Computerized flow constant-current stripping analysis for some electroactive organic compounds: application to the determination of erythromycin in urine.

The possibility of determining various electroactive organic compounds, e.g., erythromycin and riboflavin, by adsorptive preconcentration at constant potential on a mercury-coated carbon-fibre electrode in a flow system prior to stripping with a reductive constant current, is demonstrated. The advantages of measuring the electrode potential vs. time instead of current vs. electrode potential during stripping, the possibility of operating with non-deoxygenated solutions, the increased linear calibration range is in the multiple scanning mode, and the possibility of using the technique for detection in liquid chromatography, are discussed. An analytical procedure for the determination of erythromycin at the 10(-6)M level in urine after extraction with diethyl ether is described.

Constant-current stripping analysis for iron(III) by adsorptive accumulation of its Solochrome Violet RS complex on a carbon-fibre electrode.

The optimum experimental conditions with respect to pH, adsorption potential and Solochrome Violet RS concentration for the constant-current stripping determination of iron(III) have been investigated. The suggested procedure has been used for the determination of iron(III) in tap water and the accuracy investigated both by analysing standard solutions and by comparison with results obtained by atomic-absorption spectrometry. The relative precision was 6-13% and the detection limit for iron(III), set by the reagent blank, was approximately 1 mug/l. The results obtained with a mercury-film coated carbon-fibre flow electrode have been compared with those obtained with a hanging mercury drop electrode. The advantages of measuring electrode potential vs. time instead of vs. current have been evaluated.

Determination of cadmium and lead in the low ng/l range by stripping potentiometry employing medium exchange in batch mode and multiple stripping in a hanging stripping medium drop.

A novel three-in-one electrode assembly, permitting medium exchange under a controlled potential in batch mode by exploiting electrolytic conductance through a drop hanging under the electrode, has been used for the simultaneous determination of cadmium(II) and lead(II) at trace levels. Potentiostatic reduction and amalgamation of these ions are carried out in the sample and the subsequent stripping, in the stripping potentiometry mode, in a drop of a suitable medium. Several such media have been investigated, a mixture of 2.5M acetic acid and 7.5M ammonium acetate having been found to be the most suitable with respect to sensitivity, potential resolution and trace metal purity. The quiescent conditions in the drop of medium facilitated multiple stripping and thus increased sensitivity. Detection limits for cadmium(II) and lead(II) were found to be around 0.5 ng/l. (5 and 2.5pM) for an electrolysis time of 10 min. The relative precision at the concentration level 20 ng/l. was 6.4% for cadmium and 5.4% for lead. The procedure has been used for the determination of cadmium(II) and lead(II) in reference seawater samples.

Determination of cadmium, lead and copper in milk and milk powder by means of flow potentiometric stripping analysis.

A flow potentiometric stripping analysis procedure for the determination of cadmium, lead and copper in milk and milk powder samples is described. The instrumental arrangement consists of a glassy-carbon thin-layer cell through which six different solutions may be drawn by means of a peristaltic pump and magnetically operated valves. The glassy-carbon electrode is pre-coated with a film of mercury which can be employed for several analytical runs. The sample, diluted five-fold with Suprapur hydrochloric acid, is electrolysed for 0.5-4 min prior to stripping in Suprapur hydrochloric acid. Pump-rate, electrolysis time and potential, opening and closing of inlet valves and digital evaluation of stripping times are controlled automatically by the computer. The analytical results agree satisfactorily with the certified values for three milk powder reference samples. The detection limit for cadmium, lead and copper in milk samples after 4, 1 and 0.5 min of pre-electrolysis is 0.8, 4 and 8 mug l. , respectively. An analytical procedure for the determination of lead in samples containing high concentrations of tin is described.

Automated determination of lead in urine by means of computerized flow potentiometric stripping analysis with a carbon-fibre electrode.

Five different digestion procedures have been investigated. In the recommended procedure 5 ml of concentrated hydrochloric acid-nitric acid mixture (1:1) were added to a 5-ml urine sample. After 1 min the sample was diluted to 100 ml with distilled water. The sample was divided into two 50-ml samples and to one of them a standard addition of 10 microg/l. lead(II) was made. The two samples were analysed fully automatically by means of computerized flow potentiometric stripping analysis, the main features of the procedure being mercury-film precoating, electrolysis of the sample for 90 sec and subsequent stripping in 1M calcium chloride/0.1M hydrochloric acid. Tin and copper were found not to interfere if present at concentrations below 50 mg/l. but high concentrations of tin had to be masked by the addition of copper in order to form a copper-tin intermetallic compound. Complexing agents used in lead poisoning therapy did not interfere with the determination. The lead concentration in a Seronorm reference urine sample was found to be 93 microg/l. with a standard deviation of 8 microg/l. (n = 5), the certified value being 88 microg/l.

Experimental and computational study of species formed during electrochemical stripping oxidation of copper in chloride media determination of copper(II) in the ng l(-1) range by stripping potentiometry.

A novel glassy carbon electrode design, permitting medium exchange in batch mode without loss of electrode potential control, has been used for the study of copper(I) and copper(II) species formed during constant current stripping oxidation of copper in chloride media. It was found that copper(II) species dominated at chloride concentrations below about 1 mM and that soluble copper(I) species dominated at chloride concentrations above about 100 mM. In the concentration range 1-100 mM, soluble copper(I) and copper(II) species are formed as well as solid copper(I) chloride, the latter giving rise to a split peak as it is further oxidised to copper(II). The experimental results agreed satisfactorily with computer calculated equilibria data using the haltafall program. The medium exchange procedure has, furthermore, been used for the determination of copper(II) in seawater reference samples, 7.5 M ammonium acetate/2.5 M acetic acid being used as stripping medium. The detection limit, after 15 min of electrolysis, was found to be 6 ng l(-1) (0.10 mM) and the relative precision 6-10%.

Apparent Formation of an Oxidant by Electrochemical Reduction in the Mercury(0,I,II) Chloride System.

The paradoxical appearance of a cathodic reaction sometimes observed in anodic stripping voltammetry and stripping potentiometry when using mercury film electrodes in chloride media containing mercury(II) has been investigated by systematically varying relevant chemical and electrochemical parameters and comparing the results with thermodynamic equilibrium calculations. Microscopic observations of morphological changes on the electrode surface caused by potential variations were made possible by using a novel electrode design. Three conditions have to be fulfilled for the cathodic reaction to occur: (a) formation of calomel by reaction between elemental mercury on the electrode surface and mercury(II) in solution, (b) subsequent reduction of mercury(II) to elemental mercury on the calomelized electrode surface, and (c) a chloride concentration in the range 0.001-3.5 M. Different ways of avoiding the interference from the cathodic reaction in stripping voltammetry and stripping potentiometry are experimentally demonstrated, and a mechanism for the appearance of the cathodic reaction is proposed.

Mechanism for the electrochemical stripping reduction of the nickel and cobalt dimethylglyoxime complexes.

Reductive coulometric stripping potentiometry, a technique not hitherto described, has been used to establish that the reduction of Ni(II) and Co(II) dimethylglyoximates, adsorbed on a mercury film electrode, is a 10-electron process. Exhaustive adsorption of Ni(II) or Co(II) complexes, in the 0-4 µg L(-)(1) concentration range, was achieved by vibrationally promoted electrolysis for 3 min of ∼25 µL volume samples, hanging under the working electrode in a nitrogen atmosphere. The adsorbed complexes were reduced by means of a constant current of 50 µA. The technique was successfully used for the calibration-free determination of Ni(II) in certified seawater and river water reference samples.

Test atmospheres of diisocyanates with special reference to controlled exposure of humans.

An all glass apparatus for the generation of air concentrations of 2,4-toluene diisocyanate (2,4-TDI), 2,6-toluene diisocyanate (2,6-TDI) and 1,6-hexamethylene diisocyanate (HDI) was developed. The generation principle was based on gas-phase permeation with permeation membranes of silicon rubber. In an 8 m3 stainless steel test chamber, low and steady TDI-and HDI atmospheres (1-100 micrograms/m3) could be maintained. The diisocyanate concentrations were determined by an HPLC method, using the 9-(N-methylaminomethyl)-anthracene reagent utilizing UV detection. The sum of diisocyanates and their related amines were determined by sampling in 0.4 M hydrochloric acid solution, and analysis by capillary gas chromatography with thermionic specific detection. Related amines were determined by sampling in ethanol - 0.2% KOH and analysis on GC-TSD. A continuous band-tape monitor was used for the determination of diisocyanates. Losses of diisocyanates in the test chamber were evaluated by measuring the TDI and HDI concentrations at the inlet respectively the outlet of the test chamber. At the outlet of the test chamber, ca 25% of the TDI respectively HDI concentrations were recovered. With a male subject in the test chamber ca 15% of the HDI concentration was recovered. The air flow through the test chamber was ca 10 m3/h. The changes in isomeric composition of airborne TDI, at stopped flow conditions, showed that the decay of the 2,4-isomer was faster than of the 2,6-isomer. No trace of the related amine toluene diamine (TDA) was detected in the test chamber, at TDI concentrations ranging from 20 to 50 micrograms/m3. Sampling losses due to sampling connections were evaluated.(ABSTRACT TRUNCATED AT 250 WORDS)