting that 8 moles of substrate can combine with the enzyme (Figure 4 B). 

A. 2 X 10 M pyrocatechase in 0.05M tris-HCl buffer, pH 8.0, titrated with catechol under anaerobic conditions... 

Titrations were performed as above except that 2 ymoles of ferric chloride and 3 Pmoles of rhodotorulic acid were mixed and neutralized prior to introduction of 2 ymoles of catechol type siderophore dissolved in ethanol. The eq.tiivalents of standard alkali required to neutralize the solution were then noted. 

A titration vial was charged with 0.5 nil ethanol, 2 pmoles of catechol type siderophore in 0.2 ml ethanol, 2 ymoles of FeCl, in 0.1 ml methanol and 0.5 ml of water. The pH was brought to about 7 with standard 0.1 N NaOH and 0.2 ml containing 2 ymoles of competing catechol type siderophore was then added. If necessary, the pH was readjusted to 7. 

Stock 10 mM solutions of enterobactin and a jrobactin in ethanol were prepared by dissolving, respectively, 3.736 mg in 0.56 ml and 3. 21 mg in 0.5 + ml. The ferric chloride was dissolved in methanol to give a 20 mM solution. A titration vial was loaded with 0.5 ml ethanol, 0.2 ml (2 ymoles) catechol solution, 0.1 ml (2 ymoles) ferric chloride solution and 0.5 n+1 water. The pH was raised to ca. 7 by addition of 0.1 N NaOH, at which point exactly 0.2 ml (2 ymoles) of solution of competing catechol was added and, when the latter was enterobactin, the pH readjusted to -7. The neutral solutions were sealed and stored overnight at room ten jerature to assure equilibration, although the latter appeared to be achieved immediately. 

The formation constants of the ferric complexes of these synthetic catecholate ligands have been determined spectrophotometri-cally by competition with EDTA, as described above for entero-bactln. The first three (most acidic) ligand protonation constants have been determined by potentlometrlc titration of the free ligand. The second, more basic, set of protonation constants are too large to be determined readily potentlometrically. Thus the proton-dependent stability constant is expressed as... 

The reaction of silica with catechol, pyrocatechol, and 2,3-naphthalenediol has been studied by several investigators (158-162), but Bartels and Erlenmeyer appear to have been the only ones to use this reaction to characterize the rate of depolymerization of silica (163a). For example, monomeric silicic acid from ethyl silicate in a standard solution of catechol in 0.8 N HCl reacted rapidly and could be titrated to a constant pH of 8.5 with an equivalent amount of standard NaOH solution in a few minutes. An equivalent amount of silica gel required 2.5 hr, but ignited gel reacted only slightly in 5 hr. The rate of reaction, followed by a constant pH titration, provides a way to estimate the relative degree of polymerization of silica or possibly the specific surface area. 

A chemoenzymatic way to produce poly(hydroquinone) was achieved by enzymatic oxidative polymerization of 4-hydroxyphenyl benzoate, followed by alkahne hydrolysis of the resulting polymer [45]. HRP and SBP were used as enzymes. The molecular weight of the resulting poly(4-hydroxyphenyl benzoate) varied between 1100 and 2400 g/mol. The structure was said to consist of phenylene and oxyphenylene moieties, which was found by IR analysis and titration of the residual amount of phenolic groups in the polymer. Other phenol polymers have shown their potential for electronic applications as well. Besides hydroquinone, catechol has also been used as substrate for peroxidase-catalyzed polymerization. The molecular weights of the reac-... 

To 100 ml of a neutral solution containing about 20 mg of manganese add a small amount of hydroxylamine hydrochloride to prevent oxidation of the manganese. Then add 10 ml of ammonia buffer solution followed by 3 to 5 drops of catechol violet indicator and titrate immediately with 0 01 M EDTA until the colour changes from greenish-blue to reddish-purple. 1 ml O OIM EDTA = 0 0005493 g manganese. 

There are now many metallochromic indicators which may be used in complexometric titrations. For successful end-point indication the pK value of the metal-dye complex should be at least 4 units less than that of the metal-EDTA complex which is formed during the titration if the difference is less than 4 units the titrant is unable to compete satisfactorily with the indicator dye and a sluggish end-point results. The indicators which are most widely used in the methods described in this book are solochrome black (for titrations carried out in ammoniacal solution), xylenol orange (for titrations carried out between pH 4 and 7) and catechol violet (for titration in mineral acid solution at pH 1 to 2). Table 63 lists the various indicators called for in methods in the monographs. Discussion of the merits of different indicators for specific purposes will be found in various parts of the book. 

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