Acid-base Titration Experiments Research Articles

Development of Acid-Base Titration Practicum Guide Using Natural Indicators of Butterfly Pea Flowers

Sustainable Development Goals (SDGs) need to be implemented to create environmentally friendly chemistry learning. This can be done in an acid-base titration experiment using natural indicators of butterfly pea flowers. This research aims to produce a practicum guide for acid-base titrations using natural indicators of butterfly pea flowers that are valid and practicum. The research design uses the ADDIE research design, namely Analysis, Design, Development, Implementation, and Evaluation. The research results showed that butterfly pea flowers can be used as a natural indicator for acid-base titrations with color changes from pink (acidic condition) to blue (basic condition). The resulting learning media is a practicum guide which has a validity of 92% (very valid) and a practicality level of 88.75% (very practical). This data was analyzed using qualitative and quantitative analysis. The practicum guide developed is valid and practical to use as a practicum learning media in schools.

Characterizing water-soluble brown carbon in fine particles in four typical cities in northwestern China during wintertime: integrating optical properties with chemical processes

Abstract. Brown carbon (BrC) aerosol could impact atmospheric radiative forcing and play a crucial role in atmospheric photochemistry. In this study, fine particulate matter (PM2.5) filter samples were collected synchronously in four major cities in northwestern China during the winter season (December 2019–January 2020): Lanzhou (LZ), Xining (XN), Yinchuan (YC), and Ürümqi (UR), which are represented as energy-producing and heavy manufacturing cities in China. The primary aim of the study is to explore the optical properties, sources, and chemical processes of water-soluble BrC (WS-BrC). The average mass absorption efficiency at 365 nm (MAE365) of WS-BrC at these four cities was 1.24 ± 0.19 m2 g−1 (XN), 1.19 ± 0.12 m2 g−1 (LZ), 1.07 ± 0.23 m2 g−1 (YC), and 0.78 ± 0.16 m2 g−1 (UR). The properties of WS-BrC were further investigated by an acid–base titration experiment. The results showed that the MAE365 values in all cities increased with higher pH values (2–11), while the fluorescence intensities of water extracts fluctuated with pH values, being stronger under both highly acidic and basic conditions. The sensitivity to pH variation was most pronounced in the WS-BrC samples from YC and LZ, indicating the important contribution of acid or base functional group compounds in these locations. Additionally, the study revealed significant photo-enhancement (LZ) or photo-bleaching (YC and UR) phenomena of WS-BrC in different cities. These results suggest that the sources and/or chemical processes of WS-BrC varied among the cities. The sources and chemical processes of WS-BrC were further explored by a combination of parallel factor analysis (PARAFAC) on excitation–emission matrix (EEM) spectra of WS-BrC and positive matrix factorization analysis (PMF) on high-resolution mass spectra of water-soluble organic aerosol (WSOA). Six PARAFAC components were identified, including three humic-like substances (HULIS; two less oxygenated (LO) HULIS and one highly oxygenated (HO) HULIS), two protein-like or phenol-like substances (PLS), and one undefined substance. Four PMF factors were identified, including a water-soluble primary OA (WS-POA), a less oxidized oxygenated OA associated with coal combustion-induced WSOA (LO-OOA), and two highly oxidized oxygenated OAs resulting from photochemical oxidation and aqueous-phase oxidation transformations (HO-OOA1 and HO-OOA2). WS-POA was determined to be the most significant source of light absorption, accounting for 30 %–60 % based on multiple linear regression models, and it showed a significant correlation with PLS and LO-HULIS components. The loss of light absorption of WS-POA was found to occur through its conversion to LO-OOA and HO-OOAs through photochemical or aqueous reactions, with HO-OOAs being significantly correlated with the HO-HULIS component. These processes can be clearly illustrated by integrating optical properties and chemical composition using a Van Krevelen diagram and an EEM plot.

Potential Analysis of Boat Lily Leaf Extract (Rhoeo spathaceae (Sw.) Stearn) as an Alternative Indicator in Acid-Base Titration of Acid-Base Learning

The acid-base titration experiments at Senior High Schools typically use phenolphthalein and methyl orange indicators, which are relatively expensive and harmful to the environment. One of the plants containing anthocyanin pigments is boat lily (Rhoeo spathaceae (Sw.) Stearn) leaves. The research aims to determine the potential analysis of boat lily leaf extract as an alternative indicator used in chemistry learning in titration acid-base learning. The type of research used in this study is descriptive qualitative. Based on the research and discussion results, the pedagogical aspects carried out by chemistry teachers obtained results that using boat lily leaf extract indicators in acid-base titration in the acid-base chapter is appropriate and meets pedagogical terms to be applied by chemistry teachers. A review of chemistry teachers, laboratory employees, and students obtained results that the use of boat lily leaf extract indicators in acid-base titration in the acid-base chapter can be approved because it meets the aspects of ease to be applied in learning, understanding, and attractive to students, and ease of being provided in the laboratory. Based on the feasibility analysis of potential experiments, boat lily leaf extract is feasible based on financial aspects and shelf life resilience.

Coupling-to-diazonium role-convert strategy for 1,3-dimethylbarbituric acid-based dyes and configurational confirmation via NMR/UV–vis spectra

5-Amino-1,3-dimethlybarbituric acid (5-amino-diMeBA), prepared form 1,3-dimethlybarbituric acid (diMeBA) via the functional group transformation strategy, is first used as the diazonium component to produce heterocyclic dyes, in order to expand the existing research objects in this area. Role-convert synthesis has been successfully achieved from a general coupling component diMeBA to a corresponding diazonium unit 5-amnio-diMeBA, and three pyrazolone/pyridone coupled bi-heterocyclic dyes were yielded. There are multiple possible azo/hydrazone tautomers for them since three ortho-carbonyl groups could serve as the protonated sites for active protons. The configurational confirmation depends on the precise location of heterocyclic hydroxyl groups, which could be deduced by comparing the alterations of two characteristic methyl single peaks in diMeBA moiety in the 1H NMR spectra before and after deprotonation. Our results reveal that three bi-heterocyclic dyes adopt the monohydrazone form in pyrazolone with 4-OH-diMeBA and the monohydrazone form in diMeBA with 6-OH-pyridone, respectively. Further UV–Vis spectra have been carried out to compare the delocalized π-conjugated system for them, together with three BA coupled heterocyclic dyes. In addition, solvatochromic and acid-base titration experiments were performed to verify the strength of intramolecular charge transfer within the whole molecules and the monohydrazone configuration for every bi-heterocyclic dye.

Microcomputer-Based Laboratory Role in Developing Students’ Conceptual Understanding in Chemistry: Case of Acid–Base Titration

For the past five years, all Moroccan universities suspended practical work activities in the subject of chemistry for the first year due to a decision made by the Ministry of Higher Education. This study explores the effect of using a Microcomputer-Based Laboratory (MBL) on students’ performance and investigates the desirability to convert first-year university-level practicals in chemistry from experimental demonstrations in lectures to the MBL. The study sample comprised 100 students attending the first year at the faculty of sciences of the Cadi Ayyad University (UCA) in Morocco. Students in the experimental group (N = 50) received a 1 h active engagement tutorial on acid–base titration using a microcomputer-based laboratory, in contrast to the control group (N = 50) who received the same instruction with a demonstration of the experiment. The experimental and control groups got randomly selected, and the same instructor trained both groups. The two groups of responders were compared based on their performance in a written exam that assesses the skills of processing, analyzing, and interpreting qualitative and quantitative experimental results of acid–base titration. As a result, the analysis of the obtained results showed that the experimental group performed significantly better in using appropriate mathematical skills for quantitative data analysis; and they were actively engaged in the acid–base titration experiment. The obtained results, together with similar findings from other studies, indicate the positive impact of using MBL on learning outcomes and support the adoption of the proposed learning environment in laboratory educational procedures as an alternative to traditional laboratories.

Influence of secondary metabolites on surface chemistry and metal adsorption of a devitalized lichen biomonitor.

Despite the broad use of lichens as biomonitors of airborne trace elements, the surface chemistry and metal adsorption parameters of these organisms are still poorly known. The current investigation is aimed at (i) quantifying the acid-base surface properties and the first-order physical-chemical parameters of Cu2+ and Zn2+ adsorption of devitalized Pseudevernia furfuracea, a lichen commonly used in biomonitoring of airborne trace elements, and (ii) comparing the results with those available for moss biomonitors. Equilibrium constants and metal-binding site concentrations were calculated with a thermodynamic model by taking into account the presence/absence of ancillary extracellular cell wall compounds, namely melanin and acetone-soluble lichen substances. An acid-base titration experiment performed in the pH range of 3-10 showed that melanised and non-melanised P.furfuracea samples have lower pHPZC (3.53-3.99) and higher metal-binding site concentrations (0.96-1.20mmolg-1) compared to that of the mosses investigated so far at the same experimental conditions. Melanin biosynthesis increased the content of carboxyl and phosphoryl groups and reduces that of amine/polyphenols. Cu2+ and Zn2+ adsorption was unaffected by the degree of melanisation while the removal of extracellular lichen substances slightly decreased Zn2+ adsorption. Although Cu2+ and Zn2+ adsorption parameters related to P.furfuracea surfaces were 3 times lower than in the mosses, lichen samples adsorbed the same amount of Cu2+ and 30% more Zn2+. The present study contributes in understanding the role of ancillary cell wall compounds in Cu2+ and Zn2+ adsorption in a model lichen. It also provides a first comparison between the surface physico-chemical characteristics of lichens and mosses frequently used as biomonitors of trace elements.

Matching Five White Solids to Common Chemicals: A Dissolution Calorimetry and Acid–Base Titration Experiment

An experiment to match five white solids with ascorbic acid, citric acid, potassium hydroxide, sodium chloride, and urea is reported here. This 4 h experiment is an integrated practice of two experimental techniques, calorimetry and titration. Students conduct an extensive literature search on the properties of the five compounds and plan the experiment before the laboratory hours. Calorimetric results are used to identify potassium hydroxide, sodium chloride, and urea out of the five compounds. Acid–base titration is used to differentiate between the two weak acids and confirm the identity of the only base in this experiment. Additional calculations based on titration results can reveal waters of crystallization in citric acid and potassium hydroxide. The experiment helps students to understand concepts of dissolution, stoichiometry, acid–base chemistry, and thermochemistry in the context of common chemicals found in everyday life.

Configurational alterations and Cu(II)/Ni(II) coordination polymers for an asymmetrical pyrazolone/pyridone dye

In this work, an asymmetrical pyrazolone/pyridone bi-heterocyclic dye was synthesized showing good solubility in both water and organic solvents. This dye has three possible azo/hydrazone tautomers because multiple N and O heteroatoms in its molecular structure could serve as the protonated sites for two active protons. NMR techniques have been used to deduce its specific configurations in different solvents, and the results reveal that it adopts the monohydrazone form in pyridone with 5-OH-pyrazolone in MeOH and double-azo form in water. The following solvatochromic and acid-base titration experiments verify the transfer of proton sites within the multiple heteroatoms for the dye and the configurational alterations in non-aqueous and aqueous solvents. Furthermore, two one-dimensional copper(II) and nickel(II) coordination polymers have been obtained and structurally characterized, in which this asymmetrical pyrazolone/pyridone dye adopts the same depronated double-azo form. However, the two dye-metal complexes show distinguishable supramolecular architectures (layer/stair) due to the different 3d9/3d8 electronic structures for Cu(II)/Ni(II) cations and hence different spacial requirements for the coordination spheres (five-coordinate pyramid/six-coordinate octahedron).

Synthesis of sulfonated chitosan-derived carbon-based catalysts and their applications in the production of 5-hydroxymethylfurfural

A novel sulfonated chitosan-derived carbon-based catalyst was successfully prepared via isoamyl nitrite-assisted sulfanilic acid sulfonation, and its catalytic activity was examined using dehydration of fructose. The structural and chemical properties of sulfonated chitosan-derived carbon were characterized by SEM, FTIR, XRD, XPS, element analysis, N2 adsorption-desorption experiment, and acid-base titration experiment. KOH was used as activating agent in the synthesizing of carbon supports, and it was found that properly increasing the dose of KOH during activation stage had a positive effect on the subsequent sulfonation of prepared activated carbon. 4KSCC, with the highest sulfonation degree (2.04 mmol/g), exhibited high performance for the conversion of fructose to HMF in various solvent, and an optimal HMF yield of 80.9% was obtained at 140 °C in 40 min. In addition, the reusability of 4KSCC for fructose dehydration was fairly good.

Acid-base properties of dissolved organic matter extracted from the marine environment

Marine dissolved organic matter (DOM) plays a key role in the current and future global carbon cycle, which supports life on Earth. Trace metals such as iron, an essential micronutrient, compete with protons and major ions for the binding to DOM. These competitive effects and the DOM binding capacity are related to the DOM acid-base properties, which also influence DOM transport and reactivity in marine waters. Here we present the results of a complete set of acid-base titration experiments of a pre-concentrated marine DOM sample in the range 0.01 ≤ I ≤ 0.7 M and 3 ≤ pH ≤ 10. We characterize the obtained proton binding curves using a combination of the non-ideal competitive adsorption (NICA) isotherm and Donnan electrostatic model. Within the main chemical groups of marine DOM, the carboxylic distribution was accurately characterized from the obtained data (QmaxH, 1=2.52 mol·kg−1, logk¯H,1 = 3.26, m1 = 0.69 and b = 0.70). This carboxylic mode was found to be less acidic and more homogeneous than a generic fulvic acid, but the differences are consistent with the reported variability of fulvic acids of freshwater and terrestrial origin. We find that changes in temperature (down to 5.5 °C), and the presence of calcium or magnesium (at 0.01 M) resulted in no significant modification of the proton ion binding curves obtained at 25 °C and 0.7 or 0.1 M ionic strength, respectively. We demonstrate the relevance of proton binding parameters for the modelling of the system iron/marine DOM throughout a wide range of salinity and acidity conditions in the context of different future ocean scenarios.

5-Hydroxy-1-phenyl-1H-pyrazole-3-carboxylic acid based heterocyclic dyes

5-Hydroxy-1-phenyl-1H-pyrazole-3-carboxylic acid was selected as the coupling component to react with three S/N-containing heterocyclic (2-amino-4-chloro-5-formylthiophene-3-carbonitrile, 5-nitrobenzo[c]isothiazol-3- amine, 5-amino-1-ethyl-6-hydroxy-4-methyl-2-oxo-1,2-dihydropyridine-3- carbonitrile) diazo salts and three aniline derivatives with different electron-pulling/electron-pushing substituents, so as to produce six carboxylic pyrazolone based mono-/bi-heterocyclic dyes. Our results show that λmax of carboxylic pyrazolone based heterocyclic dyes depends on the substituent effects on aromatic rings and type of the heterocyclic rings. Compared with methyl pyrazolone/2,3-dimethylaniline based mono-heterocyclic dye, corresponding carboxylic pyrazolone based one with stronger D–π–A system brings a red shift of 37 nm. In contrast, carboxylic pyrazolone/thiophene and carboxylic pyrazolone/benzisothiazole based bi-heterocyclic dyes display blue shifts of 39 and 47 nm compared with relative methyl pyrazolone ones because of the role transformation for donor and acceptor (D−π−A to A−π−D) originating from the electron-withdrawing groups of S-containing heterocycles. Furthermore, different azo⇌hydrazone tautomeric transformations have been confirmed by the acid-base titration experiments. Namely, the mono-heterocyclic dyes, which would be transferred into the azo form in the alkaline condition, exhibit dominating hydrazone form in MeOH. However, the azo form of bi-heterocyclic dyes, which would convert to the hydrazone one under the acidic condition, is overwhelming in MeOH.

The effects of microcomputer-based laboratories on students macro, micro, and symbolic representations when learning about net ionic reactions

This study uses graphs of conductivity measured by a microcomputer-based laboratory (MBL) to promote students’ macro, micro, and symbolic representations when learning about net ionic reactions (NIR). A total of 54 students, aged 14–15 years old participated in this research, and were randomly divided into an experimental group (N= 27) and a control group (N= 27). The students in the experimental group were given graphs of conductivity measured by MBL, while the control group had a demonstration of acid–base titration experiments. The results reveal that the graphs of conductivity have a large effect on students’ macro, micro, and symbolic representations, that is, the students in the experimental group build more representations than the students in the control group.

Biodegradable phosphorylcholine-based zwitterionic polymer nanogels with smart charge-conversion ability for efficient inhibition of tumor cells

Zwitterionic polymer nanocarriers have attracted much attention in recent years due to their desirable biocompatibility and anti-fouling properties. However, the super-hydrophilic and neutral charge of zwitterionic polymer results in weak interactions with negatively charged cell membranes, which leads to suboptimal uptake by tumor cells. Herein, a series of biodegradable poly(2-methacryloyloxyethyl phosphorylcholine-s-s-vinylimidazole) (PMV) nanogels with uniform spherical shape was fabricated by one-step reflux precipitation polymerization, which was clean and efficient. The PMV nanogels remained in zwitterionic state at physiological pH (pH 7.4) and were converted rapidly to positive charged state at tumor extracellular pH (pH 6.5). Proton nuclear magnetic resonance spectra and acid-base titration experiment proved that the charge-conversion ability of PMV nanogels was attributed to protonation of the imidazole ring in an acidic environment. Protein stability experiment showed that PMV nanogels exhibited a protein-adsorption resistance at pH 7.4 for as long as 7 days while adsorbed protein rapidly at pH 6.5. Moreover, PMV nanogels showed a reducing-labile property, which was able to degrade into short linear polymer chains in the presence of reduction agents. Therefore, the doxorubicin (DOX) release profile was controlled finely with a low DOX leakage under physiological conditions (7.8% in 48 h) and a rapid DOX release in 10 mM glutathione at pH 7.4 (78.9% in 48 h). Confocal laser scanning microscope and flow cytometry showed that the PMV nanogels exhibit an enhanced cellular uptake by tumor cells at pH 6.5 compared with pH 7.4, which allows for a severe cytotoxic effect of DOX-loaded PMV nanogels against tumor cells.

Defluoridation by magnesia–pullulan: Surface complexation modeling and pH neutralization of treated fluoride water by aluminum

Abstract The magnesia–pullulan composite (MgOP) achieved effective fluoride removal in previous research. In the present study, an acid-base titration experiment was conducted to investigate the properties of MgOP surface and further explore the mechanism of fluoride adsorption on MgOP. Results showed that the presence of chloride ions could improve fluoride adsorption on MgOP; however, additional nitrate ions had negligible impacts. A diffuse layer model and chemical equilibrium software (Visual MINTEQ 3.1) were used to simulate the acid-base titration data. The effects of initial pH values on the rate of fluoride uptake by MgOP were also studied. Moreover, aluminum salts were added to the fluoride solution with MgOP for the pH neutralization of treated water, in which aluminum chloride was preferred.

Adsorption isotherm, mechanism, and geometry of Pb(II) on magnetites substituted with transition metals

Iron oxides are abundant in natural waters and soils and have high capacities for scavenging Pb(II) by adsorption, which affects the transport and fate of Pb on the earth's surface. We investigated the adsorption of Pb(II) on magnetites substituted with commonly incorporated transition metals such as Cr, Mn, Co, and Ni. The adsorption capacity, mechanism, and local coordination of Pb(II) were investigated by traditional macroscopic studies, i.e., acid-base titration and batch adsorption experiment, complemented with X-ray absorption fine structure (XAFS) spectrum analysis and surface complexation model (SCM). The substitution increased the surface site density, while pHpzc did not vary. Pb(II) adsorption was not suppressed by the presence of background electrolyte and improved as pH increased. The isotherms were well fit to the Langmuir adsorption model. The XAFS analysis demonstrated that Pb(II) ions were adsorbed on magnetite surface predominantly via inner-sphere complexation, where the adsorbed Pb(II) species was in bidentate binuclear corner-sharing geometry, independent of the adsorption capacity. This adsorption geometry can be applied to fit the experimental adsorption data well with the diffuse layer model (DLM). The substitutions improved the adsorption capacity in the following order: Cr>Ni>Mn>Co, and were discussed regarding the measured values of active site density and local coordination of adsorbed Pb(II). This study is the first documentation of Pb(II) adsorption on magnetite with different substitutions. The obtained results are of great significance for the understanding of Pb(II) surface complexation reactions on magnetite surface.

Surface characterization of oxidized biochar fibers derived from Luffa Cylindrica and lanthanide binding

In the last years, biochar fibers derived from Luffa Cylindrica have emerged as promising adsorbent material for harmful heavy metal ions. Based on their unique vascular structure, they exhibit a large surface area and thus high capability for the adsorption of metal ions. We investigated the structure and chemical surface composition of the carbonized fibers after treatment with 8M HNO3 prior and after Sm3+ binding in aqueous solutions. We used a large combination of classical analytical methods such as acid-base titration and batch-type adsorption experiments, as well as high-resolution microscopic and spectroscopic techniques like FTIR, XPS/REELS and SEM-EDX. The goal of the chemical oxidation is to functionalize the inner and outer surfaces of the biochar fibers by forming oxygen-containing active surface groups. Titration and spectroscopic data clearly reveal that anhydrides and carboxylic acids are acting as main surface groups to bind metal ions like Sm3+. We will show that the combination of high surface area and high density of carboxylic acid groups results in extremely high capacity of 2.4molkg−1 for the adsorption of Sm3+ ions. Therefore, the functionalization of biochar fibers offers a great potential to design advanced adsorbent materials for polluting heavy metal ions.

Tautomerism, crystal structure, and copper(ii) complexation of pyridonylazo dyes derived from methyl 4‐aminobenzoate

Four heterocyclic dyes, derived from methyl 4‐aminobenzoate and pyridine‐2,4‐dione components, with different N‐substituents in the latter [CH3, CH2CH3, CH2CH2OH, CH2CH2CH2OCH(CH3)2], were synthesised by the traditional diazotisation‐coupling strategy. Furthermore, a Cu(ii) dye–metal complex was prepared and characterised by X‐ray single‐crystal diffraction for comparison, where the configuration transformation from hydrazone to deprotonated azo forms was found before and after Cu(ii) ion complexation. In addition, the azo–hydrazone transformation could be observed by acid–base titration experiments. To the best of our knowledge, this work is the first structural study of pyridone dyes based on methyl 4‐aminobenzoate and the corresponding dye–metal complex.

Well-Shaped Sulfonic Organosilica Nanotubes with High Activity for Hydrolysis of Cellobiose

Sulfonic organosilica nanotubes with different acidity densities could be synthesized through the co-condensation of ethenyl- or phenylene-bridged organosilane and 3-mercaptopropyltrimethoxysilane followed by sulfhydryl (–SH) oxidation. Transmission electron microscopy (TEM) analysis and nitrogen adsorption-desorption experiment clearly exhibit the hollow nanotube structures with the diameters of about 5 nm. The compositions of the nanotube frameworks are confirmed by solid state 13C nuclear magnetic resonance (NMR) while X-ray photoelectron spectroscopy (XPS) shows that about 60–80% of SH groups were oxidized to sulfonic acid (SO3H). The acid contents were measured by both elemental analysis (CHNS mode) and acid-base titration experiment, which revealed that the acid density was in the range of 0.74 to 4.37 μmol·m−2 on the solid. These nanotube-based acid catalysts exhibited excellent performances in the hydrolysis of cellobiose with the highest conversion of 92% and glucose selectivity of 96%. In addition, the catalysts could maintain high activity (65% conversion with 92% selectivity) even after six recycles.

Branched polymeric prodrug/programmed cell death 4 complexes for combinational cancer therapy

Here, we demonstrate a co-delivery system constructed by integrating chemotherapeutic molecules into branched polymeric prodrug which can condense nucleic acids. Demethylcantharidate (DCA) was chosen as a model drug and premodified through nucleophilic substitution reaction by its two carboxylic groups with allyl chloride. The synthesized intermediate (DCA-dially) was then used to polymerize with tris (2-aminoethyl) amine (TAEA) through progressively ammonolysis reaction. The obtained poly (DCA-alt-TAEA) (DCAT) was used to pack PDCD4 into spherical-like nanoparticles through electrostatic interaction. Gel retardation assays implied that DCAT could integrate DNA at the weight ratio of 1 and protect it from digestion by nuclease. Acid-base titration experiments showed that DCAT obtained preferable buffering capability which was beneficial for the endosomal escape of DCAT/PDCD4 complexes. Cellular tests involving gene transfection efficiency and cytotoxicity indicated that DCA and PDCD4 co-delivered by the complexes significantly and synergistically suppressed the viability of SMMC-7721 cells. These results suggest that integrating chemotherapeutic molecules into nucleic acid-packing polymeric prodrug as cationic polymer/PDCD4 complexes is a highly efficient way to co-deliver chemotherapeutic drugs and nucleic acids for cancer therapy.

From heterocyclic hydrazone to hydrazone-azomethine dyes: Solvent and pH induced hydrazone and azo-keto transformation for a family of pyrazolone-based heterocyclic dyes

A series of pyrazolone-based full heterocyclic and half heterocyclic hydrazone dyes as well as aniline modified hydrazone-azomethine dyes has been successfully synthesized and spectrally characterized. UV–Vis spectral comparisons indicate that the aniline modified hydrazone-azomethine dyes show significant red shifts compared with their common full heterocyclic precursor and respective half heterocyclic dyes with the same substituents in the absence of the 2-aminothiophene moiety. The incorporation of CHN and NHN units in the targeting hydrazone-azomethine dyes, which results in the increase of the π-conjugated system, is responsible for the bathochromic shifts in the UV–Vis spectra. Furthermore, solvatochromism and acid-base titration experiments have been carried out to explore the possible proton transfer within the multiple heteroatoms for this family of compounds. Our results demonstrate the existence of an equilibrium between the hydrazone and azo-keto tautomers and the transformation can be driven by the polarity of solvents and pH values. In addition, the easily deprotonated azo-enol tautomer is unlikely to be formed because of the alkaline stability in both organic solvents and water for this series of pyrazolone-based heterocyclic dyes.