Njegomir Radić

Simple and Rapid Method for the Determination of Uric Acid-Independent Antioxidant Capacity

Determination of the relative contribution of uric acid level increases to the total measured antioxidative activity could be very useful for testing antioxidative products and their effect on human health. The aim of this report is to present a simple spectrophotometric method that combines the measurement of total antioxidative capacity of a sample by ferric reducing/antioxidative power (FRAP) assay, with the uricase-reaction (specific elimination of uric acid), in order to establish and correct for the contribution of uric acid in FRAP values. We measured FRAP values, with (uric acid-independent antioxidant capacity, TAC-UA) and without (total antioxidant capacity, TAC) uricase treatment, and expressed it as μmol/L of uric acid equivalents. In such way, it was possible to determine both total and uric acid-independent antioxidant capacity, plasma uric acid (UA, as the difference between TAC and TAC-UA), and the ratio of the uric acid in total antioxidant capacity (UA/TAC).

Fluoride electrode with LaF3-membrane and simple disjoining solid-state internal contact

Fluoride-selective electrode with LaF(3)-membrane and stainless steel internal contact has been prepared. The electrode has been tested in F(-) concentration range 10(-2)-10(-6) M and the results compared with those of commercial electrode and electrode with laboratory renewed internal electrolytic contact. The properties of simple disjoining solid-state contact have been investigated by means of electrochemical impedance spectroscopy (EIS). The impedance spectra show two time constants, which suggest that, at lower frequencies, proceed some Faradic reaction which probably stabilizes the potential between LaF(3)-membrane and stainless steel disk.

Ion-sensitive behaviour of silver sulphide-based solid-state copper(II) and iodide electrodes in partially aqueous systems

Abstract The response of copper(II)- and iodide-selective electrodes was evaluated in various water-organic solvent mixtures. The copper(II) electrode showed an almost Nernstian behaviour in mixtures with ethylene glycol, tert-butanol, propanol, ethanol and methanol. The experimental slopes in buffered water-acetonitrile mixtures were linear but super-Nernstian. The response of this electrode in mixtures with p -dioxane was neither Nernstian nor linear. The interference of mercury(II) in the direct potentiometric titration of copper(II) with EDTA in solution containing acetonitrile was studied. The behaviour of silver sulphide-based iodide electrode hydrophobized by PTFE in mixtures with ethanol, methanol and p -dioxane was poorer than in water. The potential response of the iodide electrode with a homogeneous membrane (Ag 2 S + AgI) was satisfactory even after prolonged soaking in p -dioxane.

Aluminium fluoride complexation and its ecological importance in the aquatic environment

A potentiometric investigation of the rate and mechanism of the reaction between aluminium (III) ions and fluoride ions in buffered aqueous solution (pH values 2 and 5) is described. Potential-time curves recorded during the reaction, using fluoride-ion selective electrode, constitute the primary data in this study. The rate of aluminium fluoride complexation is calculated under different experimental conditions. In the solutions where the analytical ratios between fluoride and aluminium are > 6, the calculated average number of fluoride ligands bound by aluminium are in the range 1.8-3.7.

Potentiometric determination of mercury(II) and thiourea in strong acid solution using an ion-selective electrode with AgI-based membrane hydrophobised by PTFE

SummaryThe potentiometric determination of mercury(II) and thiourea (TU) in strong acid solution (pH 0–1) by using an “all-solid-state” ion-selective electrode with (Ag2S 25%, AgI 25% and PTFE 50% m/m)-membrane is described. The linear response, 43 mV(pHg)−1 and 80 mV(pTU)−1, has been obtained in the concentration range from 10−2 to under 10−5 mol/l. By direct potentiometry at pH 0 mercury(II) can be determined in the presence of up to 10−3 mol/l of iron(III). The change in potential in the tested concentration range of thiourea indicates the formation of Ag(TU)1.4+at the exposed surface of the membrane. This stoichiometry is in good agreement with that calculated from the average Ag/TU ratio in the potentiometric titration. The investigated electrode can be used as a good sensor for mercury(II) and thiourea in strong acid media and a wide variety of practical analytical systems.

Indirect method for spectrophotometric determination of ascorbic acid in pharmaceutical preparations with 2, 4, 6-tripyridyl-s-triazine by flow-injection analysis

A flow-injection indirect spectrophotometric method for the determination of ascorbic acid (AA) in pharmaceutical preparations is proposed. The method is based on the reduction of iron(III) to iron(II) by the AA, and by the subsequent reaction of the produced iron(II) with 2,4,6-tripyridyl-s-triazine (TPTZ) in buffered medium (pH=3.6) to form a coloured complex (λ(max)=593nm). The three-line manifold with one reaction coil was used. The linear range of the method is from 0.08 to 10μM of ascorbic acid, with the detection limit 24nM of AA. The proposed method is simple, rapid (sampling rate of 180 samples per hour), sensitive and reproducible (RSD 0.8%, n=100). The proposed method is very selective, because only the reducing substances with standard (formal) potentials lower than 0.6V would have the thermodynamic predisposition to interfere in the proposed method. Tested reducing substances (thiol compounds) did not give serious errors when present at the same concentrations as the ascorbic acid. The proposed method can be applied for the determination of AA in pharmaceutical preparations, down to picomolar quantity.

Spectrophotometric Determination of N-Acetyl-L-Cysteine and N-(2-Mercaptopropionyl)-Glycine in Pharmaceutical Preparations

A simple spectrophotometric method for the determination of N-acetyl-L-cysteine (NAC) and N-(2-mercaptopropionyl)glycine (MPG) in pharmaceutical preparations was developed, validated, and used. The proposed equilibrium method is based on a coupled two-step redox and complexation reaction. In the first step, Fe(III) is reduced to Fe(II) by NAC or MPG. Subsequently, Fe(II) is complexed with 2,4,6-tripyridyl-s-triazine (TPTZ). Several analytical parameters of the method were optimized for NAC and MPG analysis in the concentration range from 1.0 μM to 100.0 μM. Regression analysis of the calibration data showed a good correlation coefficient (0.9999). The detection limit of the method was 0.14 μM for NAC and 0.13 μM for MPG. The method was successfully applied to quantify NAC and MPG in pharmaceutical preparations. No interferences were observed from common pharmaceutical excipients.

Kinetic Spectrophotometric Determination of Thiols and Ascorbic Acid

Abstract This paper describes a kinetic spectrophotometric method for the determination of L‐ascorbic acid (AA) and thiols (RSH). Absorbance of Fe(II)‐phen complex formed during the reaction of AA or RSH with Fe(III)‐phen was continuously measured at 510 nm by double‐beam spectrophotometer with flow cell. For determination some thiols, the catalytic effect of Cu2+ ions was used. AA and RSH can be determined in concentration ranges from 4.0×10−6 to 4.0×10−5 M and from 8.0×10−6 to 8.0×10−5 M, respectively. The applicability of the proposed method was demonstrated by determination of chosen compounds in pharmaceutical dosage forms.

Use of a multi-purpose solid-state ion-selective electrode body and an agl-based membrane hydrophobised by PTFE for the determination of I– and Hg2+

The design of a multi-purpose “all-solid-state” electrode body is described. The electrode body, which provides solid contact with the ion-selective membrane, can be used with either pressed pellet or metal disc membranes. The preparation and performance of a silver iodide-based pellet hydrophobised by PTFE are described. The pellet (Agl 25%, Ag2S 25% and PTFE 50%m/m) was mounted in the electrode body and examined as an ion-selective membrane for the determination of l– and Hg2+ ions.

Design and properties of flow-through electrode with AgI-based membrane hydrophobized by PTFE; application to flow-injection determination of thiourea

Abstract A multi-purpose solid-state iodide-selective electrode is constructed as a compact tubular sensor; the sensing hole has 2.0 mm i.d. and 1.2 mm length. The behaviour, suitability and incorporation of the tubular electrode into the conduit of a flow-injection system are described. Application of the tubular electrode is exemplified by the determination of thiourea in 0.1 M HClO 4 as supporting electrolyte by using of a flow-injection system. A rectilinear calibration graph is obtained by plotting the peak height versus concentration of thiourea. The detection limit is 20 μM under optimized flow conditions (1.2 ml min −1 , 200 μl samples, 10 −5 M silver ions in the carrier stream).