Avinoam Shenhar

Simultaneous voltammetric determination of uric and ascorbic acids in urine.

Anodic voltammetric method for simultaneous determination of uric acid (UA) and ascorbic acid (AA) in urine has been developed with the use of a commercial working rotating glassy carbon electrode. UA may be determined in a sample diluted by the buffer supporting electrolyte (HOAc+NH(4)OH; pH 5.1-5.2) approximately 100 times, and AA-in a sample diluted approximately 20 times. Before obtaining the analytical signal the electrode should be maintained in the diluted sample during 3 min at potential 0 V and the working electrode rotating 100 rpm, for achievement of the adsorption equilibrium of inhibitors from the urine matrix. For UA the electron transfer is close to reversible, for AA it is an irreversible one. Optimal voltammetric techniques are the square-wave for UA and the differential pulse for AA. Calibration curves, detection limits and recoveries for both determinations were evaluated as satisfactory.

pH-metric determination of acid values in vegetable oils without titration

A new method for determining acid values in vegetable oils has been developed for a nontitration pH-metric technique with a reagent consisting of 0.20 M triethanolamine in a 1∶1 solution of water and isopropanol. The emulsive properties of this reagent provide rapid (within a minute) and quantitative extraction of free fatty acids from an oil into the solvent phase. Acid values were determined by measuring conditional pH of the emulsified reagent in a pH-meter with an aqueous reference electrode. Three different applications of this technique are discussed.

Alternative methods for titratable acidity determination

Methods for determination of titratable acidity, other than traditional titration, i.e. methods without titration, are considered. A number of them use analytical acid-base reagents which quickly convert a mixture of strong and weak acids into a new system. This conversion makes it possible to obtain directly the analytical signal (pH, optical density, etc.) for titratable acidity calculation. These methods in stationary conditions and in flow injection analysis are discussed. Methods not using acid-base reagent (iodide-iodate, chromatographic, spectroscopic and others) are also described. The advantages of the alternative methods such as the decrease labour-consumption and analysis time, simplicity of automation and others are shown.

Design of experiments for the determination of the detection limit in chemical analysis

Abstract The uncertainty of values of the detection limit in chemical analysis is described considering the reasonable risk of false analyte detection and false nondetection, the number of analytical results and some other parameters of the experimental design. Calculations have been made of minimum number of certified reference materials or standard solutions and number of replicates needed to design the experiments for determination of detection limit based on calibration data.

Determination of water samples and ene-diols or thiols in samples inaccessible for direct K.Fischer titration

Novel reagents and the rapid technique were developed for the simultaneous determination of water and ene-diols or thiols in chemical products, drugs and other materials which are inaccessible for direct K.Fischer titration. The reagents consist of iodine, tetramethylammonium iodide or potassium iodide, base (diethanolamine, triethanolamine, sodium acetate and/or urea) in methanol mixed with N,N-dimethylformamide, or with formamide, or with dimethyl sulfoxide and N,N-dimethylformamide mixture as a solvent. The use of the reagents is based on the consecutive titration first of an ene-diol or thiol by the novel reagents and then of water by a conventional K.Fischer reagent in the same cell in a titration system protected from water vapour and oxygen, with a double burette and electrometric location of the end point in both titrations. The time for both titrations is 8-15 min.

Solid electrodes from refractory powder materials prepared for electroanalytical purposes by a novel method

Abstract A novel, simple and inexpensive method for preparation of solid working electrodes from conducting refractory powder materials at room temperature in ambient atmosphere has been developed for electroanalytical purposes. The method is based on the use of a demountable device for hand pressing of the powder mixed with a polymer solution. During the process of pressing, the electric resistance of the electrode is controlled automatically. The electrodes from carbon powder, from titanium nitride and from carbon powder mixed with cobalt(II) phthalocyanine were prepared by this method. Results of voltammetric, stripping voltammetric and chronoamperometric studies showed a number of advantages of the prepared electrodes in comparison with conventional glassy carbon and carbon-paste electrodes.

Rapid and complete extraction of free fatty acids from oilseeds for acid value determination

The rapidity and uncertainty of the acid value determination in oilseeds are defined by the rate and completeness of the free fatty acid extraction. The strategy of reagent development for extraction has been discussed, and a set of reagents that provide rapid and complete extraction has been obtained. The set consists of two reagents. Reagent A contains triethanolamine in a mixture of water, isopropanol, and heptane. Reagent B contains a strong acid and inorganic salt in water. Reagent A allows the carrying out of rapid (1–2 min) solid-liquid extraction of the free fatty acids and some other acids from oilseeds. Reagent B provides the separation of the free fatty acids only into the heptane phase (∼5 min), which can be used directly for the free fatty acid determination. Two techniques for this determination have been applied: pH-metric and titrimetric. The advantages of the proposed set of reagents are described.

The uncertainty of carbon dioxide determination in gaseous mixtures

Abstract The uncertainty of carbon dioxide determination in gaseous mixtures is evaluated for the Chromatographic technique with sample introduction via its displacement by a saturated aqueous solution of sodium chloride. It is shown that the main source of the uncertainty comes from the errors in reference mixtures preparation and other stages of calibration. The expanded uncertainty at 0.95 level of confidence is 0.1−0.2 mol% for a carbon dioxide concentration range of 0.3−100.0 mol%.

Validation of a computer program for atomic absorption analysis

The approach to validation of a computer program for an analytical instrument as a component of the analytical method (using this instrument with the program) is discussed. This approach was used for validating a new program for atomic absorption analysis. The validation plan derived from this approach was based on minimising the influence of all steps of the analytical procedure on the analytical results obtained by the method. In this way significant changes in the results may be caused only by replacement of the previous program by the new one. The positive validation conclusion was based on the comparison of the results of the analysis of suitable reference materials obtained with the new program and with its precursor in the same conditions, and also on comparison of their deviations from the accepted reference values for these materials, with the corresponding uncertainties.