Absorbance-time Curve Research Articles

Bromide Ion Impurity-Induced Reaction between Selenium(IV) and Acidic Bromate: Prototype of a Cycle with Autocatalytic Behavior.

The selenium(IV)-bromate reaction in an acidic medium using phosphoric acid/phosphate buffer was investigated by UV-vis spectroscopy monitoring the formation of bromine. In an excess of bromate, the absorbance-time curves measured at 450 nm display a characteristic sigmoidal shape having a fairly long induction period, while in the opposite case, when selenium(IV) species is used in excess, the measured data follow the rise and fall behavior. Depending on the excess of Se(IV) the final bromine-containing product is either an elementary bromine or bromide ion. Simultaneous evaluation of the measured kinetic traces clearly indicated that, surprisingly, no direct reaction takes place between the reactants. Instead of that, a trace amount of bromide ion impurity in the stock bromate solution is sufficient to drive the system via the oxidation of the bromide ion by bromate producing elementary bromine followed by the subsequent selenite-bromine reaction reestablishing the bromide ion to open a new cycle. As a result, the concentration of bromide ions increases in a sigmoidal fashion during the course of the reaction unless enough selenium(IV) species is present; hence, the overall synergetic effect observed is the autocatalytic rise of bromide ions. Therefore, the cycle mentioned above may be considered as a prototype of autocatalytic cycles. This observation prompted us to clarify the explicit difference between an autocatalytic cycle and an autocatalytic reaction.

Compatible Kinetic Model for Quantitative Description of Dual-Clock Behavior of the Complex Thiourea-Iodate Reaction.

The thiourea-iodate reaction has been investigated simultaneously by ultraviolet-visible spectroscopy and high-performance liquid chromatography (HPLC). Absorbance-time traces measured at the isosbestic point of the iodine-triiodide system have revealed a special dual-clock behavior. During the first kinetic stage of the title reaction, iodine suddenly appears only after a well-defined time lag when thiourea is totally consumed due to the rapid thiourea-iodine system giving rise to a substrate-depletive clock reaction. After this delay, iodine in the system starts to build up suddenly to a certain level, where the system remains for quite a while. During this period, hydrolysis of formamidine disulfide as well as the formamidine disulfide-iodine system along with the Dushman reaction and subsequent reactions of the intermediates governs the parallel formation and disappearance of iodine, resulting in a fairly constant absorbance. The kinetic phase mentioned above is then followed by a more slowly increasing sigmoidally shaped profile that is characteristic of autocatalysis-driven clock reactions. HPLC studies have clearly shown that the thiourea dioxide-iodate system is responsible mainly for the latter characteristics. Of course, depending on the initial concentration ratio of the reactants, the absorbance-time curve may level off or reach a maximum followed by a declining phase. With an excess of thiourea, iodine may completely disappear from the solution as a result of the thiourea dioxide-iodine reaction. In the opposite case, with an excess of iodate, the final absorbance reaches a finite value, and at the same time, iodide ion will disappear completely from the solution due to the well-known Dushman (iodide-iodate) reaction. In addition, we have also shown that in the case of the formamidine disulfide-iodine reaction, unexpectedly the triiodide ion is more reactive toward formamidine disulfide than iodine. This feature can readily be interpreted by the enhancement of the rate of formation of the transition complex containing oppositely charged reactants. A 25-step kinetic model is proposed with just 10 fitted parameters to fit the 68 kinetic traces measured in the thiourea-iodate system and the second, but slower, kinetic phase of the thiourea-iodine reaction. The comprehensive kinetic model is constituted in such a way as to remain coherent in quantitatively describing all of the most important characteristics of the formamidine disulfide-iodine, thiourea dioxide-iodine, and thiourea dioxide-iodate systems.

Kinetics and Mechanism of Permanganate Oxidation of ADA in Aqueous Perchlorate Solutions

Background: ADA as N-(2-Acetemido) iminodiacetic acid has been widely used in preparation of organic buffers and protein-free media for fibroblasts as chelating agent chicken embryo fibroblasts as tridentate chelating agent ligand. However, a little attention has been focused to the behavior of its oxidation in aqueous acidic solutions. Accordingly, a lack of information on the aqueous chemistry of oxidation of ADA in acidic solutions was recognized. Therefore, we have prompted to undertake the present work with the aims at shedding more light on the behavior of oxidation and the nature of oxidation products. Again, it aims to compensate the lack of information about the aqueous chemistry of ADA and the nature of electron-transfer for redox reactions involving permanganate ion as a multi-equivalent oxidant. Method: A spectrophotometric investigation of oxidation of ADA by permanganate ion in aqueous perchlorate solutions has been investigated. Results: Absorbance-time curves indicated the presence of two distinct stages for the oxidation process. The experimental results indicated a decrease in the reaction rates with increasing the [H+], ionic strength and F- ions. Again, the reaction rates were found to increase with increasing the added Mn2+ and temperature. Conclusion: These results indicated that the oxidation process was of acid-inhibition nature and proceeding by free-radical intervention mechanism. Again, the reactive species in the fast second stage was the Mn4+ species formed in the initial stage. The activation parameters have been evaluated from the temperature dependence of the rate constants. A tentative reaction mechanism was suggested for such redox reaction.

Determination Time of the Mononuclear Aluminum Species in the Hydroxyl Polyaluminum Solution by Ferron (7-iodine-8-hydroxyquinoline-5-sulfonic Acid)-Timed Spectrometry

A fixed time point on the curve of the absorbance variation with time determined in the hydroxyl polyaluminum solution by 7-iodine-8-hydroxyquinoline-5-sulfonic acid-timed spectrometry is usually used to quantify the mononuclear aluminum species in the hydroxyl polyaluminum solution. The right choice of the determining time of mononuclear Al must be established on the basis of the sufficient reaction between chromogenic reagent and the hydroxyl polyaluminum solution. But this question has never been noted, which resulted in the confusion of application and research of this assay. In this work, the effect of variation of the determining time on the accurate determination of mononuclear Al in the hydroxyl polyaluminum solution was first studied. The experimental results showed that 60 s was the best under the appropriate dosage of 7-iodine-8-hydroxyquinoline-5-sulfonic acid. The reasonableness of 60 s was further verified by 27Al nuclear magnetic resonance spectrometer and the absorbance–time curve obtained from the pure mononuclear Al solution with pH < 2. The above conclusion was further used to analyze the results reported in literature. The mononuclear Al determined at 60, >60, and <60 s in the literature were comparable to the corresponding results of this study.

Development of the kinetic-spectrophotometric method for quantitative determination of zopiclone in tablets by the perhydrolysis reaction

A simple and express method for the quantitative determination of zopiclone in model solutions of the substance and in “Zopiclone” tablets, 7.5 mg, by the kinetic-spectrophotometric method according to 3,3’,5,5’-tetramethylbenzidine oxidation has been developed. It is based on the system of two coupled reaction: 4-methyl-1-piperazineperoxycarboxylic acid generated in zopiclone perhydrolysis reacts with the excess of hydrogen peroxide in the weak alkaline medium with formation of coloured 3,3’,5,5’-tetramethyldiphenylquinone diimine (λmax = 420 nm, рН = 8.4). The reaction is performed spectrophotometrically by measuring the rate of change of the absorbance at 420 nm. The method was used for constructing the calibration graph. The initial rate of the reaction was obtained from the linear site of the slope of the initial tangent to the absorbance-time curve. In the pH range of 8.2-8.5 the rate of the coloured product formation becomes maximum. The calibration graph for zopiclone has a linear dependence in the range of 6-36 mg/l with the limit of detection (LOD) and quantitation (LOQ) of 1.81 and 6.04 mg/l, respectively. For five determinations of 18, 24 and 30 mg/l of zopiclone RSD is 1.81, 1.46 and 1.69%, respectively. The analytical performance of the method was validated statistically with respect to LOD, LOQ, accuracy, precision and linearity for zopiclone estimation in a pure substance and the results were satisfactory. “Zopiclone” tablets compared to the reference method contain 99.83±1.19% of C17H17ClN6O3 (RSD = 0.96% , δ = -0.17%). The assay of zopiclone in the presence of its hydrolysis products without preliminary separation is an important advantage of the method.

Addition and Redox Reactivity of Hydrogen Sulfides (H2S/HS−) with Nitroprusside: New Chemistry of Nitrososulfide Ligands

The nitroprusside ion [Fe(CN)(5)NO](2-) (NP) reacts with excess HS(-) in the pH range 8.5-12.5, in anaerobic medium ("Gmelin" reaction). The progress of the addition process of HS(-) into the bound NO(+) ligand was monitored by stopped-flow UV/Vis/EPR and FTIR spectroscopy, mass spectrometry, and chemical analysis. Theoretical calculations were employed for the characterization of the initial adducts and reaction intermediates. The shapes of the absorbance-time curves at 535-575 nm depend on the pH and concentration ratio of the reactants, R=[HS(-)]/[NP]. The initial adduct [Fe(CN)(5)N(O)SH](3-) (AH, λ(max) ≈570 nm) forms in the course of a reversible process, with k(ad)=190±20 M(-1)s(-1) , k(-ad)=0.3±0.05 s(-1) . Deprotonation of AH (pK(a)=10.5±0.1, at 25.0 °C, I=1 M), leads to [Fe(CN)(5)N(O)S](4-) (A, λ(max)=535 nm, ε=6000±300 M(-1) cm(-1) ). [Fe(CN)(5)NO](.)(3-) and HS(2)(.)(2-) radicals form through the spontaneous decomposition of AH and A. The minor formation of the [Fe(CN)(5)NO](3-) ion equilibrates with [Fe(CN)(4)NO](2-) through cyanide labilization, and generates the "g=2.03" iron-dinitrosyl, [Fe(NO)(2)(SH)(2)](-) , which is labile toward NO release. Alternative nucleophilic attack of HS(-) on AH and A generates the reactive intermediates [Fe(CN)(5)N(OH)(SH)(2)](3-) and [Fe(CN)(5)N(OH)(S)(SH)](4-) , respectively, which decompose through multielectronic nitrosyl reductions, leading to NH(3) and hydrogen disulfide, HS(2)(-) . N(2)O is also produced at pH≥11. Biological relevance relates to the role of NO, NO(-) , and other bound intermediates, eventually able to be released to the medium and rapidly trapped by substrates. Structure and reactivity comparisons of the new nitrososulfide ligands with free and bound nitrosothiolates are provided.

Photochemically induced catalysis of iodide ion and iodine in the tetrathionate–periodate reaction

The tetrathionate-periodate reaction has been studied in presence of acetate-acetic acid buffer at T = 25 +/- 0.1 degrees C and at I = 0.5 M ionic strength by a conventional diode array spectrophotometer. The stoichiometry of the reaction was found to be S(4)O + 7IO + 3H(2)O --> 4SO + 7IO + 6H(+). The profile of the measured absorbance-time curve is sigmoidal, having extremely long induction period that may be inferred as a classical type of autocatalysis, but surprisingly none of the products were proven to catalyze the reaction. Initial addition of micro amounts of iodine or iodide ion, however, strongly accelerates the reaction indicating that they are very efficient catalysts of the reaction. Moreover, it is also shown that the analyzing light beam has a great impact on the length of the induction period that questions the existence of the thermal reaction between tetrathionate and periodate. A 9-step kinetic model is presented and discussed that accurately takes the most important feature of the reaction into account.

Kinetic modelling of non-enzymatic browning in honey and diluted honey systems subjected to isothermal and dynamic heating protocols

The kinetics of non-enzymatic browning (NEB) in honey and diluted honey systems was investigated. The effect of α w (in the range of 0.54–0.99) on brown pigment formation in honey and its diluted solutions, differing in the concentration of reactant solutes was monitored upon heating at four temperatures (50, 60, 70 and 80 °C). The progression of the Maillard reaction was followed by spectrophotometric measurements at 420 nm ( A 420) and the absorbance–time curves were fitted to the logistic model. The processing temperature and water activity, or the initial reactant concentration, had a significant impact on browning kinetics and the color change of honey. Secondary models, expressing the dependence of the best fitted primary model parameters on temperature and α w, were further developed. In addition, an alternative method of expressing the temperature dependence of browning rate constants using the WLF kinetic formalism, based on the glass transition temperature, T g, was applied. The main secondary models were validated by comparing the predicted model parameters with those obtained from isothermal experiments. Finally, the derived kinetic model was further evaluated against the observed browning responses of honey under dynamic heating conditions to examine the applicability of the developed model over fluctuating temperature–time protocols.

Kinetic modelling of non-enzymatic browning of apple juice concentrates differing in water activity under isothermal and dynamic heating conditions

The kinetics of non-enzymatic browning in apple juice concentrates were investigated. The effect of a w (in the range of 0.74–0.99) and/or reactant concentration on brown pigment formation was monitored under isothermal heat treatment at four temperatures (60, 70, 80 and 90 °C) in apple juice solutions having either the same or different concentrations of reactant solutes. The extent of the Maillard reaction was evaluated by spectrophotometric measurements at 420 nm ( A 420). The absorbance–time curves were fitted to five different kinetic models (zero and first order, weibull, logistic and the parabolic model) and estimates of browning rate constants and other model parameters were obtained. Regression analysis revealed that the logistic model was the most appropriate for describing browning in apple juice. The initial reactant concentration, but not water activity, had a significant effect on the colour change of apple juice. The processing temperature also had a strong impact on browning kinetics. Secondary models, expressing the dependence of the best fitted primary model parameters on temperature and a w, were further developed and validated by comparing the predicted model parameters with the values observed in independent isothermal experiments. Finally, the derived model was further evaluated against the observed browning responses of apple juice under dynamic heating conditions, underlining the applicability of the developed model as a practical prediction tool for the study of non-enzymatic browning.

Kinetic spectrophotometric method for o-phenylenediamine in the presence of gold(III)

From the color developing reactions of o-phenylenediamine oxidizing agent and gold(III), the kinetic reactions between both of them in aquaeous solutions were studied using spectrophotometric and differential method. Light absorbances in the visible spectral range are measured as a function of mole fractions of phenylenediamine at a fixed gold(III) concentration and as a function of mole fraction of gold(III) at a fixed o-phenylenediamine concentration at periodic time internal. In the differential method, which was suggested by van’t Hoff, one deals with the actual rates of reactions as determined by measuring the slopes of absorbance–time curves. Optimum condition of the reaction were established as pH 6 at λ = 466 nm and room temperature. When the oxidation of o-phenylenediamine by gold(III) was investigated, it was observed that the following rate formula and rate constant were found: v = k [ A u + 3 ] 1 / 2 [ o -phenylenediamine ] 1 / 2 , k = 2.33 × 10 − 2 s − 1

A new heating strategy in electrothermal atomic absorption spectrosmetry for better absorbance–time curves at high atomization rate

The results previously obtained by using fast heating in electrothermal atomic absorption spectrometry are considerably improved by using a new heating sequence which can be summarized as transverse–longitudinal heating mode. The absorbance vs. time curves, obtained with the new heating mode, follow almost perfectly a simple model in which only diffusion is considered as the force acting on the atomic vapor. From the fitting of the experimental absorbance vs. time data points with theoretical values, it is possible to calculate both the absorbance, when all atoms injected are assumed to be present, and their diffusion coefficient. Both values can be calculated by a simple software approach without the operator intervention. The asymptotic absorbance calculated in this way is the maximum absorbance physically obtainable and is the basis for standardless analysis.

Kinetics and mechanism of the reactions of [FeCl4]– with PhS– or PhSH in MeCN, and the role of cations containing N–H groups

The kinetics of the reactions between [FeCl4]- and an excess of PhS- have been studied using stopped-flow spectophotometry. The associated absorbance-time curves can be fitted to two exponentials, and these first and second phases correspond to the formation of [FeCl3(SPh)]- and [FeCl2(SPh)2]-, respectively. It seems likely that the steps involving formation of [FeCl(SPh)3]- and [Fe(SPh)4]- are associated with much smaller changes in absorbance and so are not detected. The kinetics of the first phase exhibit a non-linear dependence on the concentration of PhS- indicating an associative mechanism in which PhS- rapidly binds to [FeCl4]- to form [FeCl4(SPh)]2- prior to rate-limiting dissociation of chloride and formation of [FeCl3(SPh)]-. The kinetics indicate that at high concentrations of PhS-, the five-coordinate intermediate attains stoichiometric concentrations. This is confirmed by the spectroscopic changes. The second phase shows analogous kinetics. The kinetics of the reactions between [FeCl4]- and an excess of PhSH have also been studied. For the first phase the reaction occurs at a rate independent of the concentration of PhSH, consistent with an associative mechanism in which the solvent (MeCN) is the nucleophile to form [FeCl3(NCMe)]. Subsequent rapid replacement of the coordinated solvent by PhSH yields [FeCl3(SHPh)]. The kinetics of the second phase of the reaction with PhSH exhibits a non-linear dependence on the concentration of PhSH, analogous to the kinetics observed with PhS- and consistent with an associative mechanism. The cations [NHEt3]+, [NH2Et2]+ and [lutH]+ (lut = 2,6-dimethylpyridine) form ion pairs with [FeCl4]- which undergo substitution more rapidly than free [FeCl4]-.

Reaction Rate Method for Spectrophotometric Determination of Trace Amounts of Resorcinol

A sensitive and highly selective initial reaction rate method for the determination of resorcinol is reported. The method is based on its inhibitory effect on the formaldehyde catalyzed reaction of nuclear fast red and bromate. The difference in the slope of the absorbance–time curve of catalyzed and catalyzed‐inhibited reaction Δ(slope) was proportional to the concentration of resorcinol. The effect of different parameters on the reaction rate was studied and optimum conditions were established. A linear calibration curve in the concentration range of 0.5–4.5 µg mL−1 of resorcinol with a regression coefficient of 0.9991 was obtained. The limit of detection was 0.20 µg mL−1 and relative standard deviation for 10 replicate measurements of 3.0 µg mL−1 of resorcinol was 2.5%. The method was successfully applied to the determination of resorcinol in a topical cream sample.

Activity behavior of a HPLC column including α-chymotrypsin immobilized monosized-porous particles

In this study, a polymer-based, α-chymotrypsin (CT) immobilized HPLC column was prepared as a potential material for affinity-HPLC and chiral separation applications. Monosized-macroporous particles were synthesized as the support material by a relatively new polymerization protocol, the so-called, ‘modified seeded polymerization’. The particles were obtained in the form of styrene-glycidyl methacrylate- divinylbenzene terpolymer approximately 11 μm in size. The particles were treated with aqueous ammonia to have primary amine groups on the porous surface. The amine functionalized particles were reacted by glutaraldehyde and the enzyme, CT, was covalently attached. CT carrying monosized-porous particles were slurry packed into the HPLC column 50 mm×4.6 mm in size. Since the activity behavior of immobilized CT played an important role in the enantiomeric separations performed by similar columns, the enzymatic activity behavior of the column produced by our protocol was determined. For this purpose, HPLC column was used as a packed bed reactor and the enzymatic reaction was continuously followed by measuring the absorbance of the output flow by the UV-detector of HPLC. S-shaped absorbance–time curves were obtained by monitoring the reactor output both in dynamic and steady-state periods. The columns with relatively lower immobilized enzyme content were more sensitive to the changes in the operating conditions and responded with more appreciable substrate conversion changes. The maximum reaction rate of the immobilized enzyme was estimated as approximately 25% of the free one by the mathematical model describing the activity behavior of the column. No significant loss was observed in the activity of the immobilized enzyme during the course of the experiments.

Activity behavior of a HPLC column including $alpha;-chymotrypsin immobilized monosized-porous particles

In this study, a polymer-based, α-chymotrypsin (CT) immobilized HPLC column was prepared as a potential material for affinity-HPLC and chiral separation applications. Monosized-macroporous particles were synthesized as the support material by a relatively new polymerization protocol, the so-called, ‘modified seeded polymerization’. The particles were obtained in the form of styrene-glycidyl methacrylate- divinylbenzene terpolymer approximately 11 μm in size. The particles were treated with aqueous ammonia to have primary amine groups on the porous surface. The amine functionalized particles were reacted by glutaraldehyde and the enzyme, CT, was covalently attached. CT carrying monosized-porous particles were slurry packed into the HPLC column 50mm×4.6 mm in size. Since the activity behavior of immobilized CT played an important role in the enantiomeric separations performed by similar columns, the enzymatic activity behavior of the column produced by our protocol was determined. For this purpose, HPLC column was used as a packed bed reactor and the enzymatic reaction was continuously followed by measuring the absorbance of the output flow by the UV-detector of HPLC. S-shaped absorbance–time curves were obtained by monitoring the reactor output both in dynamic and steady-state periods. The columns with relatively lower immobilized enzyme content were more sensitive to the changes in the operating conditions and responded with more appreciable substrate conversion changes. The maximum reaction rate of the immobilized enzyme was estimated as approximately 25% of the free one by the mathematical model describing the activity behavior of the column. No significant loss was observed in the activity of the immobilized enzyme during the course of the experiments.

Stopped-Flow Spectroscopic Studies on the Ligand-Exchange Reaction of Cu–(Glycine-Peptide) Complexes, Cu(H−iL), with Cysteine. Cu(II) Transport and Characterization of the Intermediate Ternary Complexes Cu(H−1L)(Cys−); L = Glycylglycine (i = 1), Triglycine (i = 2), Tetraglycine (i = 2 or 3), and Pentaglycine (i = 2 or 3)

Abstract Cu(II) complexes of glycine-peptides, abbreviated as Cu(H−iL): L = glycylglycine, triglycine, tetraglycine, and pentaglycine (i = 1–3), react with cysteine to form ternary complexes, Cu(H−1L)(Cys−), as first intermediates. The spectral parameters of the ternary complexes, which were similar irrespective of the peptides, were as follows: λmax = 332 ± 1 nm (ε = 4250 ± 50 M−1 cm−1), g|| = 2.170 ± 0.005, g⊥ = 2.00 ± 0.05, and A|| = (2.05 ± 0.01) × 10−4 cm−1, indicating that the ternary complexes have identical coordination structures: the Cu(II) coordinates with the peptides via the nitrogens from the terminal amino and the neighboring deprotonated-amide group, and with cysteine via the amino nitrogen and the thiolate sulfur. Based on the absorbance-time curves, the concentrations of each Cu(II) and Cu(I) species during the reaction were calculated. The species distribution curve clearly visualized the pathway of the Cu(II) transport from Cu(H−iL) to Cu(Cys−)2 via Cu(H−1L)(Cys−). The rate of Cu(H−1L)(Cys−) formation, which was evaluated from the initial increase in the species distribution curve, depended on the coordination modes of the Cu(H−iL) complexes. Both the Cu(H−1L), involving N,N−,O−,O* donors, and the Cu(H−2L), involving N,N−,N−,O−* donors, rapidly formed Cu(H−1L)(Cys−) complexes, where the donors asterisked represent the fourth ligand in the Cu(H−iL) complexes. The second-order rate constant, k1+, was on the order of 106 M−1 s−1, or bigger. The Cu(H−3L), involving N,N−,N−,N−* donors, reacted relatively slowly; k1+ was on the order of 104 M−1 s−1. Those results indicate that the affinity of the Cu(II) for the fourth donor in Cu(H−iL) determines the rate of metal-transport.

Spectrophotometric determination of trace amounts of hemoglobin using the oxidative decomposition reaction of a copper(II)–phthalocyanine complex

A catalytic effect of hemoglobin was found in the oxidative decomposition of a copper(II)–phthalocyanine complex (Cu–pts) with peroxomonosulfate. Based on this finding, a simple, rapid, highly sensitive and high-precision spectrophotometric method for hemoglobin was developed. In this indicator system, Cu–pts as the indicator and hemoglobin as the catalyst were simultaneously decomposed with peroxomonosulfate. Therefore, the slopes of the absorbance–time curve of the sample solution become equal to that of the blank solution after 5 min. The calibration curve was linear over the range of 12–258 μg/l. Detection limits (3 σ) were 8.13 μg/l, and the relative standard deviation was 2.05% (10 determinations) for 130 μg/l of hemoglobin. The metal ions did not significantly produce an interference in this method. The albumin had some influence but was permissible up to 6 times the weight of hemoglobin. The proposed method was applied to human blood samples with satisfactory results.

Evaluation of a predictive kinetic procedure for the enzymatic measurement of creatinine in serum.

We have evaluated the analytical performance of a predictive kinetic procedure for the enzymatic measurement of serum creatinine in comparison with equilibrium and kinetic fixed-time procedures. The procedure uses commercially available reagents and is based on calculating the total change in absorbance expected if the reaction were to proceed to equilibrium by fitting the initial part of the absorbance-time curve to a three-parameter function by non-linear regression. Total imprecision for different concentrations of creatinine ranges from 1.47 to 1.86% and is significantly better than that obtained for the fixed-time procedure. The results with the predictive procedure are less dependent on creatinine amidohydrolase activity in the reagent, pH of the reagent and temperature of analysis than are the results with the fixed-time procedure. All the procedures are interfered with by bilirubin and ascorbic acid to about the same extent. Recovery and linearity are quite acceptable, and the estimated accuracy is below 2.1%. However, in comparison with the equilibrium procedure, the predictive procedure tends to underestimate creatinine concentrations at values below 100 micromol/l, and the results obtained by the two procedures are not transferable. In conclusion, the predictive approach substantially improves the imprecision but not the specificity of the enzymatic assay of serum creatinine.

The hexacyanomanganate(IV)–hydrogen peroxide reaction. Kinetic determination of vanadium

The reaction between hexacyanomanganate(IV) and hydrogen peroxide in acidic medium is strongly catalysed by vanadium. The stoichiometry has been proved to be Δ[Mn(IV)]/Δ[H 2O 2]=1. S-shaped absorbance–time curves are obtained for a wide range of conditions, which seems to be due to the transient formation of hydroperoxyl radicals, HO 2·; therefore, the reaction is autoinduced. The reaction kinetics is first order on the hexacyanomanganate(IV) concentration. Complex dependence of the initial rate on the H 2O 2 as well as H + concentrations are observed both for the uncatalysed and for the catalysed reactions, but the rate laws are given and reaction mechanisms are proposed. The catalytic effect of vanadium has been used to determine traces of vanadium (the (IV) and (V) oxidation states are determined together). By applying the initial rate method a detection limit of 0.9 ng ml −1 (18 nM), and a linear range up to 50 ng ml −1 were found. The relative standard deviations for the 4 and 25.5 ng ml −1 levels were 5.2 and 2.5% respectively. Positive kinetic interferences from Cr(VI), Hg(I) and Ag(I) have been observed; other interferences are less severe and Cr(IIl), Fe(II) and Fe(III), among many others, do not interfere. The Mn(IV) solution must be prepared daily and its conservation needs some care (0°C) but the proposed method has been applied to the determination of vanadium in a phosphate rock (reference material, BCR No. 32) without previous separations. Recovery experiments have also been performed; excellent results were obtained in both cases.

Spectrometric pK Determination of Daphnetin, Chlorogenic Acid and Quercetin

Abstract Daphnetin, Chlorogenic Acid, Quercetin Polyphenols play an important role in the ‘immune system’ of plants. Since their charge is important for their physiological effect it is important to know their pKj values. Therefore, the pKj values of some plant contents such as daphnetin (pK = 7.36), chlorogenic acid (pK ≈ 8.5), and quercetin (pK1 = 7.03, pfro = 9.15) were determined in the pH range 7 -10 using UV/VIS spectroscopy. The decomposition kinetics of quercetin in alkaline solutions was taken into consideration by extrapolating the absorbance time curves to time zero (t = 0). The graphic method of absorbance (A) diagrams was used to determine the pKi values. In the case of quercetin the method of linear regression was also used for the determination.