Citric Acid Determination by Dual Wavelength Spectrophotometry

In this study, a dual wavelength spectrophotometric method was presented on the basis of Cu²⁺‐NH₃ complex decomposition by the citrate ion and the formation of Cu²⁺‐citrate complex. Sum of the absorbance decreasing at 600 nm (λ_max for Cu²⁺‐NH₃ complex) and the absorbance increasing at 750 nm (λ_max for Cu²⁺‐citrate complex) was used in quantification. This method is very selective, fast and inexpensive but relatively less sensitive. The calibration curve is linear in the range of 0.4–8 mmol/L with a detection limit of 0.13 mmol/L. The relative standard deviation (RSD%) for the six repeated determinations (2 mmol/L) is 1.75%. Interference studies of more than twenty common compounds, which are present in the studied samples, were carried out. Only oxalic and ascorbic acids are serious interferes and which interferences were eliminated by simple procedures. Finally, the proposed method was successfully applied in the determination of citric acid content in ORS powder and some fruit juices.     

柠檬酸辅助可控制备花状银粒子及其表面增强拉曼散射性能

以抗坏血酸为还原剂,柠檬酸为结构导向剂,一步还原硝酸银,合成了尺寸和形状可调的花状银颗粒。纳米粒子的粒径可在600～1 200 nm范围内调整,表面突起可达到10～25 nm。柠檬酸的化学性质在银纳米粒子合成多级花状银结构的过程中起着至关重要的作用。通过改变柠檬酸或抗坏血酸溶液的用量,银结构的各向异性形貌可以很容易地调节。以制备的多级花状银颗粒作为表面增强拉曼散射(SERS)基底,对浓度为10~(-10)mol·L~(-1)的罗丹明6G(R6G)仍具有较高的检测灵敏度。     

A Cu<Superscript>2+</Superscript>ion-selective fluoroionophore with dual off/on switches

A new malonamide fluoroionophore possessing two pyrene moieties was synthesized. This bispyrene exhibited the fluorescence of the pyrene monomer (λ_em = 395 nm) and intramolecular excimer (λ_em = 467 nm) emissions. The designed derivative showed the excellent ion sensing ability to Cu²⁺. The "on-off-off" and "off-on-off" fluorescence responses were demonstrated by the addition of the variable Cu²⁺ concentration. The utilization of the dual off/on responses could apply to the estimation of the rough Cu²⁺ concentration.     

A new copper containing MALDI matrix that yields high abundances of [Peptide + Cu]+ ions

The dinuclear copper complex (α-cyano-4-hydroxycinnamic acid (CHCA) copper salt (CHCA)₄Cu₂), synthesized by reacting CHCA with copper oxide (CuO), yields increased abundances of [M + xCu − (x−1)H]⁺ (x = 1–6) ions when used as a matrix for matrix-assisted laser desorption ionization (355 nm Nd:YAG laser). The yield of [M + xCu − (x−1)H]⁺ (x = 1∼6) ion is much greater than that obtained by mixing peptides with copper salts or directly depositing peptides onto oxidized copper surfaces. The increased ion yields for [M + xCu − (x−1)H]⁺ facilitate studies of biologically important copper binding peptides. For example, using this matrix we have investigated site-specific copper binding of several peptides using fragmentation chemistry of [M + Cu]⁺ and [M + 2Cu − H]⁺ ions. The fragmentation studies reveal interesting insight on Cu binding preferences for basic amino acids. Most notable is the fact that the binding of a single Cu⁺ ion and two Cu⁺ ions are quite different, and these differences are explained in terms of intramolecular interactions of the peptide-Cu ionic complex.     

Characterization and application of an electrode modified by mechanically immobilized copper hexacyanoferrate

A newly modified electrode was prepared by mechanical immobilization of copper hexacyanoferrate (CuHCF) on a graphite electrode. The modified electrode was characterized by cyclic voltammetric experiments. The effect of different background electrolytes, pHs and scan rates on the electrochemical behaviour of the electrode has been evaluated. In NH₄Cl two reversible redox peaks were observed. The first redox peak corresponding to Cu⁺/Cu²⁺ is observed only in this medium. The second redox peak corresponds to the Fe(CN)₆ ^4–/Fe(CN)₆ ^3– couple. Both anodic peaks were used for catalytic oxidation of ascorbic acid. As the anodic current for catalytic oxidation was proportional to the amount of ascorbic acid, an analytical method was developed for the determination of ascorbic acid in commercial samples. Received: 26 May 1998 / Revised: 15 March 1999 / Accepted: 20 March 1999     

Copper(II) complexing with 2′,2′-diethylheptanohydrazide

Acid-base properties of 2′,2′-diethylheptanohydrazide (DEHH) and its complex formation with copper(II) ions in water-isopropanol solutions were studied. Cationic complexes with [Cu²⁺] : [DEHH] ratios of 1 : 1 and 1 : 2 were found to be formed in weak acid media, and an uncharged complex with a [Cu²⁺] : [DEHH] ratio of 1 : 2 was found to be formed in a basic medium. log K _st values were determined. The data obtained were compared with the stability constants of copper(II) 2′,2′-diethylbenzohydrazide complexes.     

Investigation of the interaction between Co sulfide coatings and Cu(I) ions by cyclic voltammetry and XPS

The interaction between the Co sulfide coating formed on a glassy carbon electrode and Cu(I)-ammonia complexes solution was investigated by cyclic voltammetry in 0.1 M KClO₄, 0.1 M NaOH and 0.05 M H₂SO₄ solutions. It was determined that, after treating the cobalt sulfide coating formed by two deposition cycles with Cu(I)-ammonia complexes (0.4 M, pH 8.8–9.0, τ=180 s, T=25±1°C), an exchange occurs between the coating components and Cu(I). Copper(I) substitutes 75% of the Co(III) compounds present in the coating (~1.81×10^–7 mol cm^–2) because of Cu₂O (1.36×10^–7 mol cm^–2) formation. The rest of the Co(II) and Co(III) sulfide compounds are also replaced by copper with formation of Cu_{2– x} S with a stoichiometric coefficient close to 2 (~1.9). After modifying the cobalt sulfide coatings with Cu(I) ions, the total amount of metal (Co+Cu) increases, owing to the sorption of Cu(I) compounds. In addition, the number of deposition cycles decreases from 3 to 1.5 [1 cycle involves cobalt sulfide layer formation and 0.5 cycle is attributed to modifying by Cu(I) ions]. The coatings modified in the above-mentioned manner may be successfully used for plastic electrochemical metallization as Cu_{2– x} S coatings formed by three deposition cycles. Electronic Publication     

New copper-containing catalysts based on modified amorphous silica and their use in flow azide—alkyne cycloaddition

Сopper-containing catalysts supported on amorphous silica modified by amines were prepared using the chemical reduction method. The morphology of copper particles and their chemi calstate depend on the type of the reducing agent used. The use of ascorbic acid results in the formation of monodisperse submicron Cu⁰ particles 200—300 nm in size, whereas Cu⁰ particles with a size ranging from 50 to 150 nm are formed when hydrazine hydrate was used. The morphology and chemical state of the copper particles reduced with sodium borohydride depend substantially on the amount of the reducing agent: Cu⁰ nanoparticles 10—15 nm in size are formed if the reducing agent is an excess, layered Cu₂O plates are formed at the equimolar amount of sodium borohydride, and a decrease in the amount of sodium borohydride results in spherical Cu₂O particles. All the catalysts synthesized in the flow regime showed higher activity in the catalytic cycloaddition of azides to alkynes than the commercially available copper catalysts.     

Characterization and application of an electrode modified by mechanically immobilized copper hexacyanoferrate

A newly modified electrode was prepared by mechanical immobilization of copper hexacyanoferrate (CuHCF) on a graphite electrode. The modified electrode was characterized by cyclic voltammetric experiments. The effect of different background electrolytes, pHs and scan rates on the electrochemical behaviour of the electrode has been evaluated. In NH₄Cl two reversible redox peaks were observed. The first redox peak corresponding to Cu⁺/Cu²⁺ is observed only in this medium. The second redox peak corresponds to the Fe(CN)₆ ^4–/Fe(CN)₆ ^3– couple. Both anodic peaks were used for catalytic oxidation of ascorbic acid. As the anodic current for catalytic oxidation was proportional to the amount of ascorbic acid, an analytical method was developed for the determination of ascorbic acid in commercial samples.     

Electrocatalytic behaviour of citric acid at a cobalt phthalocyanine-modified screen-printed carbon electrode and its application in pharmaceutical and food analysis

Cobalt phthalocyanine-modified screen-printed carbon electrodes (CoPC-SPCEs) have been investigated as disposable sensors for the measurement of citric acid. The analyte was found to undergo an electrocatalytic oxidation process involving the Co²⁺/Co³⁺ redox couple. Calibration plots were found to be linear in the range 2 mM to 2.0 M; replicate determinations of a 5.2 mM citric acid (n = 4) solution gave a coefficient of variation of 1.43%. Additions of metal ions, such as Ag⁺, Pb²⁺, Cu²⁺, Fe³⁺ and Ca²⁺, were found not to interfere. The effects of hesperidin, cysteine, ethylenediaminetetraacetic acid (EDTA), ascorbic, formic, malic, malonic, tartaric, oxalic and trichloroacetic acids on the determination of citric acid were examined and, under the conditions employed, only oxalic acid and EDTA were found to give any significant interference. The sensors were evaluated by carrying out citric acid determinations on spiked and unspiked samples of an acid citrate dextrose (ACD) formulation, lime flesh and juice. For lime juice, recoveries were calculated to be 96.8% (% CV = 2.7%) for a sample fortified with 5% citric acid and for ACD 99.4% (%CV = 2.6%) when fortified at 2.30% citric acid. Further studies showed the possibility of determining citric acid concentrations in lime juice and fruit directly, without the need for an added electrolyte. These performance characteristics indicate that reliable data may be obtained for citric acid measurements in such samples. To our knowledge, this is the first report on the electrocatalytic oxidation of citric acid and its application using a CoPC-SPCE.     

Measurement and characterization of singlet oxygen production in copper ion-catalyzed aerobic oxidation of ascorbic acid

Production of singlet molecular oxygen (¹O₂) in the aerobic oxidation of ascorbic acid catalyzed by copper ion was measured and characterized using [4′-(9-anthryl)-2,2′:6′,2″-terpyridine-6,6″-diyl]bis(methylenenitrilo)tetrakis(acetate)-Eu³⁺ (ATTA-Eu³⁺) as a highly sensitive and selective time-resolved luminescence probe for ¹O₂. The ¹O₂ produced in the reaction was further characterized and confirmed by (i) chemical trapping of ¹O₂ with 9,10-diphenylanthracene (DPA), the corresponding endoperoxide was detected by HPLC and (ii) spin trapping of ¹O₂ with 2,2,6,6-tetramethyl-4-piperidinol (TMP-OH), the corresponding free radical of TMP-OH oxide (TMPO) was detected by electron spin resonance (ESR) spectroscopy. The effects of deuterium oxide, sodium azide and histidine on the ¹O₂ signal were investigated. The mechanism investigation of ¹O₂ production implied that the ascorbic acid-Cu(I) complex formed in the reaction could be an important intermediate for the ¹O₂ production. The reaction of ascorbic acid with copper ion monitored by ¹H NMR and absorption spectroscopy demonstrated the formation of a copper ion-ascorbic acid complex. Except for Cu²⁺ and Cu⁺-ascorbic acid systems, no detectable ¹O₂ was produced in other transition metal cation-ascorbic acid systems in the studied range.     

A Simple and Reliable Spectrophotometric Method for the Determination of Ascorbic Acid in Pharmaceutical Preparations

In this study, a simple spectrophotometric method based on the reaction between ascorbic acid and the copper(II)–ammonia complex is presented for the determination of the vitamin C content of pharmaceutical preparations. During this reaction, ascorbic acid is oxidized and the copper(II)–ammonia complex is reduced to the copper(I)–ammonia complex, and the absorbance decrease at 600 nm (_max for the copper(II)–NH₃ complex) is measured. Stirring the final solution in the presence of air leads to the primary absorbance again being obtained, which indicates that the copper(I)–NH₃ complex is quantitatively oxidized to the copper(II)–NH₃ complex by O₂. The linear dynamic range of the calibration curve is 0.8–6 mmol with a detection limit of 0.26 mmol. The relative standard deviation for eight repeated experiments is 2.4%, which shows that the proposed method has a good repeatability. Finally, this method was used in the analysis of the vitamin C content of different pharmaceutical preparations, such as multivitamin tablets and syrups, vitamin-C tablets and powders, and effervescent tablets. The obtained results are in good agreement with iodimetric data.     

EPR spectra and structures of dinuclear copper(<Emphasis Type="SmallCaps">II</Emphasis>) complexes with acyldihydrazones of benzenedicarboxylic acids

Spacer-armed dinuclear copper(II) complexes with condensation products of isophthalic and terephthalic acid dihydrazides with salicylaldehyde and 2-hydroxyacetophenone were synthesized and studied by EPR and X-ray diffraction. The compositions and structures of most of the complexes were determined by elemental analysis, thermogravimetric analysis, and IR spectroscopy. The structure of the copper(II) complex with acyldihydrazone of salicylaldehyde and 1,3-benzenedicarboxylic acid (H₄L) with the composition [Cu₂L¹·2morph·MeOH] (morph is morpholine) was established by X-ray diffraction. The Cu^II atoms are spaced by 10.29 Å and are structurally nonequivalent. One copper cation has a square-planar coordination formed by donor atoms (2 N + O) of the doubly deprotonated acylhydrazine fragment and the N atom of the morpholine molecule. The second copper atom is additionally coordinated by a methanol molecule through the oxygen atom, so that this copper atom is in a tetragonal-pyramidal coordination with the oxygen atom in the axial position. The EPR spectra of liquid solutions of the complexes based on 1,4-benzenedicarboxylic acid acyldihydrazones and 1,3-benzenedicarboxylic acid bis(salicylidene)hydrazone at room temperature show a four-line hyperfine structure with the constant a _Cu = 54.4–67.0·10⁻⁴ cm⁻¹ (g = 2.105–2.147), which is indicative of the independent behavior of the paramagnetic centers. The EPR spectrum of a solution of the complex based on isophthalic acid and 2-hydroxyacetophenone shows the seven-line hyperfine structure corresponding to two equivalent copper nuclei (g = 2.11, a _Cu = 36.5·10⁻⁴ cm⁻¹). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1898–1905, October, 2007.     

Spacered dinuclear copper(<Emphasis Type="SmallCaps">II</Emphasis>) complexes with acyldihydrazones of aliphatic dicarboxylic acids and 2-hydroxy-5-nitroacetophenone

Dinuclear copper(II) complexes with acyldihydrazones of 2-hydroxy-5-nitroacetophenone (H₄L) of the composition Cu₂(Py)_xL·mEtOH were synthesized and characterized. In these complexes, the coordination polyhedra of the copper atoms are linked to each other by a polymethylene chain of different lengths, from one to five monomer units. The structure of the [Cu₂L·4Mrf] complex (Mrf is morpholine) based on acyldihydrazone of malonic acid was established by X-ray diffraction. The copper(II) atoms in this complex are [4+1]-coordinated and are spaced by 6.94 Å. At room temperature, the signal in the ESR spectra of solutions of the complexes based on acyldihydrazones of malonic, succinic, glutaric, and adipic acids has a seven-line hyperfine structure with the constant of (35.3–38.8)·10⁻⁴ cm⁻¹ (g = 2.109–2.112) due to exchange interactions between unpaired electrons and two equivalent copper nuclei. An increase in the length of the polymethylene chain to five monomer units hinders exchange interactions, and the ESR signal of the complex based on acyldihydrazone of pimelic acid has a four-line hyperfine structure with a _Cu = 72.7·10⁻⁴ cm⁻¹ typical of mononuclear copper(II) complexes. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 229–234, February, 2007.     

A Spectrophotometric Determination of Ascorbic Acid

A new, simple and sensitive method for the spectrophotometric indirect determination of ascorbic acid in fruits, beverages, and pharmaceuticals is described. In this method, the ascorbic acid reduces Cu²⁺ to Cu⁺ and reacts with 2,9‐dimethyl‐1,10‐phenanothroline (neucoproine) to form Cu (neucoproine)⁺ complex, and it was extracted with N‐phenylbenzimidoylthiourea (PBITU) in chloroform. The apparent value of molar absorptivity of the complex in terms of ascorbic acid is (3.52) × 10⁴ L mole^?1 cm^?1 at λ_max, 460. The detection limit of ascorbic acid is 40 μg L^?1 and the method obeys Beer's law over the concentration range of 0.1–4.0 μg mL^?1. The proposed method was successfully applied for the determination of ascorbic acid in various samples. The validity of the present method was checked by the flow injection analysis (FIA) method.     

Heterogeneous Fenton reactants: a study of the behavior of iron oxide nanoparticles obtained by the polymeric precursor method

Nanoparticles composed of iron oxides and iron salts were obtained from polymeric resins produced by polymerizing Fe²⁺-citrate and Fe³⁺-citrate complexes with ethylene glycol. The citric acid:Fe molar ratio was varied to obtain different synthesis conditions. The materials were treated at 450 °C for 2 h to obtain nanoparticles, which were characterized by XRD, Mössbauer spectroscopy, FEG, CHNS, atomic absorption and surface area through N₂ physisorption. Rhodamine B photo degradation in the presence of these nanoparticles and hydrogen peroxide was carried out to analyze the possible behavior of nanoparticles as heterogeneous Fenton reactants. UV–visible spectroscopy revealed that the catalytic activity in the presence of nanoparticles obtained with a citric acid:Fe molar ratio of 12:1 was the condition that provided the best results in this work.     

Low-temperature activation of nitrogen oxide on Cu-ZSM-5 catalysts

The adsorption and activation of NO molecules on Cu-ZSM-5 catalysts with different Cu/Al and Si/Al ratios (from 0.05 to 1.4 and from 17 to 45, respectively) subjected to different pretreatment was studied by ultraviolet-visible diffuse reflectance (UV-Vis DR). It was found that the amount of chemisorbed NO and the catalyst activity in NO decomposition increased with an increase in the Cu/Al ratio to 0.35–0.40. The intensity of absorption bands at 18400 and 25600 cm⁻¹ in the UV-Vis DR spectra increased symbatically. It was hypothesized that the adsorption of NO occurs at Cu⁺ ions localized in chain copper oxide structures with the formation of mono- and dinitrosyl Cu(I) complexes, and this process is accompanied by the Cu²⁺...Cu⁺ intervalence transfer band in the region of 18400 cm⁻¹. The low-temperature activation of NO occurs through the conversion of the dinitrosyl Cu(I) complex into the π-radical anion (N₂O₂)⁻ stabilized at the Cu²⁺ ion of the chain structure, [Cu²⁺-cis-(N₂O₂)⁻], by electron transfer from the Cu⁺ ion to the cis dimer (NO)₂. This complex corresponds to the L → M charge transfer band in the region of 25600 cm⁻¹. The subsequent destruction of the complex [Cu²⁺-cis-(N₂O₂)⁻] at temperatures of 150–300°C leads to the release of N₂O and the formation of the complex [Cu²⁺O⁻], which further participates in the formation of the nitrite-nitrate complexes [Cu²⁺(NO₂)⁻], [Cu²⁺(NO)(NO₂)⁻], and [Cu²⁺(NO₃)⁻] and NO decomposition products.     

Preparation and characterization of copper chloride supported on citric acid‐modified magnetite nanoparticles (Cu2+‐CA@Fe3O4) and evaluation of its catalytic activity in the reduction of nitroarene compounds

A new, powerful and recyclable copper catalyst were prepared by heterogenization of copper chloride using of Fe₃O₄ nano particles modified with citric acid as a linker. This system can catalyze reduction of nitroaren compound to aniline derivatives in the presence of Sodium borohydride as a reduction agent in moderate to good yields. In addition, easy separation and recoverable with an external permanent magnet is the dominant properties of this catalyst (Cu²⁺‐CA@Fe₃O₄).     

Cu2+-complexes of polyamino acids in aqueous solutions catalytic activity (H202-decomposition) and structure

The decomposition of H₂O₂ (8 · 10^?3M ) catalyzed by complexes of Cu²⁺ (4 · 10^?4M ) with various oligomers and polymers of glycine, L-lysine or L-glutamic acid was investigated in aqueous solution in the pH range 5–11, at 24°C and at low ionic strength. Previous investigations have shown that the decomposition of H₂O₂ is catalyzed by Cu²⁺-complexes capable of forming Cu²⁺-peroxocomplexes. With increasing pH the catalytic activity of Cu²⁺-complexes with glycine or glycylglycine (1:1) increases while the activity of Cu²⁺-complexes with tri- or tetraglycine (1:1) is comparatively small at higher pH values apparently because in the latter cases the coordination positions of the copper become progressively occupied by the peptides. This interpretation is in accordance with the pH-dependence of the light-absorption spectra of the latter complexes. Cu²⁺-complexes with poly-α, L -lysines of various molecular weights (molar ratios Cu²⁺: lysine residues = 1:15) have a catalytic activity comparable to or higher than that of the complex Cu²⁺-ethylenediamine (1:1), indicating two available coordination positions for formation of peroxo-complexes. On the other hand, the system Cu²⁺-L -lysine (1:15) showed no significant activity probably because all coordination positions at the Cu²⁺ are occupied by lysine. Despite the excess of ligand groups over Cu²⁺ in the polylysine systems the structure of this polyamino acid apparently does not allow its full coordination with these groups under the conditions investigated. Two adjacent chelating ε-amino groups are considered as the main ligand groups of the polymer to each copper ion. The Cu²⁺-poly-α, L -glutamic acid complex examined (Cu²⁺: glutamic acid residues = 1:5) shows comparatively little activity. In this case, absorption spectra indicate formation of hydroxo-complexes at higher pH. Besides the effects of structure, the electrostatic fields of the charged polyelectrolytes polylysine or polyglutamic acid are also considered to affect the rates of catalysis.     

A new method for electrocatalytic oxidation of ascorbic acid at the Cu(II) zeolite-modified electrode

A new method is developed for the catalytic oxidation of ascorbic acid at graphite zeolite-modified electrode, doped with copper(II) (Cu²⁺A/ZCME). Copper(II) exchanged in zeolite type A acts as catalyst to oxidize ascorbic acid. The modified electrode lowered the overpotential of the reaction by ∼400 mV. First, the electrochemical behavior of copper(II), incorporated in the zeolite type A modified electrode, was studied. The results illustrate that diffusion can control the copper(II)/copper(0) redox process at the Cu²⁺A/ZCME. Then, the behavior of electrocatalytic oxidation reaction for ascorbic acid was researched. The electrode was employed to study electrocatalytic oxidation of ascorbic acid, using cyclic voltammetry and chronoamperometry as diagnostic techniques. The diffusion coefficient of ascorbic acid was equal to 1.028 × 10⁻⁵ cm² s⁻¹. A linear calibration graph was obtained over the ascorbic acid with a concentration range of 0.003-6.00 mmol L⁻¹. The detection limit (DL) of ascorbic acid was estimated as 2.76 × 10⁻⁷ mol L⁻¹. The relative standard deviations of 10 replicate measurements (performed on a single electrode at several ascorbic acid concentrations between 3.0 and 200 μmol L⁻¹) were measured between 1.0 and 2.4%.