Synthesis and spectrophotometric studies of water-soluble amino[bis(ethanesulfonate)] azobenzene pH indicators

Three azobenzene pH indicators with amino[bis(ethanesulfonate)] substituents were synthesized and studied by UV–visible absorption spectroscopy in aqueous solution. The indicators exhibit brilliant and distinct colour changes with transitions between pH 1 and 4. Significant changes were observed in the UV–visible spectra on titration with acid with pK_a values ranging from 2.2 to 2.8. The indicators demonstrate individual changes in colour as a function of pH. These novel pH indicators complement the existing library of azobenzene indicator dyes and may be useful for environmental situations with high proton concentrations.     

A comparative study of some hydroxyanthraquinones as acid-base indicators

Alizarin, Alizarin S, quinizarin and quinalizarin have been compared as acid-base indicators with Bromocresol Green and Methyl Orange as reference indicators. The chromaticity co-ordinates, complementary chromaticity co-ordinates, pK(a) values, transition pH-range, pH of maximum colour change, optimum concentration for titrations and the quality of colour change were determined. The results show all four to be good indicators with a colour change quality similar to that of Bromocresol Green.     

Colour changes of chemical indicators-III: colour specification and its accuracy

The systematic deviations involved in methods for computation of chromaticity co-ordinates have been studied for the specification of indicator colour changes. The weighted ordinale method (Deltalambda = 10 nm) has proved to be satisfactory for evaluation both of C.I.E. and complementary co-ordinates. The C.I.E. colour specification should be included among basic data about chemical indicators.     

Acid-base indicators in acetonitrile: Their pK values and chromatic parameters

A series of acid—base indicators in acetonitrile has been studied. Their chromaticity coordinates, pK_a values, transition pH ranges, pH of maximum colour change, optimum concentration for titration, and quality of colour change in acetonitrile have been determined, together with the effect of ionic strength on their properties. Various bases and their mixtures have been titrated to test the practical usefulness of the indicators.     

A comparative study of the behaviour of Crystal Violet,alizarin-9-imine and quinalizarin-9-imine as indicators in anhydrous acetic acid medium

A comparative study has been made of the behaviour of alzarin-9-imine, quinalizarin-9-imine and the widely used Crystal Violet as indicators in anhydrous acetic acid medium. The protonation constants of quinalizarin-9-imine, and the properties required by a base if it is to be titratable in acetic acid medium with use of the indicators studied, the chromaticity co-ordinates, optimal concentration for titration and the quality of the colour changes have been determined. The results obtained show Crystal Violet and alizarin-9-imine to be very good indicators in acetic acid medium but alizarin-9-imine gives better end-point location. Quinalizarin-9-imine is also a good indicator but exhibits poorer colour change quality than the other two.     

H/P NMR pH indicator series to eliminate the glass electrode in NMR spectroscopic pKa determinations

NMR titration is an efficient method to determine pK_a values of multiprotic acids in aqueous solution. While modern 1D/2D NMR techniques yield chemical shifts with increasing precision, the glass electrode-based pH measurement becomes the limiting factor to affect the precision of the resulting dissociation constants. The pH in the NMR tube can also be deduced from the actual chemical shift of an appropriate monoprotic indicator molecule. In the present work, the in situ NMR pH measurement has been extended for the entire pH range 0-12 using indicators with overlapping ranges of dissociation. In the first, calibrating ¹H/³¹P NMR titration, limiting chemical shifts and pK were determined for each indicator. An analysis of error propagation showed that the accuracy and precision of glass electrodes can be achieved at 1.8 < pH < 12 and even exceeded at pH extremes by NMR indicators, respectively. The assembled set of indicators was applied for in situ pH monitoring in the following “electrodeless” ¹H/³¹P NMR titration of a newly synthesized aminophosphinophosphonic acid. Multivariate nonlinear parameter estimation was used to calculate the pK values that were confirmed by potentiometric titrations.     

Colour specification of pyridine-2-aldehyde and 6-methylpyridine-2-aldehyde p-nitrophenylhydrazones as indicators for pH determination

The physicochemical properties, chromaticity co-ordinates and chromatic parameters for pyridine-2-aldehyde p-nitrophenylhydrazone and 6-methylpyridine-2-aldehyde p-nitrophenylhydrazone (proposed as new indicators for calorimetric pH measurements) are reported. The sensitivity and rapidity of colour change have been evaluated in terms of specific colour discrimination (SCD), pH of maximum colour change and half band-width of change of SCD (in terms of pH). The chromatic separations calculated from the co-ordinates obtained by the CIE 1976 recommendations (CIELUV or CIELAB) and the LABHNU 1977 colour space are compared and correlated with the standard deviation of colour matching.     

Spectrophotometric and potentiometric study of the acid—base equilibria of indophenols in aqueous media

Six indophenols, with redox and acid—base indicator properties, have been examined by spectrophotometric and potentiometric methods. By analysing the absorption spectra obtained at different pH values, three independent values were obtained for their K_Ox dissociation constants, which are closely related to their properties as acid—base indicators. Three of the indophenols have also been examined by acid—base and redox potentiometric titrations. All K_Ox and E° values agree well with polarographic values. The use of these substances as visual acid—base and redox indicators is discussed.     

Dynamic method as a simple approach for wide range pH measurements using optodes

In this paper, a flow system equipped with an optode has been suggested for wide range pH measurements. Triacetyl cellulose was used as the optode membrane in which different pH indicators were immobilized. For extending the pH range, the dynamic response rather than the steady-state response of the optode was measured. Since diffusion is the main process governing the system response, different parameters having influence on diffusion of the analyte into the membrane were optimized. Under the optimum conditions, wide range pH determination (up to 11 pH units) is simply achieved regardless of the pK_a of the pH indicator immobilized in the membrane. To validate the application of the method different indicators with different structures and pK_a values were tested and the results were all confirming the precision and accuracy of the method. The suggested method also has combined advantages of flow systems together with inherent advantages of kinetic systems.     

Critical study of metallochromic indicators for calcium

The visual clarity of the colour changes at the end-points in the compleximetric titration of calcium is discussed for the metallochromic indicators, arsenazo-III, calcein, calcon, eriochrome blue black B, methylthymol blue, murexide, phthalein complexone, and thymolphthalein complexone. The colour changes were specified with the help of CIE chromaticity diagrams. The sharpness of the colour changes of the indicators was studied by varying the optical (or colour) concentration. Thymolphtalein complexone and phthalein complexone are shown to be the most appropriate indicators for the visual titration of calcium with EDTA.     

Study of semicarbazones and thiosemicarbazones derived from 1,2-naphthoquinone, as acid-base indicators: Evaluation of their transition limits through the chromaticity co-ordinates

The use of 1,2-naphthoquinone-2-semicarbazone, 1,2-naphthoquinone-2-semicarbzone-4-sulphonic acid and 1,2-naphthoquinone-2-thiosemicarbazone-4-sulphonic acid as acid—base indicators has been studied. The sharpness of the indicator transitions has been investigated by means of photometric titrations and the colour quality specified with the aid of the CIE chromaticity system. The results show that the three substances are satisfactory as neutralization indicators.     

Studies on adsorption indicators : p-ethoxychrysoidine as adsorption indicator

p-Ethoxychrysoidine is studied as adsorption indicator in the direct titration of silver with iodide, bromide, thiocyanate and chloride at pH 4–5. The explanation given by Schulek and Pungor for the colour changes has been modified in the light of compound formation on the surface of the precipitate. The silver compound of p-ethoxychrysoidine has been isolated ; its behaviour provides a suitable explanation for the colour changes as well as the pH changes in the titrations.     

Acid-base indicators in N,N-dimethylformamide: chromatic parameters of transition ranges

A series of commercial indicators in N,N-dimethylformamide has been established. Their chromaticity coordinates, transition pH ranges, pH of maximum colour change, optimum concentration for titration, quality of colour change and the effect of ionic strength have been determined. A variety of acids and their mixtures have been titrated to test the indicators.     

Lanthanide doped carbon dots as a fluorescence chromaticity-based pH probe

A colorimetric and fluorescent pH probe was designed by doping carbon dots (C-dots) with Eu(III), Tb(III) and 2,6-pyridinedicarboxylic acid (DPA). The resulting nanoparticles were applied as fluorescent indicators for pH values (best detected at excitation/emission wavelengths of 272/545, 614 nm). The pH induced optical effects are due to pH induced variations in energy transfer. The fluorescence of the probe shows a continuous color variation, and a linear change with pH values in the range from 3.0 to 10.0 can be established by using a Commission Internationale de L’Eclairage (CIE) chromaticity diagram. This new kind of pH nanoprobe is more accurate than previously reported pH indicator probes because the pH value can be calculated by using chromaticity coordinates that only depend on the chromaticity. The pH nanoprobe was applied to visualize pH values in human breast adenocarcinoma cells (MCF-7).

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Graphical abstract Carbon dots modified with Eu(III) and Tb(III) complexes of 2,6-pyridinedicarboxylic acid (DPA) were prepared. The doped carbon dots were used as a pH-sensitive nanosensor. The fluorescence chromaticity of the nanoparticles changes with the variation of pH value.

     

A digital image-based method for determining of total acidity in red wines using acid-base titration without indicator

This work proposes the use of digital image-based method for determination of total acidity in red wines by means of acid-base titration without using an external indicator or any pre-treatment of the sample. Digital images present the colour of the emergent radiation which is complementary to the radiation absorbed by anthocyanines present in wines. Anthocyanines change colour depending on the pH of the medium, and from the variation of colour in the images obtained during titration, the end point can be localized with accuracy and precision. RGB-based values were employed to build titration curves, and end points were localized by second derivative curves. The official method recommends potentiometric titration with a NaOH standard solution, and sample dilution until the pH reaches 8.2-8.4. In order to illustrate the feasibility of the proposed method, titrations of ten red wines were carried out. Results were compared with the reference method, and no statistically significant difference was observed between the results by applying the paired t-test at the 95% confidence level. The proposed method yielded more precise results than the official method. This is due to the trivariate nature of the measurements (RGB), associated with digital images.     

Ruthenium and chromium complexes bearing pH-indicators as the η6-arene ligand: Synthesis,characterization, and protonation behavior

A series of ruthenium and chromium complexes bearing pH indicators as the η⁶-arene ligand, (η⁶-X)(ML_n)_y [X = methyl yellow, crystal violet lactone, phenolphthalein; ML_n = RuCp¹⁺, RuCl₂(L), Cr(CO)₃; y = 1, 2] is prepared and characterized by spectroscopic and crystallographic methods. Of the plural arene rings in the indicators, a specific arene ring can be successfully coordinated to the metal center in a selective manner under appropriate conditions (i.e. use of the precursors of different oxidation states and reaction with the non-protonated and protonated pH indicator). The obtained indicator complexes show halochromic behavior depending on pH as observed for the parent molecules but the transition pH ranges are shifted to the more acidic side because of the attachment of the electron-withdrawing metal fragments, which decrease the basicity of the attached pH indicators.     

Rapid titrimetric determination of microgram amounts of fluoride ion with spadns -thorium lake

A rapid titrimetric method has been developed for the determination of microgram amounts of fluoride ion in the range from Img-Ioomg, in 50 ml final volume. It involves the adjustment of pH, addition of 1 ml of 0.02% SPADNS indicator, dilution to volume and titration with standard 0.004M Th(NO₃)₄ until the colour obtained matches a blank containing the buffered solution of the indicator with a. trace of thorium nitrate solution. Interference by various ions was also studied. The method described for the determination of fluoride ion is very rapid and the colour change at the end point being sharp, the detection of the end point is very easy. The method is applicable to pure solutions of fluondes.     

Spectrophotometric investigations on immobilized acid-base indicators

Diffuse reflectance spectra of four acid-base indicators immobilized on Amberlite XAD-2 resin are reported as a function of pH. Adsorption modifies the acid-base properties of the indicator by broadening the pH intervals over which the dyes change colour and by shifting these intervals to higher pH values. The different behaviour in the liquid and immobilized phases may be explained by multilayered adsorption of some indicators. The occurrence of hysteresis during a pH cycle is reported.     

Colour changes of chemical indicators-VI: metallochromic indicators for direct chelometric titrations of zinc

Eriochrome Blue SE, Eriochrome Red B, Naphthylazoxine 6S, SNAZOXS, and Zincon have been studied in order to find optimum conditions for their use as metallochromic indicators in direct visual chelometric titrations of zinc. The sharpness of the indicator transitions has been investigated by means of photometric titrations and the colour quality has been specified with the aid of the C.I.E. chromaticity systems. Zincon and Eriochrome Blue SE have been found to be the most convenient for visual titrations of zinc.     

Shedding light on predicting and controlling emission chromaticity in multicomponent photoluminescent systems

Predictable colour tuning in multicomponent photoluminescent (PL) systems is achieved using mixtures of simultaneously emitting organic molecules. By mitigating the potential for energy transfer through the control of concentration, the resulting emission chromaticity of five dichromic PL systems is approximated as a linear combination of the emitting components and their corresponding brightness (χ_i, ϕ_i, and I_ex,i). Despite being limited to dilute solutions (10⁻⁶ M), colour tuning within these systems was controlled by (1) varying the composition of the components and (2) exploiting the differences in the components'' excitation intensities at common wavelengths. Using this approach, white light emission (WLE) was realized using a pre-determined mixture of red, green, and blue emitting organic molecules. Based on these results, materials and devices with built-in or programmable emission colour can be achieved, including highly sought-after WLE.

Predictable colour tuning in multicomponent photoluminescent (PL) systems is achieved using mixtures of simultaneously emitting organic molecules.