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A selective method based on high-performance liquid chromatography with electrochemical detection (HPLC-ECD) has been developed to enable simultaneous determination of three fluoroquinolones (FQs), namely danofloxacin (DANO), difloxacin (DIFLO) and sarafloxacin (SARA). The fluoroquinolones are separated on a Novapack C-18 column and detected in a high sensitivity amperometric cell at a potential of +0.8 V. Solid-phase extraction was used for the extraction of the analytes in real samples. The range of concentration examined varied from 10 to 150 ng g(-1) for danofloxacin, from 25 to 100 ng g(-1) for sarafloxacin and from 50 to 315 ng g(-1) for difloxacin, respectively. The method presents detection limits under 10 ng g(-1) and recoveries around 90% for the three analytes have been obtained in the experiments with fortified samples. This HPLC-ECD approach can be useful in the routine analysis of antibacterial residues being less expensive and less complicated than other more powerful tools as hyphenated techniques.

Nalidixic acid (NA) and its main metabolite, 7-hydroximethylnalidixic acid (OHNA), are quinolones antibacterial used as agents used for the treatment of urinary tract infection. For both compounds an adsorption process on a hanging mercury electrode (HMDE). On this basis, a square wave adsorptive stripping voltammetry (SWadSV) method has been developed for the individual and simultaneous determination of NA and OHNA. The variables that affect to accumulation process, such as concentration of perchloric acid, accumulation potential and accumulation time have been optimised by using an experimental design (concretely a Box-Behnken design with three levels) together with the response surface methodology (RSM). Calibration curves were linear in the range (0-1.38)x10(-7)molL(-1) for NA and (0-3.23)x10(-8)molL(-1) for OHNA, in the optimized conditions, with detection limits of 9.48x10(-9)molL(-1) and 8.06x10(-10)molL(-1) for NA and OHNA, respectively. The method was applied to urine samples containing only one of the analytes with satisfactory recoveries. As the voltammetric signals of these compounds show a high overlapping, different chemometric methods, such as classical least squares (CLS), partial least squares (PLS), principal component regression (PCR) and artificial neural network (ANN) have been used for the resolution of the mixture. The analysis of these compounds in urine samples were carried out using the different chemometric tools and the best recoveries were obtained by using ANN. No pre-treatment of the sample was necessary.

This paper deals with the quantification of β- and γ-tocopherol by RP-HPLC, using a C18 column as stationary phase, and MeOH:H2O (90:10, v/v) as mobile phase (flow rate, 2 mL min−1). Both analytes appear together in the same chromatographic peak and, as consequence, a partial least squares (type PLS-1) chemometric method has been developed to analyze them separately. Different calibration matrices have been done with the fluorescence spectra, excitation and emission, absorbance and derivative absorbance spectra, obtained across the peak or in the peak apex. The matrix of data constructed with the derivative absorbance spectra obtained in the peak apex is the one that gives best results. The method has been successfully applied to edible oils and dietary supplements. No pre-treatment of the samples was needed, only a dilution in 2-propanol was effected.

A method for the determination of dimethoate in olive oil by adsorptive stripping square-wave voltammetry has been developed on the base that this pesticide is hydrolyzed in basic media giving rise to an adsorptive-reductive peak at −0.780 V. Extraction of dimethoate from olive oil with ethanol-water and posterior clean-up with C18 cartridges is carried out. Response Surface Methodology has been used for the optimization of the extraction procedure. A matrix effect is observed in olive oil extract; therefore the standard addition method must be used. The detection limit is 19.00 ng g−1 and recoveries for three levels of fortification are ranged from 79.9% to 104.5%.

This paper deals with the simultaneous determination of the quaternary mixture of tocopherols (alpha-, beta-, gamma-, and delta-T) performed using fluorimetric techniques and partial least squares (PLS-1) multivariate analysis. In this study, PLS-1 was applied to matrices made up of fluorescence excitation and emission spectra (EEM) and with fluorescence excitation, emission, and synchronous spectra (EESM) of tocopherols dissolved in hexane: diethyl ether (70:30 v/v). A calibration set of 55 samples based in a central composite plus a full factorial plus a fractionated factorial design was constructed. When synthetic samples were analyzed, recoveries around 100% were obtained and detection limits were calculated using EEM and EESM. For the analysis of the oils, the samples, diluted in hexane, were cleaned in silica cartridges and tocopherols were eluted with hexane: diethyl ether (90:10 v/v). The developed method was applied to different edible oils. The results are satisfactory for alpha-, beta-, and gamma-, but they are worse for delta-T.

Principal component analysis (PCA), and soft independent modelling class analogy (SIMCA), were applied to data of content of the various triglycerides, sterols, or both data, to explore their capacity for the typification of a variety of olive oil, belonging to a Spanish origin denomination. This study has demonstrated that it is possible to characterize the oils obtained from a specific type of olives (“Manzanilla Cacerena” of North of Caceres (Extremadura––Spain)) according to their chemical composition. Best results were obtained with the content of triglycerides. The plots of PCs showed that the PC1 is related with the category variable “variety” and the PC2 is related with “maturity”. SIMCA was employed to assign unknown samples into one of two groups or classes, depending on the “variety” of olives, for those which independent PCA models were made. Comman's plot showed that different olive oils are clustered in different groups and each group could be distinguished clearly.

The voltammetric behavior of Vitamin E in the presence of olive oil is studied at the glassy carbon electrode, in a hexane–ethanol medium, with diverse techniques: sampled DC, differential pulse, and square-wave voltammetry. The influence of such variables as the hexane–ethanol proportion, sulfuric acid concentration, and instrumental parameters is studied. Separate voltammetric peaks are obtained for α-tocopherol and δ-tocopherol, but the peaks for β-tocopherol and γ-tocopherol overlap. For the simultaneous determination of α-, β+γ-, and δ-tocopherols in vegetable oils by the PLS-1 multivariate calibration method, the results using sampled DC and DPV voltammograms are compared. The DPV voltammograms are found to be the best data set. The proposed method is applied to the determination of the tocopherols in different vegetable oil samples. The olive oil samples needed a prior cleaning stage by solid-phase extraction on silica cartridges. The results are very acceptable.

Two methods for the determination of iron in olive oil by spectrophotometry and by adsorptive stripping square wave voltammetry (Ad-SSWV) have been developed. These two methods are based on the formation of a 5,5-dimethylcyclohexane-1,2,3-trione 1,2-dioxime 3-thiosemicarbazone (DCDT)-iron(II) complex in strongly acid media. In both, iron is extracted from the olive oil by using HCl. Spectrophotometric determination of iron with DCDT is based on the feature that the DCDT-Fe complex shows an absorbance maximum at 550 nm. A calibration graph has been constructed from 0 to 4000 ng mL(-)(1), and the detection limit was 115 ng mL(-)(1) (57 ng g(-)(1) in olive oil). On the other hand, the voltammetric determination of the metal is based on the appearance of a peak due to an adsorptive reductive process of the complex that it is observed when the Ad-SSWV technique is used. A calibration graph has been constructed from 0 to 30 ng mL(-)(1), and the detection limit was 0.55 ng mL(-)(1) (13.75 ng g(-)(1) in olive oil according to the proposed procedure).

We have used an artificial neural network to optimize the composition of the mobile phase for an isocratic HPLC method for the analysis of nitrophenol pesticides and related compounds, on the basic of different response functions, and have compared the results with those obtained by application of response-surface methodology. These studies resulted in the selection the mobile phase 10:30:15:45 methanol-acetonitrile-tetrahydrofuran-buffer solution (0.1m acetic acid and 0.1m sodium perchlorate); the flow-rate was 1 mL min−1. Under these conditions a chromatogram showing twelve well-resolved peaks was obtained in 14 min. Although the peaks corresponding to ethylparathion and medinoterb acetate overlapped severely, it was possible, by use, of a diode-array spectrophotometer for detection, and by combining the absorbance measured at different wavelengths as the signal, to separate the peaks corresponding to one or other of the compounds. Calibration plots were constructed for the concentration range 2–10 ppm. Detection limits, calculated by the method of Clayton et al., were approximately 0.32–0.69 ppm. The method has been applied to the analysis of these compounds in fortified river water samples, after previous preliminary preconcentration by solid-liquid extraction on a C18 cartridge.

Quinolonic and cinolonic derivatives are mainly used as antibacterials in fish-farms. In this paper we describe a careful revision of the treatment procedures of samples and a prodedure for the determination of residues of these compounds. Because of the complexity and duration of these procedures, several studies have been carried out and these have lead to a simpler and shorter method. Three approaches have been examined: lyophilization followed by extraction with chloroform, solid-liquid extraction with chloroform and solid-liquid extraction with sodium hydroxide solution, followed by liquid-liquid partition in chloroform. Some previous studies into the partition equilibrium are also included. As a result of our studies we propose a procedure with a lower number of steps than those previously described in the literature. This method has been applied to the analysis of nalidixic, 7-hydroxymethylnalidixic and oxolinic acids and cinoxacin in trout muscle. These analysis have been carried out using an HPLC system equipped with a C18 column and fluorimetric detection. The mobile phase was acetonitrile:oxalic acid. The recoveries obtained were: 70–97% for 7-hydroxymethylnalidixic acid, 75–78% for nalidixic acid, 71–95% for oxolinic acid and 72–85% for cinoxacin.

The voltammetric behavior of propyl gallate (PG), butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) has been studied at the glassy carbon electrode (static and rotating), in an acetonitrile-water medium using LS, sampled DC, DP and cyclic voltammetry. The use of a Britton-Robinson buffer (pH 2.8) containing 20 % of acetonitrile together with the sampled DC mode allows one to analyze binary and ternary mixtures of these antioxidants by applying a PLS type 1 multivariate approach. After optimization of the parameters of the chemometric procedure, the proposed method was validated with synthetic samples and applied for the determination of these antioxidants in different spiked samples of packet soup.

An HPLC method with ultraviolet and fluorimetric detection has been established for the separation and determination of six quinolonic and cinolonic antibiotics. A Nova-Pak C18 column (150 x 3.9 mm) and a Waters 486 UV and a Waters 470 fluorescence detector have been used. The influence of variables such as mobile-phase composition and flow-rate, has been studied. An acetonitrile-aqueous solution of oxalic acid 4x10(-4) M (28:72, v/v) has been selected as optimum. The wavelength for the photometric detection of the six antibiotics was 265 nm. For the fluorimetric detection two pairs of excitation/emission wavelengths, 260/360 or 270/440 nm, were selected for the determination of nalidixic acid, 7-hydroxymethylnalidixic acid and oxolinic acid, and for the determination of pipemidic acid and cinoxacin, respectively. The analytical parameters and detection and quantification limits of the method have been determined. The proposed method has been applied for the determination of the six compounds in urine, applying different procedures depending on their concentration, the results being very acceptable.

A HPLC method with coulometric detection has been established to carry out the separation of the three nitrofuran derivatives, nitrofurantoin, furazolidone and furaltadone. A Nova-Pak C18 column (150 x 3.9 mm) and a Coulochem II detector from ESA have been used. After obtaining the hydrodynamic curves of the three compounds in the porous graphite electrode a potential of -600 mV was selected as the working potential. The influence of other variables such as mobile phase composition and flow-rate were studied. The mobile phase considered as an optimum was acetonitrile-0.1 M aqueous solution of sodium perchlorate (28:72), with 0.5% glacial acetic acid. The oxygen of the mobile phase was removed with a vacuum system on-line and a nitrogen stream was used to remove the oxygen of the samples. The calibration graphs and the detection limits were established. The method proposed was used, with good results, for the determination of the three compounds in milk.

The resolution of a ternary mixture of the pesticides carbofuran, carbaryl and fenamiphos in heterogeneous media (after extraction into CHCl 3 ) by the application of different chemometric methods such as derivative ratio spectra (DD), classical least squares (CLS), principal components regression (PCR) and partial least squares (PLS) was performed. CLS, PCR and PLS were applied with the absorption spectra or with their transformations (logarithm or first or second derivative). Also, different data preprocessing algorithms were examined. Second-derivative spectra were used for DD. A comparison of the results obtained in the analysis of these compounds by the different methods was made by using analysis of variance (ANOVA). These methods were successfully applied to the analysis of spiked river water samples.

A rapid and sensitive procedure is described for the determination of the N-methylcarbamates pesticides, carbaryl, carbofuran and fenobucarb, by liquid chromatography. The pesticides are previously hydrolyzed, in a separate step, to their phenolic derivatives, which, after the separation in a high speed column in a time lower than 2.5 min, are detected in a high sensitivity amperometric cell at a potential of +0.6 V. The method presents detection limits around 0.7 ppb and has been satisfactorily applied to the analysis of these compounds in spiked river water samples.

A differential pulse voltammetric method is proposed for the determination of the carbamate pesticides carbaryl and carbofuran, based on the anodic peaks observed following their alkaline hydrolysis (formation of phenol derivatives). Linear relationships between the oxidation peaks and concentration were obtained in the ranges 5 × 10 −7 to 10 −4 M for carbaryl and 5 × 10 −7 to 5 × 10 −5 M for carbofuran, the relative standard deviations being 1.62 and 1.86%, respectively. The method was applied to the determination of these compounds in formulations. Different chemometric methods (PLS-1, PLS-2 and CLS) were examined for the resolution of the overlapped peaks of both compounds and their determination in mixtures. The best results were obtained with the PLS-1 method, which was applied to determine both compounds in river water samples.

A new and rapid method is described to determine nitrofurantoin, furazolidone and furaltadone in formulations, feed and milk by reversed phase high performance liquid chromatography. A high-speed C18 3 × 3 column is used and a mobile phase of HAc/NaAc 0.1 M (pH 3.2):acetonitrile, 90:10, was found to be the optimum to carry out the analytical separation. A photometric detection at 360 run was selected.

A multivariate calibration method, partial least squares (types PLS-1 and PLS-2), was applied to the simultaneous determination of naptalam (N-(1-naphthyl) phthalamic acid) and its metabolites N-(1-naphthyl) phthalimide and 1-naphthylamine in mixtures by UV-visible absorption spectrophotometry. The absorption and first-derivative absorption spectra of mixtures were used to perform the optimization of the calibration matrices by the PLS method. Two different experimental designs for the three-component mixtures are assayed and the results are discussed. The proposed method with the derivative spectra was applied to the determination of these analytes in river water at the ppb level.

The polarographic behaviour of furaltadone was studied in Britton-Robinson buffer solutions and perchloric acid media. Three well defined reduction waves appear in acidic media (pH < 4) whereas at higher pH the last two peaks overlap. The variation of E1/2 or Ep and Ilim or Ip with pH was studied. The diffusion-controlled nature of the waves at pH 1.87 (Britton-Robinson solution) and in 0.1 M perchloric acid was established and the irreversibility of the electrode process was verified by different criteria. The mechanism for the reduction is discussed. A method for the determination of furaltadone by differential-pulse polarography which allows 0.025–2.0 μg ml−1 (7.7 × 10−8−6.2 × 10−6 M) of this compound to be determined is proposed and was applied to the determination of furaltadone in its formulations, milk and urine.

A method, using stripping square wave voltammetry (Ad-SSWV), for the simultaneous determination of fenitrothion (FEN) and its metabolites: fenitrooxon (OXON) and 3-methyl-4-nitrophenol (3-MET) in environmental samples is reported. All three compounds produce, at mercury electrode (HMDE), an electrochemical signal due to an adsorptive-reductive process. The electrochemical approach shows a very high overlap degree for FEN and OXON voltammograms, however the adsorption kinetic profile could be used as an additional differential variable between both analytes. Second-order multivariate calibration has been tested to solve the mixture of the three compounds. The second-order assayed methods were parallel factor analysis (PARAFAC), unfolded partial least squares (U-PLS), multidimensional partial least squares (N-PLS) and the latest ones were used in combination with the residual bilinearization procedure RBL. U-PLS/RBL model was stated as the best second-order algorithm for the simultaneous determination of these three compounds up to 50 ng mL(-1) for each analyte. The detection limits and recovery values were 1.6 ng mL(-1) and 92+/-7% for FEN; 3.7 ng mL(-1) and 101+/-9% for OXON and 0.6 ng mL(-1) and 97+/-8% for 3-MET.

A simple, rapid and precise analytical method for sulfathiazole, oxytetracycline and tetracycline residues in honey has been established using micellar reversed-phase high-performance liquid chromatography with photometric detection. The mean recoveries of sulfathiazole, oxytetracycline and tetracycline were 97.7, 95.3 and 100.4%, respectively. Detection limits in honey were 0.9 μg/g for sulfathiazole, 1.2 μg/g for oxytetracycline and 3.0 μg/g for tetracycline.

Square-wave adsorptive-stripping voltammetry technique has been used to develop a method for the determination of fenthion in olive oil. Due to the fact that fenthion does not give any electrochemical signal at mercury electrode, the method has been based on a previous oxidation of fenthion to its metabolite, fenthion-sulfoxide, by using KMnO 4 . The metabolite gives rise to a peak due to an adsorptive-reductive process at −0.786 V. Fenthion is isolated from olive oil by carrying out a solid–liquid extraction procedure using silica cartridge, followed by a liquid–liquid partitioning with acetonitrile. The detection limit in olive oil is 78.8 ng g −1 and recoveries for four levels of fortification are ranged from 85% to 109%. On the other hand, it has been developed a method for the simultaneous determination of fenthion and its metabolite fenthion-sulfoxide, in river water. Pesticides are isolated from water by carrying out a liquid–liquid partitioning with trichloromethane. The detection limits are 0.41 ng g −1 and 0.44 ng g −1 , for fenthion and fenthion-sulfoxide, respectively. Recoveries for three levels of fortification are ranged from 96% to 103% for fenthion and 94% to 104% for fenthion-sulfoxide.

Basic studies on the photochemical behaviour of trans-resveratrol and its photoproduct are reported. Photometrically and fluorimetrically calculated acidity constants of the former were determined. The usefulness of the determination of resveratrol by photochemically induced fluorescence and second-derivative photochemically induced fluorescence was also examined. The very weakly fluorescent trans-resveratrol is converted into a highly fluorescent photoproduct by irradiating hydroethanolic solutions of trans-resveratrol containing 40% v/v of ethanol for 60 s with intense UV radiation. The photoproduct presents excitation and emission maxima centred at 260 nm, and 364 and 382 nm, respectively. Under these conditions, a linear relationship between fluorescence intensity and trans-resveratrol concentration was found between 6.6 and 66 ng mL−1. Optimum conditions for the extraction of trans-resveratrol from an aqueous phase at pH 5.0 with diethylether were a phase ratio (aqueous/organic) of 2, a shaking time of 60 s and a buffer concentration of 0.15 mol L−1. An extraction recovery of 100% was reached under these conditions. The optimized extraction procedure was applied to the analysis of resveratrol in wine samples, employing the amplitude between 356 and 364 nm of the second-derivative photoinduced emission spectrum as analytical signal. It was found that there is not matrix effect and recoveries around 100% were obtained at different fortification levels.

A rapid high-performance liquid chromatographic method has been developed to determine piromidic acid in trout muscle tissue and in urine, in the presence of nalidixic, 7-hydroxymethylnalidixic, oxolinic and pipemidic acids and cinoxacin. A Nova-Pak C18 column was used with acetonitrile-4x10(-4) M oxalic acid (40:60, v/v) as the mobile phase. A post-column change of pH was made with NaOH. Fluorimetric detection at 456 nm (lambda ex 275 nm) was used. The instrumental detection limit was 5.91 ng/ml, based on height of peak. Pretreatment of the urine samples was not necessary and fish samples were extracted with sodium hydroxide solutions and cleaned by means of an extraction with chloroform. Detection limit was 147 ng/ml for urine and 5.91 ng/g for trout muscle. Good separation without interference from any other components was obtained. Recovery was better than 87% in urine and better than 72% in trout muscle tissue.

The polarographic behaviour of the sulfonamides sulfadiazine, sulfamerazine and sulfamethazine was studied in Britton–Robinson buffer solutions. These compounds show well defined reduction waves and differentiated peaks in differential-pulse polarography (DPP) with Ep values near –700, –790 and –1140 mV, respectively, in an acidic medium (approximately pH 2), the behaviour of the sulfonamides in response to variations in pH and concentration is very similar. However, when two or three of these compounds are present together, the signal obtained in DPP does not agree with the superposition of the signal of each one of them when present alone. This fact can be explained by the change that the dc wave corresponding to any of the sulfonamides undergoes owing to the presence of one or two of the others, thus making the direct analysis of their mixtures by DPP impossible. The ability of partial least squares multivariate methods to resolve this problem was examined. In general, good results were obtained and the method was applied to the analysis of mixtures of these sulfonamides in veterinary and pharmaceutical formulations.

1,3-Cyclopentanedione bis(4-methylthiosemicarbazone) monohydrochloride produces colored solutions with chlorate ions in strongly acid medium. The yellow color obtained has been used to propose a spectrophotometric method of C10 3 − determination in the concentration range 0.5–6.0 ppm (molar absorptivity 1.26 × 10 4 liters mol −1 cm −1 at a wavelength of 397 nm).

The kinetic method is based on the catalytic effect of copper(II) on the atmospheric oxidation of 1,3-cyclopentanedione bis(4-methylthiosemicarbazone) monohydrochloride. The reaction is followed spectrophotometrically at 390 nm. The kinetic parameters of the reaction are reported and a rate equation is suggested. The calibration graphs are linear in the range 0.05–0.5 μg Cu ml−1. Interference by other ions is studied.

The 2-oximinocyclohexanone thiosemicarbazone forms, in basic medium, a violet complex with Mn(III) whose stoichiometry is 3:1 (reagent:Mn(III)). The molar absorptivity has a value of 3000 liters · mol−1 · cm−1. By means of the formation of this complex manganese can be determined between 9 and 400 μg with a relative error (95% confidence level) of 0.20%. The effect of foreign ions has been examined and the method has been applied to Mn(II) determination in Lincolnshire iron ore.

Cyclopentane-1,3-dione bis (4-methylthiosemicarbazone) monohydrochloride produces coloured solutions with bromate ions in perchloric acid medium. The yellow colour obtained (molar absorptivity 1.8×104 1.mol−1.cm−1 at a wavelength of 400 mm) has been used for the spectrophotometric determination of trace amounts of bromate.

1,3-Cyclopentanedione bis(4-methylthiosemicarbazone) monohydrochloride produces colored solutions with iodate ions in acid medium. The yellow color obtained has been used to proposed Spectrophotometric methods for determination of IO 3 − in the concentration range 1.0–11.0 ppm in acetic acid medium (molar absorptivity 1.08 × 10 4 liters mol −1 cm −1 at a wavelength of 415 nm) and 0.5–8.0 ppm in perchloric acid medium (molar absorptivity 2.05 × 10 4 liters mol −1 cm −1 at a wavelength of 400 nm).

The spectrophotometric study of reddish cyclohexane-1, 3-dione bis (4-methylthiosemicarbazone)-Zn(II) was made in dimethylformamide-water solution (λmax= 475 nm, ∑ = 3.3×104 1.mole−1. cm−1. Sandell sensitivity = 2×10−2 μg Zn(II).cm−2, stoichiometry 1:1, and apparent stability constant 6.1×104). A new method for the spectrophotometric determination of Zn(II) is proposed for concentrations between 0.1 and 2.5 ppm. The relative error (95% confidence level) is 0.7% for 1.0 pprn of Zn(II). The extraction with ethylacetate of the reddish complex was also studied spectrophotometrically (λmax = 493 nm, ∑ in organic phase = 4.8×104 1.mole−1.cm. Sandell sensitivity = 3.4×10−4 μg Zn(II).cm−2, stoichiometry 1:1, apparent extraction constant 1.4×104). A new method for the extraction-spectrophotometric determination of Zn(II) is proposed for concentrations, in aqueous phase, between 0.02 and 0.30 ppm. The relative error (95% confidence level) is 1.0% for 0.15 pprn of Zn(II).

A simple method is proposed for the determination of L-ascorbic acid in pharmaceutical preparations, foods and urine. The procedure is based on the rapid reduction of the iron(III) ion in the presence of L-ascorbic acid. The iron(II) formed is measured by reaction with 2-oximinocyclohexanone thiosemicarbazone (2-OCHT). The method has been used to determine L-ascorbic acid above 0.33 µg ml–1, and the detection limit is 0.10 µg ml–1. The results of the determination of L-ascorbic acid obtained using this procedure were comparable to those given by the 2,6-dichlorophenolindophenol (DIP) method.

The iron (II)-5,5-dimethyl-1,2,3-cyclohexanetrione-1,2-dioxime-3-thiosemicarbazone complex can be extracted from a medium of high acidity into amyl alcohol. Measurement of the absorbance at 550 nm gives a determination limit for iron of 0.015 mu/ml. The method has been applied to the determination of iron in water, wine, beer and fruit juice.

A rapid, simple and selective method is described for the spectrophotometric determination of iron in acids based on the formation of an iron-5,5-dimethylcyclohexane-1,2,3-trione-1,2-dioxime-3-thiosemicarbazone complex.