A naphthyridine-based receptor for sensing citric acid

A naphthyridine-based charge neutral receptor has been designed and synthesized. Its complexation with a series of carboxylic acids involved in the Krebs cycle has been studied by ¹H NMR, UV-vis and fluorescence methods. The receptor shows strong binding to citric acid (K_a = 1.60 × 10⁵ M⁻¹) and is also able to distinguish diastereomeric maleic acid from fumaric acid by fluorescence.     

Efficient utilization of inulin and glycerol as fermentation substrates in erythritol and citric acid production using <Emphasis Type="Italic">Yarrowia lipolytica</Emphasis> expressing inulinase

Inulin and glycerol were used as substrates for efficient erythritol and citric acid production by newly engineered Yarrowia lipolytica strains. Hydrolysis of inulin by the Y. lipolytica Wratislavia K1 strain was established by expressing the Kluyveromyces marxianus INU1 gene. Erythritol was produced in two stages: inulin was used for biomass formation, followed by erythritol biosynthesis initiated by glycerol addition. The highest titer of erythritol obtained, 120.9 g L^?1 with the yield of 0.6 g g^?1, was produced by the K1 INU 6 strain. Moreover, the K1 INU 6 strain in fed-batch culture produced a high amount of citric acid: 105.2 g L^?1 after 235 h from 200 g L^?1 of inulin. Maximum activity of inulinase during this culture was 14000 U g^?1 of cell dry mass. The presented study proves the potential of new Y. lipolytica transformants for efficient erythritol and citric acid production from inexpensive raw materials such as inulin and glycerol.     

Synthesis and characterization of functional l-lactic acid/citric acid oligomer

Oligomers of l-lactic acid and citric acid (PLCA) were synthesized by reacting lactic acid with citric acid in the presence of stannous chloride. The chemical compositions of these multicarboxylated oligomers were verified by FT-IR and ¹H-NMR spectroscopy. The thermal characteristics of the oligomers, such as glass transition temperature T_g, melting temperature T_m and melting enthalpy, were confirmed by DSC. The crystallinity of the oligomers were determined by DSC and WXRD. Meanwhile, the acid-base surface characteristics of PLCA have been determined by contact angle. The results implicated that these oligomers may be used to entrap the cospecies on PLLA surface in tissue engineering.     

Study on the selective leaching of low-grade phosphate ore for beneficiation of phosphorus and rare earths using citric acid as leaching agent

In the study the organic/inorganic chemical leaching and enrichment technology were used for selective extraction of the dolomite which co-existed in the Zhijin low-grade phosphate ore for beneficiation phosphorous and rare earths (RE) by using citric acid as leaching agent. The effects of acid concentration, reaction time, reaction temperature, liquid/solid ratio, and particle size on P₂O₅ and rare earths grade and the recovery ratio of them were investigated. The results show that under the optimized experimental conditions (acid concentration 9%, reaction time 240 min, reaction temperature 40°C, liquid/solid ratio 50: 1, and ore particle size 0.18–0.125 mm) the P₂O₅ grade can be increased from 15.47 to 34.82%, and P₂O₅ recovery rate comes up to 88.02%. The rare earths are mainly enriched in the leaching residues. Meanwhile, the recovery rate of rare earths is 72.08%. ΣREO grade can be increased from 978.06 × 10^–4 to 1998 × 10^–4%. In addition, the reaction kinetics of the chemical reaction between citric acid and dolomite are also discussed, the results show that the leaching process is controlled by chemical reaction. The activation energy for leaching was found to be 36.6337 KJ mol^–1 and k₀ was 3.67×104 s^–1, and the rate of the leaching based on the chemical reaction-controlled process could be expressed as 1–(1–a)^1/3 = 3.67 × 10⁴e^–36.63/RTt. Compare to the conventional process, the method provided in this study not only has advantages including higher phosphate concentration and rare earth grade, and higher recovery rate, but also using less amount of chemicals. Meanwhile, the citric acid can be recycled, avoiding discharge wastewater.     

Quantitative analysis of malic and citric acids in fruit juices using proton nuclear magnetic resonance spectroscopy

¹H NMR spectroscopy was applied to the quantitative determination of malic and citric acids in apple, apricot, pear, kiwi, orange, strawberry and pineapple juices. Aspartic acid was studied as a potential interference. The effect of the sample pH on the chemical shifts of signals from malic, citric and aspartic acids was examined and a value of 1.0 was selected to carry out the determination. Integration of NMR signals at 2.89-2.95 and 3.00-3.04 ppm were used for calculating the concentration of malic and citric acids, respectively. At this pH the integrated signals were not overlapped. Sodium 3-(trimethylsilyl)tetradeuteropropionate (TSP) was used as an internal reference. The obtained results applying NMR procedures to analyze the juices from different fruits were compared to those obtained using enzymatic methods and both were in close agreement. The intra- and inter-day repeatability was tested for apple juice (7.86 g l⁻¹ malic acid, 0.32 g l⁻¹ citric acid) and apricot juice (5.06 g l⁻¹ malic acid, 4.79 g l⁻¹ citric acid) obtaining coefficients of variation lower than 3.4% for intra-day measures (n = 10) and lower than 3.8% for inter-day measures (n = 20).     

Simultaneous production of citric acid and erythritol from crude glycerol by Yarrowia lipolytica Wratislavia K1

This study shows a possible microbial process for utilization of crude glycerol generated by the biodiesel industry for citric acid and erythritol production. Simultaneous production of citric acid and erythritol under nitrogen-limited conditions with glycerol as the carbon source was achieved with an acetate negative mutant of Y. lipolytica Wratislavia K1 in fed-batch cultivations. The effect of the initial glycerol concentration (from 30–180 g dm⁻³) on the citrate and erythritol production was investigated. As a result of the experiments, maximum citric acid production (110 g dm⁻³) and a very high amount of erythritol (81 g dm⁻³) were determined after 168 h of fed-batch cultivation with the initial glycerol concentration of 150 g dm⁻³ and the total glycerol concentration of 250 g dm⁻³. In addition, the citric acid to isocitric acid ratio of the products from this strain was 35.5:1. Presented at the 34th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 21–25 May 2007.     

Gas chromatographic determination of isocitric and malic acid in the presence of a large excess of citric acid

Derivatization yields of different esters (methyl, ethyl, n-propyl, isopropyl, n-butyl and isobutyl) of citric, malic and isocitric acids with various acylating agents, such as acetic anhydride, propionic anhydride, trifluoroacetic anhydride and heptafluorobutyric anhydride, were investigated. The formation of acylated malic and isocitric acid esters is rapid and quantitative whereas the acylation of citric acid esters was slow and partial in most instances. It was found that selective analytical conditions can be achieved with the O-heptafluorobutyryl n-butyl esters. The analytical applicability of the separation and determination of the malic, isocitric and citric acid contents of model solutions and of pressed lemon juice is discussed. Concentrations of 1.2 × 10^?3 ?9.0 × 10^?3g per 100 g of isocitric acid and 4.2 × 10^?3 ?3.5 × 10^?2 g per 100 g of malic acid in the presence of 9.3 × 10^?1 g per 100 g of citric acid were measured with relative standard deviations of 7.5 and 4.2%, respectively.     

A thermochemical study of acid-base interactions in aqueous solutions of citric acid

The heat effects of interaction of a solution of citric acid with HNO₃ and KOH were measured calorimetrically at 298.15 K and ionic strengths of 0.5, 1.0, and 1.5 (KNO₃). The heat effects of acid dissociation were calculated using the universal HEAT program. The standard thermodynamic characteristics of step dissociation of citric acid were determined.     

Complexation of citric and tartaric acids with Na and K ions in aqueous solution

The formation of MTar^–, MCit^2– and M₂Cit^– complexes (M is Na⁺ or K⁺) was established in reactions of aqueous solutions of citric and tartaric acids with sodium or potassium chloride solutions; their stability constants were determined by potentiometric titration in aqueous solution at 298.15 K and ionic strength 0.1 and 0.3 mol/l with tetraethylammonium chloride (TEACl) as a supporting electrolyte. Heat effects of reactions between citric acid solutions and sodium or potassium chlorides were measured by calorimetric method at 288.15, 298.15, and 308.15 K and at the ionic strength 0.1, 0.2, and 0.3 M TEACl. The increasing ionic strength was found to decrease exothermic effect of complex formation processes, while the temperature produced the opposite effect. Extrapolation to zero ionic strength was used to find thermodynamic stability constants and standard heat effects of complex formation reactions in solutions of oxyacids. The changes in entropy and heat capacity were calculated as well as standard enthalpies of formation of Na and K complexes of the indicated oxyacids in aqueous solution at 298.15 K.__________Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 4, 2005, pp. 253–261.Original Russian Text Copyright © 2005 by Zelenina, Zelenin.     

Simultaneous dual wavelength spectrophotometric determination of citric and ascorbic acids using an artificial neural network

In this study, a very simple spectrophotometric method for the simultaneous determination of citric and ascorbic acid based on the reaction of these acids with a copper(II)-ammonia complex is presented. The Cu²⁺-NH₃ complex (with λ_max = 600 nm) was decomposed by citrate ion and formed a Cu²⁺-citrate complex (with λ_max = 740 nm). On the other hand, during the reaction of ascorbic acid with copper(II)-ammonia complex, ascorbic acid is oxidized and the copper(II)-ammonia complex is reduced to the copper(I)-ammonia complex and the absorbance decreases to 600 nm. Although there is a spectral overlap between the absorbance spectra of complexes Cu²⁺-NH₃ and Cu²⁺-citrate, they have been simultaneously determined using an artificial neural network (ANN). The absorbances at 600 and 740 nm were used as the input layer. The ANN architectures were different for citric and ascorbic acid. The output of the citric acid ANN architecture was used as an input node for the ascorbic acid ANN architecture. This modification improves the capability of the ascorbic acid ANN model for the prediction of ascorbic acid concentrations. The dynamic ranges for citric and ascorbic acid were 1.0–125.0 and 1.0–35.0 mM, respectively. Finally, the proposed method was successfully applied to the determination of citric and ascorbic acids in vitamin C tablets and some powdered drink mixes. The text was submitted by the authors in English.     

A quinoline-based tripodal fluororeceptor for citric acid

The quinoline-based tripodal fluororeceptor 1 has been designed and synthesized for the detection of citric acid in less polar solvents. Receptor 1 shows monomer emission quenching followed by excimer emission upon hydrogen bond-mediated complexation of citric acid. In comparison, receptor 2, in presence of the same acid, gives rise to a decrease in the monomer emission of the naphthyl moiety without showing any peak for the excimer. Receptor 1 is found to bind citric acid more strongly than receptor 2 in CHCl₃.     

Calorimetric investigation of order–disorder transition in Cu0.6Pd0.4 and Cu0.85Pd0.15 alloys

Order–disorder phase transitions in Cu_0.6Pd_0.4 and Cu_0.85Pd_0.15 alloys have been investigated using differential scanning calorimetry and drop calorimetry. The differential scanning calorimetry measurements show that the transition in both these alloys are reversible in nature and the enthalpy increment measurements reveal that these transitions are first order in nature. The transition temperature of first-order phase transition in Cu_0.6Pd_0.4 and Cu_0.85Pd_0.15 alloys have been evaluated to be 884(±2) and 799(±2) K, respectively, from drop calorimetric measurements. The latent heat of first-order phase transition in Cu_0.6Pd_0.4 alloy were evaluated to be 31.2(±0.6) and 28.9(±0.5) J g^?1, by enthalpy increment and differential scanning calorimetry measurements, respectively. Similarly, the latent heat of first-order phase transition in Cu_0.85Pd_0.15 alloy were evaluated to be 23.1(±0.6) and 21.3(±0.5) J g^?1, by enthalpy increment and differential scanning calorimetry measurements, respectively. The solidus temperatures of Cu_0.6Pd_0.4 and Cu_0.85Pd_0.15 alloys were found to be 1,457(±2) and 1,360 K, respectively.     

1:1 Molybdenum(VI) citric acid complexes

Summary The complexes obtained by crystallisation from an aqueous solution of molybdate and citric acid (H₄cit) at pH 4–8 have Mo:cit ratio 11. The new complexes have been characterised by a variety of methods in the solid state and in solution. On this basis two dimeric species, [(MoO₂)₂O(citH)₂]^4– and [(MoO₂)₂O(cit)₂]^6–, and a monomeric species [MoO₃(cit)OH₂]^4– are proposed.     

Purification and properties of carnitine acetyltransferase from citric acid producingAspergillus niger

Carnitine acetyltransferase was purified from the citric acid producingA. niger mycelium with a protein band showing a relative molecular weight of 77,000 and a pH optimum of 7.3. TheK _m values for the purified enzyme for acetyl-CoA and for carnitine were 0.1 mM and 1 mM, respectively. Carnitine acetyltransferase was located both in the mitochondria and in the cytosol. Both mitochondrial and cytosolic enzyme were purified using ammonium sulfate precipitation, Mono Q and Superose 12 separation. Regarding the localization, except for maximum velocity, there were no differences observed in substrate specificity and inhibition. Inhibition of the enzyme with micromolar concentrations of Cu²⁺ could contribute to a greater citric acid biosynthesis. Carnitine acetyltransferase can be considered as an enzyme necessary for the transport of acetyl groups through mitochondrial membrane in both directions.     

Influence of borax and citric acid on the hydration of calcium sulfoaluminate cement

The hydration of calcium sulfoaluminate cement in the presence of borax and citric acid has been studied using isothermal calorimetry, X-ray diffraction, thermogravimetric analysis, Fourier transfer infrared spectrometry and mercury intrusion porosimetry. The results suggest that the formation of complex between calcium and citrate in the solution adsorbed on the solid surface delayed the C₄A₃ \( \bar{S} \) and anhydrite dissolution and AFt nucleation. At the same time, the presence of borate replaced sulfate anion in ettringite (AFt) to form B-AFt. These reactions have an effect on the setting times and the mechanical strength values. The setting times of the calcium sulfoaluminate cement increased when the mineral admixtures increased in the cement. The effect of borax and citric acid on the strength development at different hydration stages presented different results, namely a decreased strength in an early stage and an increased strength at 3 and 28 days.     

Reference value standards for pH measurements in 5, 15 and 30% (w/w) acetonitrile/water solvent mixtures at temperatures from 288.15 to 308.15 K

Reference value standards, pH (RVS), for 0.05 mol kg^?1 potassium hydrogenphthalate (KHPh) reference buffer solutions in 5, 15 and 30% (w/w) acetonitrile/water mixed solvents at temperatures from 288.15 to 308.15 K are determined from reversible e.m.f. measurements of the cell Pt¦H₂¦KHPh + KCl¦AgCl|Ag|Pt. Values of the first ionisation constant of o-phthalic acid (H₂Ph; benzene-1,2-dicarboxylic acid) in these mixed solvents are also determined from analogous measurements. The consistency of the results is analysed by multilinear regression of the quantity p(a_HγCl) as a function of both solution composition and temperature. The standard pH (RVS) values determined are given by the equation pH (RVS) = 4.0080 + 6.330x + 16.177x² ? 115.3x³ + 0.3089u ? 201.0ux² + 909ux³ + 13.04v, where x is the mole fraction of acetonitrile in the mixed solvent, u = z/(1 + z), v = [ln(1 + z) ? u], z = (T ? θ)/θ, and θ = 298.15 K.     

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.     

Thermal behaviour of citric acid and isomeric aconitic acids

Thermal decomposition of citric acid, trans- and cis-aconitic acid has been studied using the TG-MS, TG-FTIR and DSC techniques. The measurements were carried out in an argon atmosphere over a temperature range of 293–673 K. The influence of the acid structures and configurational geometry on stability of the transition products and pathways of thermal transformations of the studied compounds studied is discussed.     

Influence of citric acid as chemical modifier for lead determination in dietary calcium supplement samples by graphite furnace atomic absorption spectrometry

Citric acid was used as a chemical modifier for Pb determination by graphite furnace atomic absorption spectrometry in dietary supplement samples (calcium carbonate, dolomite and oyster shell samples) and its efficiency was compared to the use of palladium. Pyrolysis and atomization curves were established without use of chemical modifier, with the addition of 20, 100 and 200 μg of citric acid, and with 3 μg of palladium. The citric acid modifier made possible the interference-free Pb determination in the presence of high concentrations of Ca and Mg nitrates. Acid sample digestion involving closed vessels (microwave-assisted and conventional heating) and acid attack using polypropylene vessels at room temperature were compared. All digestion procedures presented similar results for calcium carbonate and dolomite samples. However, for oyster shell samples accurate results were obtained only with the use of closed vessel systems. Analyte addition and matrix-matched standards were used for calibration. The characteristic mass for Pb using citric acid and palladium were 16 and 25 pg, respectively. The relative standard deviation (RSD) was always less than 5% when citric acid was used. The relative and absolute limits of detection were 0.02 μg g^− 1 and 8 pg with citric acid and 0.1 μg g^− 1 and 44 pg with the Pd modifier, respectively (n = 10, 3σ). The recovery of Pb in spiked calcium supplement samples (10 μg l^− 1) was between 98% and 105%. With the use of 100 μg of citric acid as chemical modifier, problems such as high background absorption and high RSD values were minimized in comparison to the addition of 3 μg of palladium.     

Indirect atomic absorption spectrometric determination of citric acid by continuous precipitation

A continuous precipitation and filtration flow system for the separation of citric acid by precipitation with lead and indirect flame atomic absorption spectrometry is proposed. The precipitate is formed by injecting the lead solution into a carrier containing the sample and is subsequently retained on a filter. By using this reversed precipitation flow-injection configuration, citric acid was determined in the range 2–40 μg mL^–1, with a relative standard deviation of 2.9% at a sampling frequency of 60 samples h^–1. This method has been applied to the determination of citric acid in fruit juices, carbonated soft drinks and sweets.