C. Martinez Lozano

Simultaneous Flow Injection Determination of Diquat and Paraquat in Foodstuffs, Natural Waters and Biological Fluids

Abstract The reduction of diquat and paraquat with alkaline sodium dithionite has been applied to the simultaneous determination of both herbicides using a flow injection system. A dual-channel manifold with two detectors in parallel was used. The sample bolus is split into two subplug, one of the sub-boluses merges with a 0.1% sodium dithionite stream and routes to a spectrofluorimeter where diquat is determined. The other sub-bolus merges with a 0.5% sodium dithionite stream and routes to a spectrophotometer for the measurement of paraquat. The system allows 70 samples to be analysed per hour. Applications of the method to the determination of diquat and paraquat in real samples are reported.

Determination of paraquat in biological and environmental samples by a photokinetic method

Abstract A fast and sensitive photokinetic method for the determination of paraquat (MV 2+ ) (1−27×10 −5 M) is described, based on the rate of photoreduction of MV 2+ by EDTA, sensitized by acridine yellow in the absence of oxygen. The rate of photoreduction, which is a linear function of the concentration of MV 2+ is monitored polarographically by recording the limiting current of p - benzoquinone, which is reduced by the radical monocation MV +· generated in the photochemical reaction. The results obtained by the application of the fixed-time, fixed-concentration change and initial-rate kinetic methods are evaluated. An alternative method for monitoring the rate of the process is by measuring the time necessary for the total reduction of p -benzoquinone. The end-point is detected with two platinum electrodes at an applied voltage of 100 mV. The procedure has been successfully applied to the determination of paraquat in commercial herbicides, waters, and flowers and in spiked soils and blood sera.

Determination of periodate with photoreduced thionine

Abstract A method for the microdetermination of periodate in the presence of iodate is presented. It is based on reduction of periodate to iodate by leucothionine generated in situ by photochemical reaction between EDTA and blue thionine. Biamperometric curves are used for evaluation, representing reduction time for the periodate versus the current generated in the oxidation of the T red in excess after the end-point. A polarographic study of the process is presented. An applied potential of 100 mV is suitable for the biamperometric measurements with Pt—Pt electrodes. The method is applicable for 0.4–45 ppm periodate. Iodate does not interfere up to IO - 3 : IO - 4 ratios of 100:1.

Photo-Oxidation of phenosafranine and neutral red in the presence of iron(III): Determination of trace amounts of iron and phosphate

SummaryPhenosafranine (Ph) and Neutral Red (NR) in the presence of iron(III) are decolorized by illumination with light of wavelength shorter than 360 nm. The decolorization mechanism involves ·OH radicals, formed during the photo-excitation of hydrolysed iron(III) species. In the presence of oxygen, iron(III) acts as a catalyst in the photochemical process. The photo-oxidation of Ph and NR is followed by measuring the decrease in absorbance of the dyes, and linear calibration graphs in the range 1–11 ppm of iron are obtained. The decrease in the fluorescence of phenosafranine has also been used to measure the photo-oxidation of this dye, and is linearly related to iron concentration in the range 0.1–1.1 ppm. Phosphate strongly inhibits the iron(III)-catalysed photodecolorization and this can be made the basis of a method for its determination (concentration limit 1×10−5M).ZusammenfassungPhenosafranin (Ph) und Neutralrot (NR) werden in Gegenwart von Fe(III) durch Belichtung unter 360 nm entfärbt. Der Mechanismus dieses Vorgangs erfordert die Gegenwart von OH-Radikalen, die sich bei Belichtung von hydrolysierten Fe(III)-Verbindungen bilden. In Gegenwart von Sauerstoff wirkt Fe(III) als Katalysator der photochemischen Reaktion. Die Photooxydation von Ph und NR läßt sich durch Abnahme der Absorbanz der beiden Farbstoffe verfolgen. Dabei ergeben sich lineare Eichkurven für 1–11 ppm Fe. Die Abnahme der Fluoreszenz von Ph dient ebenfalls zur Messung der Photooxydation dieses Farbstoffes und verläuft für 0,1–1,1 ppm Fe linear. Phosphat hindert die Fe(III)-katalysierte Photoentfärbung, was als Grundlage für die Phosphatbestimmung bis zu 1×10−5M dienen kann.

Determination of oxidants with photoreduced methylviologen

SummaryDetermination of Oxidants with Photoreduced Methylviologen Optimal conditions for photogeneration of the reduced form of methylviologen are described in detail. The generation is done by photochemical reaction of the system 3,10-dimethyl-5-deazaisoalloxazine-methylviologen-electron donor (EDTA, oxalate or arginine). The methylviologen radical cation is suitable as a reductimetric titrant. A method has been developed for the determination of micromolar quantities of organic and inorganic substances byin situ photochemical generation of the reduced form of methylviologen. Spectrophotometric end-points can be utilized because of the intense blue colour of the titrant. Amperometric end-points can also be utilized. A precision of 1.5% can be obtained with both detection methods.

Spectrophotometric determination of gold with 2-(p-methoxyphenyl)-5,7-diphenyl-1,3,4-thiadiazolo-(3,2-a)-pyridinium chloride

SummaryIn 0.4M hydrochloric acid medium, gold(III) forms with 2-(p-methoxyphenyl)-5,7-diphenyl-1,3,4-thiadiazolo(3,2-a)pyridinium chloride (TDZP) an ion-association compound which is extractable into isoamyl acetate. The extracted ion-pair, which has an Au:TDZP mole ratio of 1∶1, is used for the spectrophotometric determination of gold (ε=3.80 x 104l·mole−1·cm−1) in the concentration range 1–25μg of gold per 5 ml of organic solvent. The interference of a number of foreign ions has been investigated. The method is applicable to the determination of gold in ores.ZusammenfassungIn 0,4 M Salzsäure bildet Gold(III) mit TDZP eine Ionenassoziationsverbindung, die sich mit Isoamylacetat extrahieren läßt. Diese im Molverhältnis Au:TDZP=1∶1 zusammengesetzte Verbindung dient zur spektrophotometrischen Goldbestimmung (ε=3,80 x 104l·Mol−1·cm−1) im Konzentrationsbeieich 1–25μg Au/5 ml organisches Lösungsmittel. Die störende Wirkung von Fremdionen wurde untersucht. Das Verfahren eignet sich zur Goldbestimmung in Erzen.

Indirect photokinetic microdetermination of fluoride

Abstract A catalytic-kinetic method for the determination of nanogram amounts of fluoride based on its inhibiting action on the catalyst in the iron(III)-catalyzed photooxidation of thionine is described. The rate measurements are accomplished very simply by measuring the decreasing of absorbance after illumination for a fixed time.

Coulometric determination of organic compounds with methylviologen cation-radical

Abstract The scope of coulometrically generated viologen radical-cation as a reagent for the determination of organic compounds such as o-nitrophenol, p-nitrophenol, p-benzoquinone, and dichloroindophenol was investigated. Samples of the order 10−5 to 10−4 μmol could be determined with about 1% precision and accuracy. These organic compounds react quantitatively and rapidly with the viologen radical-cation, which can be electrolytically generated with 100% current efficiency. A sensitive biamperometric end point detection can be applied.

Titration of oxidants with the reduced species of phenosafranine and neutral red, generated by electrochemical and photochemical techniques

SummaryThe generation of the reduced species of phenosafranine and Neutral Red by coulometric and photolytic methods is described. The electrogeneration is done at a platinum or mercury cathode, and the photogeneration by photochemical reaction between EDTA and the dye. The redox potentials for phenosafranine and Neutral Red are very low. The reduced dyes are suitable as reductimetric titrants and have been applied in coulometric and photochemical titrations of periodate, chromium(VI), iron(III), cobalt(III) and 2,6-dichloroindophenol.ZusammenfassungDie Herstellung der reduzierten Formen von Phenosafranin und Neutralrot mit coulometrischen bzw. photolytischen Methoden wurde beschrieben. Ersteres erfolgt an einer Platin- oder Quecksilberkathode, letzteres durch photochemische Reaktion zwischen EDTA und dem Farbstoff. Die Redox-potentiale für Phenosafranin und für Neutralrot sind sehr niedrig. Die reduzierten Farbstoffe eignen sich als reduktometrische Titranten und wurden für coulometrische und photochemische Titrationen von Perjodat, Chrom(VI), Eisen(III), Kobalt(III) und 2,6-Dichlorindophenol verwendet.