Enrique Barrado | Universidad de Valladolid (original) (raw)

Papers by Enrique Barrado

Journal of AOAC INTERNATIONAL, 2002

An amperometric multisite detection flow injection analysis (FIA) system was developed for sequen... more An amperometric multisite detection flow injection analysis (FIA) system was developed for sequential determination of 2 analytes with a single sample injection and single detector. Tubular composite carbon electrodes with an inner diameter similar to that of the FIA manifold tubing were constructed so that measurements could be made without impairing the sample plug hydrodynamic characteristics. The electrochemical behavior of the tubular voltammetric cell in a low-dispersion FIA manifold and the behavior of the FIA system incorporating this type of voltammetric cell intended for multisite detection were evaluated by performing measurements with potassium hexacyanoferrate(II). Feasibility of the approach was demonstrated in the sequential determination of salicylic and acetylsalicylic acids in pharmaceutical products at a fixed potential of 0.98 V. The system allows sequential determination of salicylic acid concentrations ranging from 1.0 × 10−5 to 5.0 × 10−5M and acetylsalicylic ...

Nutricion Clinica y Dietetica Hospitalaria

By examining fatty acid profiles, the different ingredients used in ice cream production may be c... more By examining fatty acid profiles, the different ingredients used in ice cream production may be characterized. When these profiles were compared to those of other fats (oils, milk, etc.), many of the commercial brands of ice cream tested were found to contain coconut oil or coconut itself. In contrast, our data indicate that the smaller cottage industry-type ice creams examined were elaborated from milk or milk products (butter or cream). These qualitative findings were confirmed by multivariate statistical analysis (factor and cluster analyses). In the cluster diagrams presented, it can be seen that that many of the industrial samples grouped in the vicinity of coconut oil. Within the cottage ice cream grouping, appeared a group close to one clustering milk and butter, while ice cream made of almond and nougat appeared in the almond oil zone, showing great similarity with other vegetable oils (olive, sunflower etc.).

Analusis, 1999

A flow injection system for the determination of high levels of glucose in parenteral solutions b... more A flow injection system for the determination of high levels of glucose in parenteral solutions based on amperometric detection of hydrogen peroxide produced in the glucose oxidase reaction was developed. The determinations were made without any sample pre-treatment. The flow injection manifold includes two dialysis units to perform dilution of the samples and to maintain the enzyme in a closed re-circulation loop. Due to the hydrodynamic characteristics of the analytical system, the variables were optimized using a parameter design of Taguchi. Under optimal conditions, a linear response for glucose concentrations, between 0.1 and 1.0 M was achieved. A good reproducibility (r.s.d. < 3%) and a sampling rate of 30 determinations/hour were observed. The results obtained are in a good agreement with those obtained using the spectrophotometric enzymatic method.

Water Research, 1996

ABSTRACT A procedure for purifying waters polluted with metal ions has been designed. The method ... more ABSTRACT A procedure for purifying waters polluted with metal ions has been designed. The method is based on the precipitation of metals as magnetic ferrite from the alkalinised solution containing iron(II). The working conditions were optimised by using a Taguchi L9(34) experimental design in order to minimise the total residual concentration (TRC) of metal ions in solution. A statistical analysis of the experimental data revealed the most influential factor to be the Fe(II)/metal concentration ratio (F), with a 29.5% contribution, followed by pH (P, 5.2%) and time (H, 2.3%). On the other hand, temperature (T) had little effect on the purification efficiency (1.0%), whereas noise (N, KMnO4) was found to contribute by as much as 22.1%. Maximal purification efficiency (99.99%) is achieved when wastewater samples are treated for 3 h at 50°C and pH 10 in the presence of iron(II) in a ratio Fe(II)/total metal of 15. In these conditions, the process efficiency is also the least influenced by variability in the sample composition, which validates the proposed procedure.

Electrochimica Acta, 2014

The electrode reaction of Tb(III)/Tb couple in the eutectic LiCl-KCl, at Cd liquid electrodes (i.... more The electrode reaction of Tb(III)/Tb couple in the eutectic LiCl-KCl, at Cd liquid electrodes (i.e a Cd pool and a Cd coated W electrodes) was investigated in the temperature range of 673-823 K. In both electrodes, the electrochemical reduction of Tb(III) was observed at less cathodic potential values than at the surface of an inert W electrode, due to the decrease of Tb activity in the metal phase. Cyclic voltammetry, using a Cd bulk electrode, suggest a quasi-reversible behaviour of the system Tb (III)/Tb (dissolvedinliquid Cd) , and the values of the kinetic parameters, k 0 and a, as well as the reversible half wave potential, E r 1/2 , have been obtained. The differences between the equilibrium potential adopted by a Tb electrode and the E r 1/2 observed with the same Tb(III) solution at the Cd pool electrode, were used to calculate approximate values of the excess Gibbs energy change of Tb in liquid metal, and hence the activity coefficient of Tb in Cd. The formation of intermetallic compounds was also studied. Electromotive force, emf, measurements for five intermetallic compounds in two-phase coexisting states were carried out using a Cd coated tungsten electrode. The activities and relative partial molar Gibbs energies of Tb were obtained for TbCd 6 , TbCd 45/11 , TbCd 3 , TbCd 2 and TbCd. The formation energy of each intermetallic compound, and the global formation constants were also calculated. The linear dependence of the Gibbs free energies with temperature yields to the enthalpies and entropies of formation of the five intemetallic compounds. Analysis of the samples after electrolysis runs by scanning electron microscopy (SEM) with energy dispersive X-ray allowed the identification of TbCd 6 , TbCd 3 , TbCd 2 and TbCd.

Water Research, 1990

Abstract We have analysed the contents and speciation of Zn, Cd, Pb, Cu, Ni and Co in sediments t... more Abstract We have analysed the contents and speciation of Zn, Cd, Pb, Cu, Ni and Co in sediments taken in the Pisuerga river, that flows through the town of Valladolid and is polluted by industrial and municipal effluents. All heavy metals were determined by voltammetric methods, and their mean total contents were (in μgg−1): Zn, 245.49; Cd, 1.05; Pb, 18.77; Cu, 66.53; Ni, 46.51; and Co, 11.41. We also determined the % H2O, % loss at 800°C, % CaCO3, % organic matter and % organic N for each sediment. All the data were examinated by principal components analysis in order to explain the behaviour of each metal and sampling point. The speciation was carried out by Tessier's method to find five metal categories: (a) adsorptive and exchangeable, (b) bound to carbonates, (c) bound to reducible phases, (d) bound to organic matter and sulphides and (e) residual metals. Cd and Pb, and to a lesser degree Zn, appeared in fractions (a), (b) and (c), whereas Cu, Ni and Co were mainly found in fractions (c) and (d). The residual metal fractions, taken as a guide for pollution, were found to be similar to the ones obtained in rivers belonging to our same European zone.

Water Research, 1994

ABSTRACT The copper complexation capacity (CC) and the conditional formation constant of the resu... more ABSTRACT The copper complexation capacity (CC) and the conditional formation constant of the resulting complexes, β′, have been measured in the Pisuerga River at its passage through the town of Valladolid, by means of a voltammetric titration of the water at “natural pH” (acetate buffer) using DPASV as analytical technique. The effect of both sampling site and sampling time on CC and β′ have been evaluated, finding that only sampling site has a significant effect on CC, whereas both factors affected β′. We have also found a significant correlation (at P = 0.05) of CC with Chemical Oxygen Demand and no correlation between CC and Dissolved Organic Carbon. The CC of Pisuerga River has been monitored during a 127-week period, finding a progressive increase of its level with the time, a significant correlation with the stream flow, as well as a periodical behaviour that has been elucidated with the aid of signal processing techniques (smoothing and Fourier transform), finding long term variations than can be attributed to seasonal changes in the river stream flow.

Talanta, 1990

A method for determmatton of arsemc(V) m the range between 1 x IO-' and 1 2 x 10m6M has been deve... more A method for determmatton of arsemc(V) m the range between 1 x IO-' and 1 2 x 10m6M has been developed These levels are reached by means of the multtphcatton factor ytelded by use of 12-molybdoarsemc actd and the great senstttvtty of polarographtc determmatton of the MO(W) m the heteropoly actd The procedure has been successfully apphed to determmatton of arsemc m copper alloys, after selecttve extractton of phosphorus

Talanta, 1985

Sag-An indirect polarographic method has been developed for the dete~ination of zirconium by fo~a... more Sag-An indirect polarographic method has been developed for the dete~ination of zirconium by fo~ation of moly~o~rconophospho~~ acid, its extraction with MIBK or a mixture of diethyl ether and I-butanol, stripping with alkali and measurement of the MO(W) by its catalytic effect on the polarographic reduction of hydrogen peroxide. Depending on the extractant, a detection limit of 2.5 or 8.9 pg/l. can be achieved.

Talanta, 2009

Eight amino acids (ethanolamine, glycine, alanine, ␤-aminobutyric acid, leucine, methionine, hist... more Eight amino acids (ethanolamine, glycine, alanine, ␤-aminobutyric acid, leucine, methionine, histidine and asparagine) were identified and quantified in Spanish wines by high performance liquid magnetochromatography (HPLMC) with UV-V spectrophotometry. For this method, the amino acids are first complexed with mono(1,10-phenanthroline)-Cu(II) to confer them paramagnetic properties, and then separated by application of a low magnetic field intensity (5.5 mT) to the stationary phase contained in the chromatographic column. Principal components analysis of the results obtained grouped together the wine samples according to their denomination of origin: "Ribera del Duero", "Rueda" or "Rioja" (Spain). Through cluster analysis, a series of correlations was also observed among certain amino acids, and between these groupings and the type of wine. These clusters were found to reflect the role played by the amino acids as primary or secondary nutrients for the bacteria involved in alcoholic and malolactic fermentation.

Journal of Non-Crystalline Solids, 2005

A new way of obtaining iron oxides embedded in amorphous silica gel matrices using Fe2+, oxygen a... more A new way of obtaining iron oxides embedded in amorphous silica gel matrices using Fe2+, oxygen and tetraethoxysilane (TEOS) as precursors is described. The product of this new method was compared to the compound produced when hydrochemically synthesized iron ferrite (magnetite) was directly embedded in silicon oxide (SiO2). The two xerogels were characterized and found to differ substantially. The compound

International Journal of Environmental Analytical Chemistry, 1990

ABSTRACT

Food Chemistry, 2003

... Lower quantities give rise to slow growth, abnormal behaviour or erosion of the caudal fin ( ... more ... Lower quantities give rise to slow growth, abnormal behaviour or erosion of the caudal fin ( [Kaushik, 1990]). ... Tissues were homogenised in a Servall Omni Mixer unit, placed in Petri dishes and dried in a heating cabinet at 104±2 °C for 4 h 30 min. ...

Food Chemistry, 1994

The proximate composition (water, carbohydrates, proteins, fats and ash and also sodium, potassiu... more The proximate composition (water, carbohydrates, proteins, fats and ash and also sodium, potassium and chloride ions) of some legumes in the diet of a clinical hospital, over a 10-year period, has been determined. From the average values, or their graphical representations, large differences between several kinds of legume can be observed, but these differences are clearer and better explained by chemometrics methods (factor analysis and varimax rotation) which produce groupings of the samples into three types (fresh, canned and frozen) according to two factors: nutrients and salt.

Electrochimica Acta, 2008

ABSTRACT The electrochemical behaviour of terbium at a tungsten electrode, in the eutectic LiCl–K... more ABSTRACT The electrochemical behaviour of terbium at a tungsten electrode, in the eutectic LiCl–KCl molten was investigated in the temperature range 673–823K, by means of cyclic voltammetry, chronopotentiometry and chronoamperometry. It was found that during cathodic polarization, deposition of metallic Tb from the chloride mixture onto the tungsten surface proceeds in a single step, and electrocrystallization plays an important role in the whole process. Experimental current–time transients followed the theoretical models based on instantaneous nucleation with three-dimensional growth of the nuclei whatever the applied overpotential. From chronopotentiometric measurements, the diffusion coefficient of the Tb(III) ions was determined by applying the Sand equation and modifying the immersion dept of the working electrode in stages. The validity of the Arrhenius law was also verified and the activation energy for diffusion was found to be 33.4±1.5kJmol−1.The standard apparent potential value of the Tb(III)/Tb(0) system has been determined by potentiometry at several temperatures, and the activity coefficient of Tb(III) in the molten chloride media based on a pure supercool reference state has also been calculated.

Electrochimica Acta, 2009

The electrochemical behaviour of TmCl 3 solutions was studied in the eutectic LiCl-KCl in the tem... more The electrochemical behaviour of TmCl 3 solutions was studied in the eutectic LiCl-KCl in the temperature range 673-823 K using inert and reactive electrodes, i.e. W and Al, respectively. On an inert electrode, Tm(III) ions are reduced to metallic thulium through two consecutive steps: Tm(III) + 1e ↔ Tm(II) and Tm(II) + 2e ↔ Tm(0) The electroreduction of Tm(III) to Tm(II) was found to be quasi-reversible. The intrinsic rate constant of charge transfer, k 0 , as well as of the charge transfer coefficient,˛, have been calculated by simulation of the cyclic voltammograms and logarithmic analysis of the convoluted curves. Electrocrystallization of thulium plays an important role in the electrodeposition process, being the nucleation mode affected by temperature. The diffusion coefficients of Tm(III) and Tm(II) ions have been found to be equal. The validity of the Arrhenius law was verified by plotting the variation of the logarithm of the diffusion coefficients vs. 1/T. The electrode reactions of Tm(III) solutions at an Al electrode were also investigated. The results showed that for the extraction of thulium from molten chlorides, the use of a reactive electrode made of aluminium leading to Al-Tm alloys seems to be a pertinent route. Potentiometric titrations of Tm(III) solutions with oxide donors, using a ytria stabilized zirconia electrode "YSZE" as a pO 2− indicator electrode, have shown the formation of thulium oxychloride and thulium oxide and their corresponding solubility products have been determined at 723 K (pk s (TmOCl) = 8.0 ± 0.3 pk s (Tm 2 O 3) = 18.8 ± 0.7).

Electrochimica Acta, 2012

This work concerns the electrochemical extraction of scandium from molten chlorides. In this way,... more This work concerns the electrochemical extraction of scandium from molten chlorides. In this way, the electrodeposition of scandium has been investigated in the molten LiCl-KCl with the eutectic composition on a tungsten electrode in the 673-823 K temperature range by several electrochemical techniques (i.e. cyclic voltammetry, chronopotentiometry and chronoamperometry). It has been found that during cathodic polarization, deposition of metallic Sc from the chloride mixture onto the tungsten surface proceeds in a single step, and that electrocrystallization plays an important role in the whole process. Experimental current-time transients followed the theoretical models based on instantaneous nucleation with three dimensional growth of the nuclei. From chronopotentiometric measurements, the diffusion coefficient of the Sc(III) ions was determined by applying the Sand equation and modifying the immersion dept of the working electrode in stages. The activation energy for diffusion was found to be 38.18 ± 1.95 kJ mol −1. The standard apparent potential value of the Sc(III)/Sc(0) system has been determined by potentiometry at several temperatures, and the activity coefficient of Sc(III) in the molten chloride media based on a pure supercool reference state has also been calculated.