Double indication in catalytic—kinetic analysis; Determination of iron(III), cyanide and molybdenum(VI)

The reactions in catalytic—kinetic methods are followed simultaneously with two independent indication systems. The information delivered by the two indication methods can be used alone or in combination for the determination of the catalyst or the inhibitor. The following examples illustrate the method: the determination of iron(III) by its catalytic action on the decomposition of hydrogen peroxide (thermometric and biamperometric indication)in the range 10–100 ng Fe/6 ml; the determination of cyanide which inhibits the catalytic activity of copper on the decomposition of hydrogen peroxide thermometric and biamperometric indication) in the range 2–60 μg CN^-/7 ml; and the determination of molybdenum based on the Landolt-type system iodide—bromate— ascorbic acid (thermometric and photometric indication) in the range 0.8–40 μg Mo/8 ml.     

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 ).     

Catalytic determination of molybdenum(VI) by means of an iodide ion-selective electrode and a landolt-type hydrogen peroxide-iodide reaction

A trace amount of molybdenum(VI) can be determined by using its catalytic effect on the oxidation of iodide to iodine by hydrogen peroxide in acidic medium. Addition of ascorbic acid added to the reaction mixture produces the Landolt effect, i.e., the iodine produced by the indicator reaction is reduced immediately by the ascorbic add. Hence the concentration of iodide begins to decrease once all the ascorbic acid has been consumed. The induction period is measured by monitoring the concentration of iodide ion with an iodide ion-selective electrode. The reciprocal of the induction period varies linearly with the concentration of molybdenum(VI). The most suitable pH and concentrations of hydrogen peroxide and potassium iodide are found to be 1.5, 5 and 10mM, respectively. An appropriate amount of ascorbic acid is added to the reaction mixture according to the concentration of molybdenum(VI) in the sample solution. A calibration graph with good proportionality is obtained for the molybdenum(VI) concentration range from 0.1 to 160 μM. Iron(III), vanadium(IV), zirconium(IV), tungsten(VI), copper(II) and chromium(VI) interfere, but iron(III) and copper(II) can be masked with EDTA.     

Katalytisch-kinetische bestimmung von katalase, kupfer, molybdän und jodid mit hilfe eines biamperostaten

Biamperometrically observable reactions may be evaluated by the “potentiostat”-method with the application of a current-to-voltage transducer. The catalase-catalyzed autodecomposition of hydrogen peroxide, the copper-catalyzed oxidation of iodide with peroxydisulfate, the molybdenum(VI)-catalyzed oxidation of iodide with hydrogen peroxide, and the iodide-catalyzed oxidation of arsenic(III) with cerium(IV) are used to illustrate this concept. The catalysts, catalase, copper(II), molybdenum(VI) and iodide, as well as azide as an inhibitor for catalase, can be determined in the p.p.b.-p.p.m. range.     

Catalytic—kinetic determination of iodide,manganese and molybdenum with the use of an absorptiostat

The kinetic—catalytic determination of several catalysts by use of an absorptiostatic method is described. The determination of iodide in the range 0.5–5.0 $\text{μg}\text{45 ml}$ is based on its catalytic action on the cerium(IV)-arsenic(III) reaction. The catalytic action of manganese(II) on the reaction of malachite green with periodate was used for its determination in the range 1–10 $\text{μg}\text{45 ml}$. The determination of molybdenum(VI) in the range 10–100 $\text{μg}\text{40 ml}$ is based on its accelerating action on the oxidation of iodide with hydrogen peroxide.     

Catalytic determination of molybdenum with improved sensitivity by means of the hydrogen peroxide/iodide/ascorbic acid landolt reaction

Modified catalytic procedures are described for the determination of 0–0.1 and 0–1 mg l^?1 molybdenum in solutions. Reaction times of the hydrogen peroxide/iodide/ascorbic acid Landolt reaction are evaluated from conductometric or potentiometric traces. Calibration graphs are based on the ratio of the reaction times for the blank and the sample, t(0)/t(c), plotted against the concentration of molybdenum. The shapes of the conductometric and potentiometric traces are explained.     

Eine katalytisch-kinetische strömungsmethode auf papier. Die bestimmung von molybdän

A new catalytic-kinetic flow method is described, in which a capillary support, filter paper, serves as medium for the reaction as well as for the transport. The catalyst to be determined and the two reactants migrate through a filter paper strip. On a suitable place of the filter paper itself, the concentration of one of the reactants is continuously measured. The example of the molybdenum catalyzed reaction between hydrogen peroxide and iodide illustrates the procedure. The course of the reaction is followed potentiometrically with an iodide-selective electrode.     

Determination of molybdenum at μ l−1 levels by catalytic spectrophotometric flow injection analysis

A flow injection analytical method based on the catalytic action of molybdenum on the oxidation of iodide by hydrogen peroxide in acidic medium is proposed. The triiodide formed is measured spectrophotometrically at 350 nm. Molybdenum is determined in natural water samples without preconcentration at a sampling rate of 90 h^?1 with 200-μl sample injections. The detection limit is 0.7 μ l^?1 and the calibration curve is linear over the range 1–1000 μ l^?1. The relative standard deviation is 0.83% for 50 μ l^?1 molybdenum and 1.9% for 13 μ l^?1 molybdenum.     

Flow-injection catalytic determination of molybdenum with blamperometric detection in a microprocessor-controlled system

The flow-injection determination of molybdenum(VI) is based on its catalytic effect on the oxidation of iodide by hydrogen peroxide. The triiodide ion formed in this reaction is detected amperometrically in a flow-through cell containing two platinum wire electrodes polarized at 100 mV. After optimization of the measuring conditions, the detection limit is 1.2 μg l^?1 Mo(VI) and the linear range extends to 1 mg l^?1. Interference of various metal ions and their removal is described. The procedure was tested on the determination of molybdenum(VI) in soil extracts.     

校正计算－催化动力学方法同时测定钨(Ⅵ)和钼(Ⅵ)

利用某些试剂(柠檬酸、酒石酸、草酸及丙二酸)对钨(Ⅵ)、钼(Ⅵ)催化作用抑制程度的差异,对钨(Ⅵ)、钼(Ⅵ)进行了同时测定。先对一组标准钨(Ⅵ)、钼(Ⅵ)混合溶液进行校正计算并建立了线性和非线性的两种模型,再对含量在0.012～0.20μg/mL范围内的未知钨(Ⅵ)、钼(Ⅵ)含量的混合溶液进行了浓度预报。以碘离子选择电极量测催化反应过程中体系的电位变化,并以此值表示催化反应的速率。     

Thermometric titration of cadmium with sodium diethyldithiocarbamate, with oxidation by hydrogen peroxide as indicator reaction

A new method of end-point indication is described for thermometric titration of cadmium with sodium diethyldithiocarbamate (DDTC). It is based on the redox reaction between hydrogen peroxide added to the system before titration, and the first excess of DDTC. Amounts of cadmium in the range 10-50 mumoles are titrated within 1% error.     

Deprotection of dithioacetals with 30% hydrogen peroxide catalyzed by tantalum(V) chloride–sodium iodide or niobium(V) chloride–sodium iodide

The reaction of dithioacetals with 30% hydrogen peroxide in the presence of catalytic amounts of tantalum(V) and iodide ion effectively produced carbonyl compounds in high yields. Dithioacetals also can be deprotected using the niobium(V) catalyzed oxidation of iodide ion by hydrogen peroxide under mild conditions.     

Determination of organic peracids and hydrogen peroxide in mixtures

A simple method for the determination of organic peracids and hydrogen peroxide in mixtures is presented. The method is based on the instantaneous reaction of peracids with neutral potassium iodide and on the formation of a stable complex between hydrogen peroxide and titanyl ions. This complex is decomposed with sodium fluoride and the ensuing reaction with iodide is accelerated with molybdic acid. The influence of the different additives on the analytical results has been studied.     

Catalytic determination of molybdenum in plants by flow-injection spectrophotometry with ion-exchange separation

The catalytic effect of molybdenum n the oxidation of iodide by hydrogen peroxide is utilized. A cation-exchange resin column is incorporated into the system to remove interfering ions; during sample processing, the interfering ions are eluted towards waste. Plant materials are ashed and solutions are injected. The proposed system can handle about 40 samples per hour with molybdenum contents in the 1.0–40.0 μg 1^?1 range. Results are precise (r.s.d. usually <%) and in agreement with those obtained by graphite-furnace atomic absorption spectrometry.     

Determination of free chlorine in water by chemiluminescence reaction with hydrogen peroxide

The analytical utility of the hydrogen peroxide—hypochlorite singlet oxygen chemiluminescence reaction for the determination of hypochlorite in water is investigated. Effects of pH and hydrogen peroxide concentration are discussed and interference data for over 35 species in the absence and presence of hypochlorite are provided. The limit of detection is 4 μg l^-1 with a usable non-linear calibration curve up to about 200 μg l^-1. The new method is shown to be relatively free from interferences and to give results for tap water comparable to a standard colorimetric method based on a reaction with N, N-diethyl-p-phenylenediamine.     

On the mechanism of the pseudocatalatic degradation of hydrogen peroxide by lactoperoxidase/iodide

Hydrogen peroxide is catalytically disproportionated by lactoperoxidase in the presence of iodide ions, Km = 55 microM in 100 mM sodium phosphate, pH 7.00, 25 degrees C. Products formed are water and molecular oxygen. The reaction is competitively inhibited by hydrogen sulfite, Ki = 0.24 mM in 100 mM sodium phosphate, pH 7.00, 25 degrees C. The stoichiometry of the reaction is identical with the corresponding catalase reaction but the mechanism differs. A mechanistic model for lactoperoxidase-iodide dismutation of hydrogen peroxide is discussed.     

Kinetic–catalytic–spectrophotometric determination of low concentrations of molybdenum in white wines

The paper describes the determination of the molybdenum content in white wines based on its catalytical action on the kalium iodide oxidation by hydrogen peroxide in acid medium.

The optimum reaction conditions (the catalyst, KI and H₂O₂ concentrations, the pH value, the order of the reagent additions, the temperature) have been found by studying the effect of the reaction variables.

The influence of some metallic ions (Ca²⁺, Mg²⁺, Zn²⁺, Cd²⁺, Fe²⁺ and Fe³⁺) and complexing anions (F⁻, C₂O²⁻₄, EDTA⁴⁻) on the catalyzed reaction rate was elucidated.

The molybdenum concentration was estimated by the tangent, fixed-time and fixed-absorbance method. The obtained average values for molybdenum content in white wines are within the 1.77×10⁻⁷–1.83×10⁻⁷ mol l⁻¹ range.     

Kinetic determination of microquantities of D(−)-arabinose

A kinetic method for the determination of microquantities of d(?)-arabinose is presented. The method is based on the accelerating effect of d(?)-arabinose on the reaction between molybdenum(VI) and hydrogen peroxide in solution containing 50 vol% of acetonitrile. In order to find optimum experimental conditions for the determination of d(?)-arabinose, the kinetics of the reaction between molybdenum(VI) and hydrogen peroxide in the presence, as well as in the absence, of d(?)-arabinose was studied. d(?)-Arabinose was determined photometrically by following the rate of the colored reaction product formation. The concentrations of d(?)-arabinose which were determined ranged from 46 to 135 μg/ml, and the standard deviation was lower than 10%.     

Epoxidation of olefin with alkyl hydroperoxide catalyzed by molybdenum complex supported on activated carbon

The catalytic activities of unsupported molybdenum compounds and those supported on activated carbon were compared for the epoxidation of cyclohexene with t-butyl hydroperoxide. Two types of molybdenum compounds were obtained by oxidizing molybdenum powder with hydrogen peroxide. The epoxide was produced without byproducts for both supported and unsupported molybdenum compounds. The reaction was expressed by second order kinetics. By supporting the molybdenum compound on activated carbon, the reactivity was increased ~ six-fold.     

Simple and rapid determination of hydrogen peroxide using phosphine-based fluorescent reagents with sodium tungstate dihydrate.

A simple batch method for the fluorometric determination of hydrogen peroxide using phosphine-based fluorescent reagents has been developed. A rapid, mild and selective derivatization reaction was achieved by adding sodium tungstate dihydrate to the reaction mixture of hydrogen peroxide and a phosphine-based fluorescent reagent. When 4-diphenylphosphino-7-methylthio-2,1,3-benzoxadiazole was used as a reagent, the derivatization reaction was completed after 2 min at room temperature. The calibration curve was linear between 12.5 and 500 ng hydrogen peroxide in a 10 microL sample solution. This method is accurate and has potential for on-line applications.