Kinetic study of the iodate—iodide and chlorate—iodide reactions in acidic solutions,and a method for the microdetermination of bromide

The iodate—iodide and chlorate—iodide reactions were studied spectrophotometrically in acidic solutions by stopped-flow techniques. Intermediate products(I⁺)were followed; reaction rate constants and activation energies of the reactions were determined. A method of determining bromide was developed on the basis of its accelerating effect on the iodate—iodide reaction ; microamounts of bromide in the range 16–320 μg (10^-4–2 × 10^-3M) were determined with relative errors and relative standard deviation of about 2%.bl]     

Spectrophotometric determination of periodate and glycerol with 5,5-dimethyl-1,3-cyclohexanedione bisthiosemicarbazone monohydrochloride

A direct spectrophotometric method for the determination of periodate (1—10 ppm) is based on the formation of a yellow oxidation product of 5,5-dimethyl-1,3-cyclohexanedione bisthiosemicarbazone monohydrochloride in aqueous solution at pH 0.5—1.2. The molar absorptivity at 415 nm is 1.72 × 10⁴ l mol^-1 cm^-1. The relative standard deviation is 0.8% for 8.0 ppm periodate. A modification to this procedure is used for the indirect spectrophotometric determination of glycerol (2–10 ppm), via the Malaprade reaction.     

Controlled anodic generation of the catalyst in kinetic continuous flow analysis

Controlled anodic dissolution of copper in a separate generator cell yields well-defined concentrations of catalyst, depending on the voltage applied. This adjustable generation of copper catalyst makes it possible to determine iron over a wide range of concentration (10–1500 μg Fe³⁺ ml^-1) via the iron(III)—thiosulphate reaction. By the copper(II)-catalyzed hydrogen peroxide—hydroquinone reaction, EDTA can be determined as an inhibitor (0.5–5 μg ml^-1) and cadmium(II) as a reactivator (1–10 μg ml^-1). As zinc(II) forms complexes with 2,2'-bipyridine, which activates copper in this reaction, it can be determined (5–50 μg Zn²⁺ ml^-1) by measuring the decrease in activation. The electrogeneration of silver ion as a catalyst is also described. The sulphanilic acid—peroxodisulphate reaction is catalyzed by silver(I), which is again activated by 2,2'-bipyridine. Zinc(II) can be determined (0.29–2.9 mg Zn²⁺ ml^-1) by the same principle as in the copper(II)-catalyzed reaction.     

Enzyme analysis by means of the air-gap electrode-determination of urease and arginase by monitoring of the initial reaction rate

The air-gap electrode has been used in kinetic chemical analysis, illustrated by the assay of the activities of the enzymes urease and arginase. The determinations are based on monitoring the initial reaction rates of the selective release of ammoniacal nitrogen, which in the arginine—arginase system was ensured by adding excess of urease. The reaction rates measured are in the range 2.5 · 10^-5—1.5 · 10^-3M min^-1; the relative standard deviation is ca. 2.8 %.     

The application of the ph-stat method to metal ion-catalyzed reactions

The pH-stat method, which is well known in organic chemistry and biochemistry, is used for the kinetic determination of metal ion catalysts. Indicator reactions that involve protons can be followed by controlled addition of standard base or acid. This is illustrated by the following examples: determination of copper(II) (0.03–0.3 μg ml^-1) with the indicator reaction ascorbic acid—peroxydisulphate; determination of molybdenum(VI) (0.2–2.5 μg ml^-1) with the indicator reaction thiosulphate—hydrogen peroxide; determination of zirconium(IV) (0.2–2 μg ml^-1) with the indicator reaction iodide—hydrogen peroxide; and determination of vanadium(V) (0.2–2 μg ml^-1) with the indicator reaction iodide—bromate. For one example, the copper—ascorbic acid—peroxydisulphate reaction, it is shown that the pH-stat method has distinct advantages over closed systems, giving considerably better sensitivity for the determination of copper (0.5–5 ng ml^-1 ).     

Mechanism and kinetics of hypophosphite oxidation by permanganate ion

The kinetics of the permanganate‐hypophosphite redox reaction has been studied over a wide range of pH by using a stopped‐flow technique. It has been found that the reaction leads to the formation of phosphite and orthophosphate, and that the molar ratio of the final products depends on pH. The proposed mechanism of the process was based on the assumption that the first step in the oxidation of hypophosphite is a fast reversible reaction in which an intermediate complex (O₃MnOH₂PO₂)²⁻ is formed. The observed dependence of the reaction rate on pH was attributed to the influence of hydrogen and hydroxyl ions on the decomposition of this complex. The rate constants and the equilibrium constants of elementary reactions are given. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 737–743, 1999     

Cation exchange kinetics of pyridinecarboxylic acids

The sorption—desorption kinetics of nicotinic and isonicotinic acids on strong acid cationites (sulfocationite CU-2-8, sulfonated polycalixarene) were considered. The rate of the processes, following both the cation exchange mechanism H⁺ ? Cat⁺ and the ion exchange mechanism accompanied by the chemical reaction of protonation of pyridinecarboxylic acid zwitterions, is determined by a slow diffusion of components in the polymeric phase. The interdiffusion coefficient in sulfocationite CU-2-8 for the process H⁺ ? Cat⁺ was equal to ~1?10^–11 m² s^–1, in sulfonated polycalixarene the corresponding value was in the range of (1—4)?10^–12 m² s^–1.     

Study on the determination of trace rhenium (VII) by the adsorption differential pulse polarography

The determination of trace rhenium (VII) by differential pulse polarography in the system of H₂SO₄-(NH_sOH)₂ · H₂SO₄-TeO^2?₄ is markedly improved by the addition of Nitron, which is adsorbed on the surface of mercury electrode. The limit of detection is down to 2 × 10¹⁰ M. The adsorptive peak potential is ?0.80 V (vs. SCE). In the ranges of 5 × 10¹⁰—10^?8, 1 × 10^?5—10^?7 and 1 × 10^?7—10^?6M, there are good linear relationships between the peak current increment and the concentration, of which the relative standard deviations are 9.5, 6.6, 1.8% respectively with the correlation coefficients of linear regression of 0.995–0.999. The results relating to this polarographic wave show that it is an adsorption-catalytic wave. The mechanism of the electrode reaction is discussed.     

Determination of glutamate pyruvate transaminase and pyruvate with an amperometric pyruvate oxidase sensor

Pyruvate oxidase (E.C. 1.2.3.3.) is immobilized by adsorption on a porous acetylcellulose membrane, and combined with an oxygen electrode to provide a sensor for pyruvate (0.1–0.8 mM). The response time is 2 min. Glutamate pyruvate transaminase (0.5–180 × 10^-3 I.U. ml^-1) is determined by its effect on pyruvate production by the alanine—α-ketoglutarate reaction. The sensor is stable for more than 10 days and 100 assays.     

Influence of preliminary UV irradiation on the thermal decomposition of copper hypophosphite

The effect of the influence of preliminary UV irradiation on the kinetics of the thermal decomposition of crystalline copper hypophosphite has been discovered, revealing a decrease in the induction period. Spectral dependence of the effect was determined by absorption of copper hypophosphite in the 255 nm charge transfer band. Comparison of the quantum efficiency of the disappearance of ions of bivalent copper 10^?1–10^?2 ions per quantum and the quantum output of hydrogen 10^?4–10^?5 mole per quantum as well as acid formation during photolysis shows that a proton (acid) as well as the univalent copper is the main primary product of photolysis. Formation of acid in the course of photolysis participating in the nucleation during the investigated topochemical process of thermal decomposition is the reason for the discovered effect.     

Double indication in catalytic-kinetic analysis: simultaneous photometric and thermometric indication of the iodine-azide reaction in closed and flow systems

The iodine—azide reaction catalyzed by sulphur-containing compounds is followed simultaneously by optical and thermometric measurements in closed and flowing systems. In the closed system, thiosulphate can be determined in the range 32.4–324 μg ml^-1, by observing the turbidity caused by the nitrogen formed during the reaction and the temperature changes. With the flow apparatus, thiosulphate can be determined in the range 112–1120 μg ml^-1 by continuously mixing the sample and reagent solutions. H₂S in nitrogen 5–100 ppm) is measured by sweeping the gas into the reaction Cuvette. In a third flow procedure, H₂S is liberated continuously from sodium sulphide solutions (0.1–10 μg S^2- ml^-1) by ascorbic acid, and swept to the measuring cuvette with nitrogen.     

Chiral phosphinyl analogues of 2-C-arylmorpholinols: 2-aryl-3,5-diphenyl-[1,4,2]-oxazaphosphinanes

(2R,3R,5R)-2-Aryl-3,5-diphenyl-[1,4,2]-oxazaphosphinanes 6, analogues of C-arylmorpholinol 3, were prepared with diastereomeric excess higher than 94%, via a three step sequence: (i) diastereoselective addition–cyclization reaction from methyl hypophosphite and a chiral imine 10, (ii) pallado-catalyzed arylation, and then (iii) a selective inversion of configuration at the phosphorus center.     

Determination of cadmium,indium and zinc in nickel—base alloys by atomic absorption spectrometry with introduction of solid samples into furnaces

Atomic absorption spectrometry with an induction furnace is used for the determination of cadmium (0.002–2 μg g^-1), indium (0.6–350 μg g^-1) and zinc (0.05–26 μg g^-1) in 0.8–35 mg samples of nickel—base alloys dropped into the furnace. A resistively-heated furnace is employed for the determination of lower concentrations of indium (<0.6 μg g^-1). Standardised alloys were used for calibration. Accuracy, precision and detection limits are described for numerous nickel—base alloys. With alloys containing zinc, > 0.1 μg Cd g^-1 and >0.6 &,mu;g In g^-1, the relative standard deviations are 12%, 8% and 7%, respectively. Calculated detection limits for cadmium, indium and zinc are 2 ng g^-1, 10 ng g^-1 and 10 ng g^-1, respectively.     

Thermal decomposition of metal complexes. X. uranyl nitrate—phenylurea complexes

The kinetics and mechanism of the thermal association process (solid—solid interaction) and the reverse dissociative process

(where n-2–5;=m1–4; n+ m= 3–6) is discussed. The “ activation energy” and the reaction order have been evaluated. It was found that the associative reaction is chemically controlled whereas the dissociative process is physical in nature; therefore no mutual agreement was found between theE^*_a values relative to the two associative and dissociative processes.     

Reagentless lactate sensor based on cytochrome b2

The sensor is based on cytochrome b2 adsorbed on the semi-conducting N-methyl-phenazinium—7, 7,8,8-tetracyanoquinodimethane complex held on a platinum electrode. At an applied potential of —0.03–0.4 V (vs. Ag/AgCl), steady-state currents are reached in 0.5–0.7 min. The optimal pH is 6.6. Lactate in the range 10^-5 –lO^-3 M can be determined. The sensor is useful for 3–9 days depending on the enzyme source.     

Improved determination of hydrogen peroxide by measurement of peroxyoxalate chemiluminescence

Chemiluminescence from the reaction of bis-(2,4,5-trichloro-6-carbopentoxyphenyl) oxalate with hydrogen peroxide in the presence of triethylamine in t-butanol—water has been investigated as a means of determining hydrogen peroxide. Increasing the percentage of t-butanol increases the signal-to-background ratio but reduces the absolute magnitude of the emission signal. The sensitivity is greatest in aqueous solutions at pH 8; the response is linear from the detection limit (2 × 10^-8 M) to 10^-3 M. The system is also shown to respond linearly to uric acid concentrations in the range 1–4 × 10^-6 M, when uricase is used to catalyze uric acid oxidation to yield hydrogen peroxide.     

A urea complex of copper(II) hypophosphite at 293, 100 and 15 K

The crystal structure of poly­[copper(II)‐di‐μ‐hypophosphito‐μ‐urea], [Cu(H₂PO₂)₂(CH₄N₂O)]_n, has been determined at 293, 100 and 15 K. The geometry of the hypophosphite anion is very close to ideal, with point symmetry mm2. Each Cu atom lies on an inversion centre and is coordinated to six O atoms from four hypophosphite anions and two urea mol­ecules, forming a tetragonal bipyramid. The unique urea molecule lies on a twofold axis. Each hypophosphite anion in the structure is coordinated to two Cu atoms. The hypophosphite anions, urea mol­ecules and Cu^II cations form polymeric ribbons. The Cu^II cations in the ribbon are linked together by two hypophos­phite anions and a urea mol­ecule, which is coordinated to Cu via an O atom. The ribbons are linked to each other by N—­H?O hydrogen bonds and form polymeric layers.     

Infrarotspektroskopische untersuchungen an metallcupferronaten im bereich 4000-32 cm-1

The preparation and empirical interpretation of the m.i.r. and f.i.r. spectra of the cupferronates of Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Sn²⁺, Pb²⁺ und Bi³⁺ in the range 4000—32 cm^-1 is described. The bands in the range 1700—500 cm^-1 and the C—H valency vibrations can be assigned to vibrations of the pure ligand; differentiation of individual metal chelates in this range is therefore difficult. In contrast, the far i.r. region (below 500 cm^-1) contains many absorption bands (the metal chelate fingerprint region) which differ in position and intensity. The position of the bands with the highest v—M—O character, which can be recognised on the basis of the large metal-dependent frequency shifts, depends linearly on the second ionization potential of the metal atom, except for the Mn and Sn chelates. The relative absorptions, E_M—O/E, of the far i.r. bands of these cupferronates allow classification of the chelates into groups corresponding to the metal—ligand bonding involved (tetrahedral, octahedral and planar). The characteristic value used for this classification (the quotient of the absorption at the M—0 frequency and the absorbance of the strongest remaining band in the far i.r.) covers the range 0.92–2.17 for tetrahedral chelates, and 5.30–6.20 for planar chelates, with the octahedrally coordinated chelates having intermediate values.     

Formic acid as a reagent for alkaline permanganate : Potentiometrrc determination of quadrivalent selenium

Se⁺⁴ can be determined by mixing with KMn0₄ in l N NaOH, stirring the mixture at room temperature and measuring the potential until equilibrium, which needs ~10–15 min. Excess KmnO₄ is then determined with formate.In the direct oxidation of Se⁺⁴ with MnO₄^- in the cold, and in the presence of 2.5 N NaOH and 10% NaCl, MnO₄^- → MnO₄^-2. At 90°C, and in the presence of 0.1 N NaOH 10% NaCl and 2— 3 ml of 0.5% AuCl₃, MnO₄^- → MnO₂. The reaction which is rather slow is accelerated by the above reagents.Reduction of MnO₄^- with Se⁺⁴ in l— 3 N NaOH yields MnO₄^-2.Like the indirect method, the direct potentiometric procedures yield good results.     

Thermal lensing spectrophotometric analysis with ion-pair solvent extraction

Thermal lensing spectrophotometry is applied to the determination of iron(II) with 4,7-diphenyl-1,10-phenanthroline disulfonic acid in aqueous solution, and in chloroform by ion-pair extraction with trioctylmethylammonium chloride. A phase-sensitive detection system with digital processing was used, the optimum modulation frequency being 5–10 Hz. A baseline drift of 0–03% was achieved. In water, the enhancement factor (sensitivity relative to conventional spectrophotometry) was 70 at an exciting power of 800 mW, and 2 × 10^-9 M iron(II) was determined. In chloroform 2 × 10^-10 M iron(II)—complex could be detected, the enhancement factor being 1200.