Spectrophotometric analysis of simultaneous protolysis equilibria

Simultaneous protolysis equilibria can be spectrophotometrically analysed easily by means of absorbance diagrams. The dissociation constants K(1) and K(2) can be determined directly from distinct points on the absorbance diagram by correlation with a photometric titration plot of absorbance vs. pH. Furthermore, from the absorbance diagram the ratio K (2)K (1) can be evaluated from the ratio of corresponding sides, and pH-measurement is not necessary. The titration systems of a mixture of 2-nitroso-l-naphthol-4-sulphonic acid with P-nitrophenol, and a mixture of P-nitrophenol with o-nitrophenol have been analysed by these methods, and are described as examples.     

Spectrophotometric determination of aqueous acidity constants of three azo dyes

The two successive steps acidity constants of three azo dye derivatives D1–D3, were determined by a spectrophotometric method using 0.10 M KNO₃ as supporting electrolyte in water solvent. The electronic spectra of the compounds were recorded in the course of their pH-metric titration with a standard base solution. The protolytic equilibrium constants can be solved very satisfactorily from absorbance data, by combining the data obtained in the spectrometric determinations with graphic methods as the absorbance diagrams. An A-diagram shows the relative absorbance changes at two wavelengths as a function of the pH. For a one-step system, the absorbance at any wavelength must be proportional to the absorbance at any other wavelength, so an A-diagram for all wavelength combinations for such a system must be linear (rank, s=1). However, if a system is governed by two or more equilibria, the A-diagrams corresponding to multi-step titrations will change direction every time a new equilibrium becomes dominant in the system. Analysis of the each uniform sub-region can then be used in evaluating of the corresponding equilibrium. A-diagrams for all wavelength combinations suggest that these systems have the rank two(s=2). The results revealed that the K_a(1) and K_a(2) values of different azo dyes follow the order D3>D2>D1.      

Mathematical treatment of absorbance versus pH graphs of polybasic acids

Mathematical relationships determining the shape of absorbance vs. pH graphs of polybasic acids are derived, and new graphical and numerical techniques for evaluating the acidity constants of two-step overlapping equilibria are examined.     

Determination of chromophore charge states in the low pH color transition of the fluorescent protein Rtms5 via time-dependent DFT

The red fluorescent protein Rtms5^H146S displays a transition from blue (absorbance λ_max 590 nm) to yellow (absorbance λ_max 453 nm) upon titration to low pH. The pK_a of the reaction depends on the concentration of halide, offering promise for new expressible halide sensors. The protonation state involved in the low pH form of the chromophore remains, however, ambiguous. We report calculated excitation energies of different protonation states of an RFP chromophore model. These suggest that the relevant titration site is the phenoxy moiety of the chromophore, and the relevant base and conjugate acid are anionic and neutral chromophore species, respectively.     

Determination of individual stability constants of coexisting molecular complexes formed by unresolvable isomers

Some biogenic substances undergo steric isomerism or tautomerism in which the isomers are unresolvable. Naturally, the isomers are expected to form molecular complexes with distinct stabilities. This paper proposes the method of determining individually the stability constants of complexes formed under isomerisation and complexation equilibria: the NMR titration curves are modelled and formulated for fast and slow isomeric equilibria, the process of least-squares fittings is explained, and the application to supramolecular complexation of unresolvable isomers is discussed as compared with NMR titration data reported for α- and β-D-glucosamine anomers.     

Ultraviolet absorbance titration for determining stability constants of humic substances with Cu(II) and Hg(II)

We describe an ultraviolet (UV) absorbance titration method that can be used to determine complexing capacities (C_L) and conditional stability constants (log K) of humic substances (HSs) with metal ions such as Cu(II) and Hg(II). Two fulvic acids (FA) and one humic acid (HA) were used for this study. UV absorbance of HSs gradually increased with the addition of Cu(II) or Hg(II) after blank correction, and these increases followed the theoretical 1:1 (ligand:metal ion) binding model. The results from the absorbance titration calculation for HSs with Cu(II) and Hg(II) compared well with those from fluorescence quenching titration. The titration of the model compound l-tyrosine with Cu(II) proved the validity of this method, and the K and C_L were within 2.3% and 7.4% of the fluorescence quenching titration. The results suggest that the UV absorbance titration can be used to study the binding capacities of HSs and/or dissolved organic matter (DOM) with trace metals. The advantages and disadvantages of the absorbance titration method were also discussed.     

Quantitative treatments of protonation equilibria shifts in micellar systems.

The paper reviews the theoretical models and equations used for the interpretation of weak acid dissociation equilibria in the presence of micellar systems. The models and the equations of the literature can be rewritten and reinterpreted in terms of the generalized pseudophase model, for which the basic equations are reported. Some new relationships for the titration of weak acids in micellar systems are presented.     

31P nuclear magnetic resonance titrations: Simultaneous evaluation of all pH-dependent resonance signals

The simultaneous evaluation of ali pH-dependent resonance signals (or multiplets) of an NMR titration offers a substantially increased accuracy and significance. The number of linearly independent titration equilibria is determined by graphical matrix rank analysis. The chemical shifts of all pH-dependent resonance lines are plotted against each other (chemical shift or CS diagrams) indicating whether a single or more titration equilibria are NMR spectrometrically observable and how far they overlap with each other. An iterative curve-fitting program allowing the simultaneous evaluation of all (pH) curves is available, from which pK values and chemical shifts of all species can be calculated. The starting pK values for the iteration need only be estimated very approximately (accuracy ±1–2 units). The titration end-points do not have to be experimentally accessible. The different methods for the simultaneous evaluation of all pH-dependent NMR signals are exemplified in the ³¹P NMR titration of thiamine pyrophosphate. In this case either the observed resonance lines (two doublets in a broad band proton decoupled spectrum) or the calculated chemical shifts for this AB system can be evaluated. A titration of sodium pyrophosphate was performed and evaluated for comparison.     

Spektralphotometrische charakterisierung von ternären komplexen in lösung durch indirekte analyse mit hilfe linearer transformationen

Mathematical transformations are presented which enable one to characterize mononuclear ternary complexes in solution from experimental measurements of the absorbance as a function of the ligand concentration or the pH. In addition to getting the molar ratios and the proton balance, one can also find values for the formation constants and the molar absorption coefficients. Both simple equilibria and substitution equilibria between binary and ternary complexes are considered, with the proviso that only one equilibrium system is involved over the concentration range covered. Suggestions are given as to how the evaluation may be carried out in practice, and factors to be considered when using the transformations are pointed out.     

Determination of carbonate and hydrogencarbonate by titration using sequential injection analysis

A fully automated spectrophotometric sequential injection titration system has been developed for the determination of carbonate and hydrogencarbonate in water samples. The sample is titrated against hydrochloric acid using phenolphthalein and methyl orange as indicators; the absorbance is measured at 430 nm. A logarithmic relationship is obtained between the peak width and the absorbance. The logarithmic range for carbonate is 0.0008-0.01 M, and for hydrogencarbonate is 0.001-0.01 M. Reproducibility is good (<2% for carbonate and <1.5% for hydrogencarbonate) and the results obtained agree well with those obtained using a potentiometric titration. The sample throughput is 12 samples h⁻¹.     

Existence field of the σ phase of the Co-Cr-Ni-V quaternary at 800°C

We studied phase equilibria in the Co-Cr-Ni-V system at 800°C by means of microstructure examination, X-ray powder diffraction, and electron probe microanalysis. We also constructed sections through the Co-Cr-Ni-V isothermal tetrahedron containing 10 and 20 at % vanadium.     

Investigation of the United stable tetrahedron LiF-KBr-K2MoO4-KF of the quaternary reciprocal system Li,K‖F,Br,MoO4

The united stable tetrahedron LiF-KBr-K₂MoO₄-KF of the quaternary reciprocal system Li,K‖F,Br,MoO₄ was studied by differential thermal analysis. The melting points and ternary eutectic and peritectic compositions in the secant triangle LiF-KBr-K₃FMoO₄ were identified. The melting points of the samples corresponding to the quaternary invariant equilibrium points (eutectic and peritectic points) and their compositions in the stable tetrahedron LiF-KBr-K₃FMoO₄-KF were determined. The volumes of crystallizing phases in the united stable tetrahedron were determined; the phase equilibria at invariant points were described.     

Application of principal component analysis to the study of multiple equilibria systems : Study of copper(II)/salicylate/mono-, di- and triethanolamine systems

Principal component analysis is applied to the study of multiple equilibria systems based on copper (Il) ion with salicylate and mono-, di- or triethanolamine as ligands. A procedure based on evolving factor analysis is described which allows the model-free determination of distribution plots and of the individual spectra of the various species formed in the course of a spectrophotometric titration. The results are compared with those obtained by using least-squares methods.     

Phase and extraction equilibria in the water—ethoxylated nonylphenol—sodium sulfate system

Russian Chemical Bulletin - Phase equilibria in the water-ethoxylated nonylphenol (neonol AF 9–12)-sodium sulfate system were studied at 25 °C using isothermal titration and isothermal...     

Process monitored spectrophotometric titration coupled with chemometrics for simultaneous determination of mixtures of weak acids

A new spectrophotometric titration method coupled with chemometrics for the simultaneous determination of mixtures of weak acids has been developed. In this method, the titrant is a mixture of sodium hydroxide and an acid-base indicator, and the indicator is used to monitor the titration process. In a process of titration, both the added volume of titrant and the solution acidity at each titration point can be obtained simultaneously from an absorption spectrum by least square algorithm, and then the concentration of each component in the mixture can be obtained from the titration curves by principal component regression. The method only needs the information of absorbance spectra to obtain the analytical results, and is free of volumetric measurements. The analyses are independent of titration end point and do not need the accurate values of dissociation constants of the indicator and the acids. The method has been applied to the simultaneous determination of the mixtures of benzoic acid and salicylic acid, and the mixtures of phenol, o-chlorophenol and p-chlorophenol with satisfactory results.     

Stable tetrahedron LiF-KCl-KBr-K2MoO4 of the quinary reciprocal system Li,K‖F,Cl,Br,MoO4

The stable tetrahedron LiF-KCl-KBr-K₂MoO₄ of the quinary reciprocal system Li,K‖F,Cl,Br,MoO₄ was studied by differential thermal analysis, and phase equilibria were determined.     

Investigation of equilibria in solution. Determination of equilibrium constants with the HYPERQUAD suite of programs

A new suite of 10 programs concerned with equilibrium constants and solution equilibria is described. The suite includes data preparation programs, pretreatment programs, equilibrium constant refinement and post-run analysis. Data preparation is facilitated by a customized data editor. The pretreatment programs include manual trial and error data fitting, speciation diagrams, end-point determination, absorbance error determination, spectral baseline corrections, factor analysis and determination of molar absorbance spectra. Equilibrium constants can be determined from potentiometric data and/or spectrophotometric data. A new data structure is also described in which information on the model and on experimental measurements are kept in separate files.     

Phase equilibria in the stable tetrahedron LiF–KF–KBr–Li<Subscript>2</Subscript>CrO<Subscript>4</Subscript> of the quaternary reciprocal system Li,K||F,Br,CrO<Subscript>4</Subscript>

Phase equilibria in the stable tetrahedron LiF–KF–KBr–Li₂CrO₄ of the quaternary reciprocal system Li,K||F,Br,CrO₄ were studied by differential thermal analysis. The characteristics of a quaternary eutectic were found.     

Some studies on the determination of indicator transition and its use in the titration of weak basic samples

In the titration of a weak basic sample, especially in the dilute solution, the transition color of the indicator is not sharp at the equivalence point. In order to avoid the indicator error, the color transition near the equivalence point must be known accurately. The color transition of an acid-base indicator can be calculated from complementary tristimulus data. To ensure accuracy, however, the chemical stoichiometric relationships about the acid-base equilibria are considered. A general expression for determination of the equivalence point is presented and factors influencing the titration are discussed.     

A note on the application of significance tests based on the R-factor ratio to the selection of species in complex-formation equilibria

The use of the significance test based on the R-factor ratio for the selection of species in complex formation equilibria studied by least-squares analysis of potentiometric titration data is discussed.