Hydrolysis and redox factors affecting analysis of common phenolic marker compounds in botanical extracts and finished products

Many of the marker compounds analyzed in herbal products are redox-active phenolic molecules, which are commonly found in plants as components of glycosides and starch polymers. Variability in degree of sample hydrolysis can occur due to differences in water content, pH, and temperature. Sonication versus shaking during extraction can also influence hydrolysis and oxidation of sensitive compounds. Some traditional botanical extract marker compounds are esters and glycosides of phenolics such as echinacoside from Echinacea while others are free phenolics, such as quercetin from glycosides in Ginkgo. Optimizing hydrolysis conditions maximizes free quercetin levels, but lowers echinacoside levels. Furthermore, acidic hydrolysis conditions mimic stomach conditions encountered by oral supplements and protect resulting free phenolics from oxidation. Oxidative degradation of botanical phenolic markers can be initiated by light, sonication, oxygen, basic pH conditions, heat, redox-active solvents, and formulation additives. Some phenolic markers reversibly cycle through multiple oxidation states creating a formula-specific equilibrium of oxidation states. Finished product formulations that include easily oxidized phenolics, carbonates, phosphates, and transition metals affect sample hydrolysis degree and redox equilibria, and quantitation. By recognizing and controlling hydrolysis and oxidation variables, more accurate and rugged methods can be developed allowing for improved botanical standardization and finished product analysis.     

结合PBL的思维导图法在大学化学“四大平衡”教学中的应用

Taking the chemical equilibria including acid-base equilibrium, complexation equilibrium, precipitation equilibrium and redox equilibrium as an example, the application of the mind mapping combined with problem-based learning in university chemistry teaching is introduced.     

论化学平衡中的独立等量关系

物料平衡等式(MBE)、电荷平衡等式(CBE)和质子平衡等式(PBE)是化学平衡定量解析中的常用等量关系。本文从理论上证明了CBE可以由MBE导出,因此在计算中不是一个必不可少的独立条件。由于H~+来源的多样性,在涉及酸碱的化学平衡中,CBE比MBE的计算效率更高。这些结论与计算实践相一致,即在配位平衡、氧化还原平衡和沉淀平衡的计算中,MBE为主,CBE起辅助作用;在酸碱平衡计算中,CBE为主,MBE为辅。PBE也不是一个独立等量关系,而且仅适用于酸碱平衡,可以完全被效率更高的CBE代替。本研究明确了化学平衡中的独立等量关系,不仅有助于提高化学平衡定量解析的效率,而且使其理论结构更加简洁明晰。     

Determination of equilibrium constants of alkaline earth metal ion chelates with Dowex A-1 chelating resin

A complexation chemistry model is applied to chelating ion-exchange systems and a method is presented for the determination of equilibrium constants for metal ion chelates with these resins. Protonation constants for the iminodiacetic based chelating resin Dowex A-1 were determined from potentiometric pH-data. Equilibrium constants were determined for 1:1 beryllium, magnesium, calcium, strontium and barium chelates with the resin in a wide pH range by measuring the concentrations of respective metal ions in the aqueous phase with direct current plasma atomic emission spectrometry (DCP-AES). A batch technique was used for the equilibrium experiments. At pH below 7 protonated 1:1 species were also found to be formed with the resin. From the obtained equilibrium constants, theoretical distribution coefficients were calculated as function of pH for respective metal ion resin system.     

Precipitation of mixed Ca–Ba,Ca–Cd and Ca–Mn carbonates with distinct morphologies under cooperativity of divalent metal ions and protein

According to the basic principles of biomineralization, a homogeneous precipitation process involving BSA as the matrix was used to precipitate mixed Ca–Ba, Ca–Cd and Ca–Mn carbonate particles with distinct morphologies and structures at room temperature. The structures and morphologies of these carbonates were investigated using SEM and XRD. It was found that there were distinct differences when two different kinds of divalent metal ions were simultaneously precipitated in the reacting solutions. Moreover, according to the results of FT-IR and TG-DTA, we could conclude that bovine serum albumin also affected the diverse morphologies of Ca–M carbonates besides the influence of divalent metal ions. A self-assembly process coupled with an Ostwald ripening mechanism was also discussed based on a series of time-dependent SEM observations.     

Retention model for the separation of anionic metal-EDTA complexes in ion chromatography

A retention model is derived for complex anions eluted from an anion-exchange column with multiple ionic eluents containing hydrogencarbonate, carbonate, and hydroxyl species and the sample solution, containing transition metals, anions and complexing ligand. The theory is based on the generalized ion-exchange equilibrium, protonation and complex-formation equilibria. The unknown parameters of chromatographic ion-exchange equilibrium constants for sample and eluent species are determined from the experimental retention data by iterative minimization, using a non-linear regression algorithm. The model was utilized to predict the retention behaviour of CdEDTA²⁻, CoEDTA²⁻, MnEDTA²⁻ and NiEDTA²⁻ ions. The capacity factors of complex ions were determined for wide ranges of pH values and eluent concentrations. Good agreement was obtained between the observed and predicted retentions.     

Kinetics and mechanism of the formation of glycol esters: Benzoic acid–ethylene glycol system

Esterification-hydrolysis and condensation-glycolysis are the principal equilibria entailed in a system comprised of benzoic acid, ethanediol, ethanediol monobenzoate, ethanediol dibenzoate, and water. These equilibria are characterized by three interrelated equilibrium constants that pertain to the hydrolysis of both ethanediol monobenzoate and ethanediol dibenzoate and to the condensation of ethanediol monobenzoate. The rate constants related to these reactions and to the corresponding reverse reactions were all found to be functions of the concentration of carboxy groups. Antimony triacetate, a well-known transesterification catalyst, catalyzed the condensation reaction strongly but did not affect the esterification reaction. Based on a mechanism that entailed the considered three principal equilibrium reactions, a set of both the kinetic and thermodynamic parameters was obtained by a nonlinear regression procedure. With these parameters the nonlinear rate equations were integrated numerically, using the fourth-order Runge-Kutta method. Excellent agreement between the values thus calculated and the experimental data were obtained in all cases except when considerable concentrations of both benzoic acid and ethanediol dibenzoate were present in the system. It has been concluded that in these instances the formation of the dibenzoate is paralleled by a pyrolysis reaction that is strongly catalyzed by carboxy groups.     

Determination of riboflavin in pharmaceutical products by automatic discrete-sample analysis

A procedure for the determination of iron, copper, nickel, cobalt, manganese and chromium down to 0.01 μg g^-1 in sodium calcium silicate glass, sodium borosilicate glass, sodium carbonate and calcium carbonate is described. The analytical procedure depends on the separation at pH 6 of the metal diethyldithiocarbamates into isobutyl methyl ketone, and their determination by flameless atomic absorption spectrometry, with a Massmann-type graphite furnace. The limiting factors on the detection limits attainable are discussed and related to the purity of the acids used for sample solution, sample contamination during chemical separation and the sensitivity of the analytical technique.     

Coprecipitation of Nanocomposites Based on Colloidal Particles of Sulfur and Carbonates of Alkaline-Earth Metals from Polysulfide Solutions

It has for the first time been shown that the action of carbon dioxide on solutions of alkaline-earth metal polysulfides causes a reaction yielding nanoparticles of sulfur and calcium, barium, and strontium carbonates. It has been found that, initially, particles of sulfur and a corresponding carbonate are synthesized with average sizes of about 20–25 nm; then, the particles are enlarged (aggregated) with the precipitation of a composite, which consists of hydrophobic particles of sulfur and the carbonate (the latter become hydrophobic due to the adsorption of neonol present in the reaction mixture). It has been shown that only sulfur exhibits antifungal activity in the composites, while carbonates have no effect on pathogenic fungi. The composite consisting of sulfur and calcium carbonate nanoparticles has shown the highest biological activity during germination of wheat seeds.     

Chemical equilibrium of glycerol carbonate synthesis from glycerol

In this paper, the chemical equilibrium for the glycerol carbonate preparation from glycerol was investigated. The chemical equilibrium constants were calculated for the reactions to produce glycerol carbonate from glycerol. The theoretical calculation was compared with the experimental results for the transesterification of glycerol with dimethyl carbonate. Transesterification of glycerol with cyclic carbonates or alkyl carbonates is thermodynamically favourable for producing glycerol carbonate from glycerol according to the equilibrium constant. Increasing temperature can increase the chemical equilibrium constant for the reaction of glycerol with dimethyl carbonate. For the reaction of glycerol with ethylene carbonate, increasing temperature can decrease the chemical equilibrium constant. The reaction of glycerol with carbon dioxide is thermodynamically limited. High temperature and low pressure are favourable to the reaction of glycerol and urea.     

Easy separation of optically active products by enzymatic hydrolysis of soluble polymer-supported substrates

The easy separation of optically active compounds from enzymatic kinetic resolution products by simple precipitation using poly(ethylene glycol)(PEG)-supported carbonates is disclosed. The water-soluble substrate was prepared by the immobilization of (±)-1-phenylethanol onto a middle-molecular weight (av M_w 5000) monomethoxy PEG (MPEG) through a carbonate linker. The enantioselective hydrolysis using Lipase from porcine pancreas (PPL; Type II, Sigma) in a mixed solvent (hexane/buffer = 9:1) proceeded to afford the corresponding optically active compounds. In this system, the separation of the products was achieved by a simple procedure without laborious column chromatography. A hydrophobic spacer between the MPEG moiety and the carbonate linker affected both the reactivity and enantioselectivity, and the substrate with a phenylethyl spacer was hydrolyzed with the highest enantioselectivity (E value = 270).     

Modeling manganese removal in chelating polymer-assisted membrane separation systems for water treatment

The removal of manganese from groundwater, using water-soluble chelating polymers such as polyacrylic acid (PAA) in combination with ultrafiltration (UF), was investigated. The effects of the solution pH and polymer dosages on the manganese removal were evaluated, and the removal efficiency was modeled considering the relevant chemical equilibria. In the absence of polymer, manganese removal with UF membranes alone was negligible at acidic pH values, but the removal increased substantially when polyacrylic acid (PAA) was added to the feed solution. The increase can be attributed to the formation of Mn²⁺–PAA chelates which are rejected by the membranes. A mathematical model was developed to explain this phenomenon based on chemical equilibria, including complex formation and precipitation. The chelation number (i.e., the number of carboxyl groups in the PAA binding to a single metal ion) and the equilibrium constants for metal–PAA chelation reactions were obtained by fitting experimental data at acidic pH in single-divalent metal systems. The model was able to predict Mn removal in chelation/UF systems at various pH levels and polymer dosages, and to account for the competitive interactions of PAA with the target (Mn²⁺) and background species (Ca²⁺, Mg²⁺) in multi-component systems. The predicted Mn removal efficiency was most sensitive to the chelation number.     

Local Light-Controlled Generation of Calcium Carbonate and Barium Carbonate Biomorphs via Photochemical Stimulation

Photochemical activation is proposed as a general method for controlling the crystallization of sparingly soluble carbonates in space and time. The photogeneration of carbonate in an alkaline environment is achieved upon photo-decarboxylation of an organic precursor by using a conventional 365 nm UV LED. Local irradiation was conducted focusing the LED light on a 300 μm radius spot on a closed glass crystallization cell. The precursor solution was optimized to avoid the precipitation of the photoreaction organic byproducts and prevent photo-induced pH changes to achieve the formation of calcium carbonate only in the corresponding irradiated area. The crystallization was monitored in real-time by time-lapse imaging. The method is also shown to work in gels. Similarly, it was also shown to photo-activate locally the formation of barium carbonate biomorphs. In the last case, the morphology of these biomimetic structures was tuned by changing the irradiation intensity.     

Surface modified precipitated calcium carbonates at a high degree of dispersion

Calcium carbonates of different degrees of surface hydrophobicity were obtained when sorbic acid or polyoxyethylene glycol were present during the precipitation. In the presence of trace amounts of divalent cations carbonates with high surface hydrophilicity and improved monodisperse character are obtained. Surface modification of the calcium carbonates by several proadhesive compounds can markedly improve the chemical affinity of the carbonates to polymers. Very effective are isostearoyl titanate and two polyoxyethylene compounds. Precipitated calcium carbonates modified with 2 to 3 percent (wt/wt) of isostearoyl titanate increased the tensile strenght of butadiene-styrene rubber by approximately 100%. Polyurethane is only strengthened when 30 wt/wt of a filler are introduced independently of the type proadhesive compounds. The best strength and hysteresis of polyurethanes are obtained with calcium carbonate modified with 2 percent (wt/wt) of polyoxyethylene glycol.     

Stable Prenucleation Calcium Carbonate Clusters Define Liquid–Liquid Phase Separation

Liquid–liquid phase separation (LLPS) is an intermediate step during the precipitation of calcium carbonate, and is assumed to play a key role in biomineralization processes. Here, we have developed a model where ion association thermodynamics in homogeneous phases determine the liquid–liquid miscibility gap of the aqueous calcium carbonate system, verified experimentally using potentiometric titrations, and kinetic studies based on stopped‐flow ATR‐FTIR spectroscopy. The proposed mechanism explains the variable solubilities of solid amorphous calcium carbonates, reconciling previously inconsistent literature values. Accounting for liquid–liquid amorphous polymorphism, the model also provides clues to the mechanism of polymorph selection. It is general and should be tested for systems other than calcium carbonate to provide a new perspective on the physical chemistry of LLPS mechanisms based on stable prenucleation clusters rather than un‐/metastable fluctuations in biomineralization, and beyond.     

Growth behavior and kinetics of self-assembled silica-carbonate biomorphs

Upon slow crystallization from silica-containing solutions or gels at elevated pH, alkaline-earth carbonates spontaneously self-assemble into remarkable nanocrystalline ultrastructures. These so-called silica biomorphs exhibit curved morphologies beyond crystallographic symmetry and ordered textures reminiscent of the hierarchical design found in many biominerals. The formation of these fascinating materials is thought to be driven by a dynamic coupling of the components' speciations in solution, which causes concerted autocatalytic mineralization of silica-stabilized nanocrystals over hours. In the present work, we have studied the precipitation kinetics of this unique system by determining growth rates of individual aggregates using video microscopy, and correlated the results with time-dependent data on the concentration of metal ions and pH acquired online during crystallization. In this manner, insight to the evolution of chemical conditions during growth was gained. It is shown that crystallization proceeds linearly with time and is essentially reaction controlled, which fits well in the proposed morphogenetic scenario, and thus, indirectly supports it. Measurements of the silica concentration in solution, combined with analyses of crystal aggregates isolated at distinct stages of morphogenesis, further demonstrate that the fraction of silica coprecipitated with carbonate during active growth is rather small. We discuss our findings with respect to the role of silica in the formation of biomorphs, and moreover, prove that the external silica skins that occasionally sheath the aggregates--previously supposed to be involved in the growth mechanism--originate from secondary precipitation after growth is already terminated.     

Trace metal speciation by adsorptive stripping voltammetry of metal chelates of solochrome violet RS

Adsorptive stripping voltammetry has become one of the most sensitive methods for trace metal determinations. The growing application of the method to natural water systems prompted an investigation into the fraction of the metal concentration that contributes to the adsorptive stripping response. Recent procedures for trace measurement of iron, titanium and gallium, based on chelation with solochrome violet RS (SVRS) are coupled to systematic ligand competition experiments. Tannic acid, EDTA, NTA, glycine, cysteine, carbonate and chloride ions are used as model natural ligands. It is shown that adsorptive stripping voltammetry measures the free ion and metal displaced from complexes by the “analytical” ligand. The exact fraction of the metal measured thus depends on the thermodynamic stability of the metal-SVRS chelate (compared to that of natural complexes), and on the relative concentrations of the competing ligands. The method offers possible distinction between metal complexes, based on their thermodynamic stabilities. The use ofa large excess of the “analytical” ligand can lead to measurement of the total metal content. Implications of these results relative to the use of this procedure for studying the speciation of trace elements in natural waters are discussed.     

Gas chromatography carbonate microassay of calcified tissue

The aim of the study is to assay the carbonates held in the mineral phase of calcified tissues.An apparatus is presented for the manipulation of the CO₂ released during acid attack of biological and mineral carbonate samples before injection into a gas chromatograph. The gasses are assayed for CO₂ by means of a calibration curve, previously established under the same experimental conditions using carbonate standards.This analytical technique allows very small quantities of sample to be assayed: a few hundred micrograms in the case of mineral carbonates and a few milligrams for biological material. In spite of the low quantities involved the quantitative results obtained are of high accuracy.     

Synthesis of symmetrical organic carbonates via significantly enhanced alkylation of metal carbonates with alkyl halides/sulfonates in ionic liquid

[reaction: see text] We report a new phosgene-free method for the synthesis of symmetrical organic carbonates via alkylation of metal carbonate with various alkyl halides and sulfonates in 1-n-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6], as an ecofriendly reaction media. Alkylation of metal carbonate in various ionic liquids with 1-bromo-3-phenylpropane (1a) as a model reactant has thoroughly been investigated. Potassium and cesium carbonates appeared to be the most suitable metal carbonate due to their high solubility in ionic liquids. Besides good to excellent yields, this simple and convenient methodology is devoid of highly toxic and harmful chemicals such as phosgene and carbon monoxide, which is an additional advantage.