Catalytic adsorptive stripping voltammetry versus electrothermal atomic absorption spectrometry in the determination of trace cobalt and chromium in human urine.

Two methods of the determination of cobalt and chromium in human urine of non-occupationally exposed populations-highly sensitive catalytic adsorptive stripping voltammetry (CAdSV) and electrothermal atomic absorption spectrometry (ET-AAS)-are evaluated and compared. The CAdSV methods are based on adsorptive accumulation of a cobalt-nioxime (1,2-cyclohexanedione dioxime) or a chromium-DTPA (diethylenetriammine-N,N,N',N'',N''-pentaacetic acid) complexes on a hanging mercury drop electrode, followed by a stripping voltammetric measurement of the catalytic reduction current of the adsorbed complex in the presence of sodium nitrite in case of cobalt or in the presence of sodium nitrate in case of chromium determination. In the CAdSV procedure UV-photolysis was used for the sample pre-treatment; the ET-AAS determination did not require any separate preliminary decomposition of the analyte urine samples. The accuracy of the procedures was checked by the analysis of commercially available quality control urine samples. The detection limits (3sigma) were 0.13mugl(-1) for Co and 0.18mugl(-1) for Cr in ET-AAS determination and 0.007mugl(-1) for Co and 0.002mugl(-1) for Cr in CAdSV measurements. Precision (R.S.D.) was less than 5% for both methods. The study has shown that the CAdSV is a more reliable and sensitive technique for the determination of very low cobalt and chromium contents in urine, the detection of which is not possible when using the AAS technique.

Determination of arsenate and organic arsenic via potentiometric titration of its heteropoly anions.

Determination of arsenate based on its conversion to molybdoarsenate heteropoly anions followed by potentiometric titration is described. The titration is realized on the ion-pairing principle using cetylpyridinium chloride (or an analogous titrant containing a lipophilic cation), and is monitored by a carbon paste electrode, although other liquid-polymeric membrane-based electrodes can also be used. Calibration plots of the titrant end-point consumption versus concentration of arsenic were constructed and used to evaluate the content of arsenic in aqueous samples. The method could be applied in the analyses of samples with quite low arsenic content (amounts approximately 10 mug As in 50 cm(3) could be titrated). Organic arsenic was determined analogously after the Schöniger combustion of the sample and conversion of its arsenic to arsenate.

Studies on the potentiometric thallium(III)-selective carbon paste electrode and its possible applications.

Construction, performance characteristics and applications of a carbon paste thallium(III) ion-selective electrode are described. The electrode, which is based on ion-associate compounds formed between cetylpyridinium and chlorothallate(III) complexes dissolved in tricresyl phosphate as pasting liquid, showed near-Nernstian response over the concentration range of 5.8 x 10(-6) - 2.9 x 10(-3) mol/L. Potentiometric titrations of thallium(III) with cetylpyridinium chloride were affected by higher concentrations of excess halides, probably due to the formation of higher halogenothallates.

Trace iron determination in aminoisophthalic acid using differential-pulse cathodic stripping voltammetry at carbon paste electrodes.

Application of differential-pulse cathodic stripping voltammetry using a carbon paste electrode (consisting of carbon powder and liquid paraffin) have been investigated for trace determination of iron in 5-aminoisophthalic acid (AIPA). Samples were dissolved in 1 M HC1, pH was adjusted to 4-5 after addition of EDTA. Voltammetric measurements were performed after filtration. No sample decomposition (mineralization) was necessary. The method showed a good linearity between current and concentration from 3 x 10(-7) to 5 x 10(-5) mol dm-3 of iron, with a detection limit of 3 x 10(-7) mol dm-3 (resp. 1 ppm in solid AIPA). The results agreed well to those obtained by atomic absorption spectrometry (AAS) using electrothermic atomisation. For AAS measurement, however, microwave digestion of samples was necessary.

Some aspects of the use of heteropoly anions in elemental analysis by simple potentiometric ion-pair formation-based titration.

The possibility of the use of heteropoly anions formed by addition of phosphate to the solutions of either molybdate or tungstate in the determination of phosphorus by the simple method of potentiometric titration was studied. The heteropoly anions are titrated on the ion-pairing principle using a titrant containing a lipophilic counter-ion, i.e. 1-(ethoxycarbonyl)pentadecyltrimethylammonium bromide (Septonex), the titration being monitored by carbon paste electrode, although other liquid membrane-based electrodes can also be used. Calibration plots of the titrant end-point consumption vs. concentration are not linear, but allow one to evaluate the content of phosphorus. Similar procedures, when optimized, should be elaborated for determination of numerous other elements forming heteropoly anions.

Application of stripping voltammetry to trace lead analysis in intermediates and final products of syntheses of pharmaceuticals.

Applications of differential pulse anodic stripping voltammetry using a new pen-type renewed hanging mercury electrode have been investigated for trace analysis of lead in pharmaceutical substances and intermediates of their syntheses, such as procaine hydrochloride, 4-aminobenzoic acid, methyl 4-aminobenzoate, 2-(4-chlor-3-aminobenzoyl) benzoic acid, benzyl 2-naphthyl ether, 5-aminoisophthalic acid, 3-aminobenzoic acid, 5-hydroxyisophthalic acid, and N, N'-dibenzylethylenediamine diacetate. Samples were dissolved in 1 M HCI or 1 M NaOH and the electrochemical scan was carried out. No sample mineralization was necessary. The method showed a good linearity up to 50-100 ppm Pb with a detection limit less than 100 ppb. The results agreed well, but were more precise than those obtained by atomic absorption spectrometry using air/acetylene flame atomisation.

Determination of thallium(I) in solutions containing cations interfering in potentiometric ion-pair formation-based titrations.

The possibility of the use of simple potentiometric coated-wire sensors in the analysis of mixtures of ions, the determination of which is based on the ion-pairing principle, was studied. Attention was especially paid on the determination of thallium(I) in the presence of alkali metal ions, mercury(II), copper(II) and silver(I). It was found that thallium(I) can selectively be titrated with sodium tetraphenylborate in diluted solutions. Interferences of ions of the alkali metals are practically negligible if their concentrations are lower than 10(-3) mol dm(-3). Interference of Cu(II) is minimized using EDTA as a masking agent, Hg(II) and Ag(I) ions are sufficiently screened by additions of sodium cyanide. In mixtures containing higher concentrations of alkali metals, thallium(I) can be oxidized to thallium(III) and selectively titrated as TlCl(-)(4) with a cationic titrant.

Voltammetric determination of mercury(II) at a carbon paste electrode in aqueous solutions containing tetraphenylborate ion.

The determination of mercury(II) ions can be achieved by monitoring the decrease in the oxidation peak of the tetraphenylborate ion in the presence of this metal ion at a carbon paste electrode. The reaction between mercury(II) and the tetraphenylborate ion results in the formation of diphenylmercury, thus providing the method with good selectivity over other metal ions. Using anodic stripping voltammetry in a neutral electrolyte, a linear dependence of the decrease of peak height was observed on increasing the mercury(II) concentration in the range 1 x 10(-6)-8 x 10(-9)M mercury(II). Zinc(II), cadmium(II), lead(II), nickel(II), cobalt(II), tin(II), potassium(I) and ammonium(I) ions did not interfere at a 1000-fold concentration excess. Iron(III) and chromium(III) did not interfere at a 250-fold and 50-fold concentration excess, respectively. Following masking procedures, copper(II), bismuth(III) and silver(I) did not interfere at a 100-fold concentration excess. The method can be used to determine the concentration of mercury(II) in natural waters contaminated by this metal.

Dissociation constants of some o,o'-substituted azo dyes.

The dissociation constants, pK(a) (H(2)L(-)) and pK(a)(HL(2-)), were determined spectrophotometrically in aqueous media at ionic strength 0.2 for six o,o'-substituted azo dyes, namely C.I. 13900, C.I. 15685, C.I. 18744, C.I. 18760, C.I. 18821 and Calmagite.

The use of ion-selective electrodes in the determination of drug substances.

The use of ion-selective electrodes and potentiometric techniques in the analysis of drug substances are reviewed. Sensors for potentiometric measurements, potentiometric ion-pair and complex formation-based titrations, titrants, and applications are discussed. Other potentiometric methods used in the analysis of pharmaceuticals are discussed.

Determination of sulphated glycosaminoglycans by automated potentiometric titration with simple coated-wire electrodes.

A method for the determination of sulphated glycosaminoglycans is based on their precipitation with (1-ethoxycarbonyl)pentadecyltrimethylammonium bromide (Septonex), the excess of which is back-titrated with sodium tetraphenylborate. The titration is monitored by a simple coated-wire ion-selective electrode with a plasticized poly(vinyl chloride) membrane on aluminium wire. Under certain conditions the results are almost independent of the relative molecular mass of glycosaminoglycans. The method has been applied to the determination of the active ingredient in the pharmaceutical preparation, heparon injection.

Colour changes of chemical indicators-VI: metallochromic indicators for direct chelometric titrations of zinc.

Eriochrome Blue SE, Eriochrome Red B, Naphthylazoxine 6S, SNAZOXS, and Zincon have been studied in order to find optimum conditions for their use as metallochromic indicators in direct visual chelometric titrations of zinc. The sharpness of the indicator transitions has been investigated by means of photometric titrations and the colour quality has been specified with the aid of the C.I.E. chromaticity systems. Zincon and Eriochrome Blue SE have been found to be the most convenient for visual titrations of zinc.

Colour changes of chemical indicators-III: colour specification and its accuracy.

The systematic deviations involved in methods for computation of chromaticity co-ordinates have been studied for the specification of indicator colour changes. The weighted ordinale method (Deltalambda = 10 nm) has proved to be satisfactory for evaluation both of C.I.E. and complementary co-ordinates. The C.I.E. colour specification should be included among basic data about chemical indicators.