Guillermo López-Cueto

Simultaneous kinetic spectrophotometric determination of o-, m-and p-aminophenol using partial least squares calibration

The oxidative coupling reactions of p-aminophenol with m-and p-aminophenol in the presence of octacyanomolybdate(v) were used as the basis of a differential kinetic method for the simultaneous determination of o-, m-and p-aminophenol (o-, m-and p-AP) in binary and ternary mixtures. The influence of Mov concentration, acidity, ionic strength and temperature was studied in order to establish optimum conditions. Several oxidants, including Ce4+, CrVI, MnVII, H2O2 and metaperiodate and FeIII, MnIV, MoV and WV cyano complexes were tried. The MoV cyano complex was chosen because it has a suitable redox potential and gives rise to kinetically useful runs. The reaction must be implemented under sub-stoichiometric concentrations of oxidant and so the concentration of each aminophenol in the mixture influences the reaction of the others, originating in a system with a high degree of non-linearity. As a result, most methods of data treatment cannot be used and multivariate calibration (PLS) was applied to the data acquired by a diode-array spectrophotometer between 340 and 500 nm. PLS calibration with a number of factors ranging between six and nine allowed satisfactory results for all kinds of mixtures. Aminophenols can be determined over the concentration range 0.1–1.0 mmol l–1 for o-AP, 0.3–1.0 mmol l–1 for m-AP and 0.25–3.0 mmol l–1 for p-AP with errors of less than 10% in all instances. No systematic errors were detected.

Individual Kinetic Determinations Using Partial Least SquaresCalibration

The slow oxidation reactions of cysteine, tyrosine and tryptophan by hexacyanomanganate(IV) were used to investigate partial least squares calibration (PLS) as a method to be applied to the kinetic determination of individual species. All the three reactions are first order in hexacyanomanganate(IV) concentration and the reactions with cysteine and tyrosine are first order in amino acid concentration. The stoichiometry was studied in each case. The determination results were always compared with those obtained by applying the initial rate method to the same kinetic runs. When suitable kinetic curves are obtained and the initial rate is easily determined, both methods give similar results; however, when the initial rate is not easily determined (too fast reaction, slope that changes very quickly, etc.) the results obtained with PLS are better (better precision and wider dynamic range). Moreover, and in contrast to the initial rate method, PLS allowed tryptophan to be determined in the presence of some different feed matrices, provided the calibration set was of the same nature as the samples. This advantage is of general application and makes the analytical method more robust. PLS was tested on uncatalysed reactions but it could equally be applied to catalysed reactions.

Linear and non-linear multivariate calibration methods for multicomponent kinetic determinations in cases of severe non-linearity

Both p-aminophenol (p-AP) and p-phenylenediamine (p-PDA) undergo oxidative coupling reactions in the presence of octacyanomolibdate(V) as oxidant. The kinetics of these reactions can be followed spectrophotometrically. When they happen to be in the same reaction mixture, relevant characteristics are (1) related to reaction conditions; the reagent must be in deficit and (2) related to the reaction products; ‘cross-coupling’ reactions are also involved. All this gives rise to systems with severe non-linearities. This is the reason why they have been used here to test the predictive ability of several linear and non-linear algorithms in cases of special complexity. Principal component regression (PCR), partial least squares regression (PLS) and three different forms of applying artificial neural networks (ANNs), with and without reduction of variables, have been tested. All the models were applied to the same calibration and validation sets. Typical prediction errors ranged between 2 and 5%. Not very different results were found when independent accuracy and precision studies were accomplished, regardless of whether linear or non-linear calibration methods were used. This is probably due to differences in the spectroscopic as well as the kinetic behaviour of both analytes. These differences allowed the construction of ANN calibration models with only 30 original variables (kinetic profiles of 10 data points at three selected different wavelengths).

New way of application of the bromate–bromide mixture in kinetic analysis☆

Abstract The bromine generated in situ by the bromate–bromide mixture has usually been used in kinetic analysis to determine either individual species that give reaction with bromine or the total concentration of mixtures of those analytes. In this paper the bromate–bromide mixture is proposed as a reagent for multicomponent kinetic determinations. Mixtures of acetylsalicylic acid (ASA) and paracetamol (AAP) have been used to test the discriminating ability of reactions with bromine. The reaction between bromine and AAP is fast and the reaction between bromine and ASA is negligible. However, ASA slowly hydrolyzes to yield salicylic acid (SA) and acetic acid, and the reaction between bromine and SA can be considered fast in the reaction conditions. This complex behaviour contains enough information to resolve mixtures of ASA and AAP after applying partial least squares regression (PLS) to the kinetic photometric data. After optimization, errors around 5–6% were found for the determination of ASA and AAP. Quimiometric considerations on the calibration models are made. The method has been successfully applied to the determination of ASA and AAP in pharmaceuticals.

The hexacyanomanganate(IV)-hydrogen peroxide reaction. Kinetic determination of vanadium.

The reaction between hexacyanomanganate(IV) and hydrogen peroxide in acidic medium is strongly catalysed by vanadium. The stoichiometry has been proved to be Delta[Mn(IV)]/Delta[H(2)O(2)]=1. S-shaped absorbance-time curves are obtained for a wide range of conditions, which seems to be due to the transient formation of hydroperoxyl radicals, HO(2).; therefore, the reaction is autoinduced. The reaction kinetics is first order on the hexacyanomanganate(IV) concentration. Complex dependence of the initial rate on the H(2)O(2) as well as H(+) concentrations are observed both for the uncatalysed and for the catalysed reactions, but the rate laws are given and reaction mechanisms are proposed. The catalytic effect of vanadium has been used to determine traces of vanadium (the (IV) and (V) oxidation states are determined together). By applying the initial rate method a detection limit of 0.9 ng ml(-1) (18 nM), and a linear range up to 50 ng ml(-1) were found. The relative standard deviations for the 4 and 25.5 ng ml(-1) levels were 5.2 and 2.5% respectively. Positive kinetic interferences from Cr(VI), Hg(I) and Ag(I) have been observed; other interferences are less severe and Cr(IIl), Fe(II) and Fe(III), among many others, do not interfere. The Mn(IV) solution must be prepared daily and its conservation needs some care (0 degrees C) but the proposed method has been applied to the determination of vanadium in a phosphate rock (reference material, BCR No. 32) without previous separations. Recovery experiments have also been performed; excellent results were obtained in both cases.

Improved selectivity in multicomponent determinations through interference modelling by applying partial least squares regression to kinetic profiles

Binary and ternary mixtures of cysteine, tyrosine and tryptophan were used to show how kinetic methods together with partial least squares regression (PLSR) can be used to overcome interferences in multicomponent determinations. Two methods were compared: an equilibrium method which uses the characteristic UV spectra of amino acids and a kinetic method which follows the reaction kinetics of the oxidation of the amino acids by hexacyanomanganate(IV). In both cases, multi-wavelength detection with a diode-array spectrophotometer was used. The results obtained from the two methods are critically compared. Lower concentrations of tyrosine and tryptophan can be determined by the equilibrium method because of their high absorptivities. On the other hand, the kinetic method, in conjunction with PLSR, can be considered an example of how selectivity can be improved. This allowed the resolution of mixtures of tyrosine and tryptophan in the presence of several complex spiked matrices (aqueous extracts of feed samples that interfere to different extents) without any previous separation.

Promoting effect of iodide on the hexacyanomanganate(IV)—arsenic(III) redox reaction, for kinetic determination of iodide

The rate of the reaction between hexacyanomanganate(IV) and arsenic(III) in an acid medium is strongly accelerated by iodide. The reaction kinetics indicates that the iodide activity decreases throughout the reaction, probably because manganese(IV) oxidizes iodide to iodate (an inactive form). This behaviour is defined as promotion, rather than catalysis, and this rate-modifying effect has been used to determine iodide by a kinetic method. A linear calibration plot was obtained by a two-point fixed-time procedure. A detection limit of 0.2 ng/ml, a quantification limit of 0.6 ng/ml and relative standard deviations of 5.5 and 13% for the 6.7 and 0.6 ng/ml levels respectively have been found. Positive kinetic interferences from osmium(VIII) and iodate have been observed, and copper(II), silver(I) and mercury(II) inhibit the iodide activity by precipitaton. The method has been applied to determination of iodide in sodium arsenite (reagent grade) and table salt. The method has been validated by recovery experiments.

Multicomponent determinations by partial least-squares regression analysis of fast-reaction multiwavelength profiles obtained by continuous addition of a reagent

Partial least-squares regression (PLS) analysis of multiwavelength reaction profiles, obtained by continuous addition of a reagent to a binary mixture of analytes, giving fast reactions, is presented. The methodology has proved to be useful for the simultaneous determination of the mixture components. Octacyanomolibdate(V) (Mo(V)) has been used as a reagent for mixtures of hydroquinone (HQ) and pyrogallol (PG) in acidic medium. When the reaction profiles are followed spectrophotometrically at several wavelengths, and they are handled with PLS, the mixture can be resolved. The final spectrum of the mixture, after reaction with Mo(V), is enough to predict the HQ concentration, but in the case of PG successive spectra along the reaction are necessary to predict properly the concentration. The concentrations for HQ in the calibration set ranged between 1.8x10(-6) and 1.7x10(-5) M and for PG between 2.5x10(-6) and 2.5x10(-5) M. The concentration ratio, [HQ]:[PG], in the validation set ranged between 0.17 and 4.10. Mean validation errors of 1.6% for HQ and 4.2% for PG were found. Satisfactory results were obtained when independent studies of accuracy and precision were accomplished. The effect of p-phenylenediamine and aminophenols was studied as interferences and, in general, hydroquinone is more affected than pyrogallol. The tolerated concentration of interferences ranged between 10(-6) and 10(-4) M.

Enhanced decompositions of some cyano complexes during diode array spectrophotometric measurements

Acidic solutions of the Fe(III), Mo(V), W(V) and Mn(IV) cyano complexes are known to decompose photochemically. These reactions do not take place to an appreciable extent when the cyano complexes are subjected to spectrophotometric measurements by means of conventional spectrophotometers, but they do take place when diode array spectrophotometers are used. The reactions are autoaccelerated, at least in the cases of Mo(V), W(V) and Mn(IV), and consist of photoinduced reductions of the metal ion. This effect should be taken into account in all cases where the measured signal has to be used for quantitative purposes, but its significance must be emphasized mainly in the field of kinetic analysis, where it can lead to large errors. The extent of these reactions, and therefore the size of the errors, can be lowered by using UV cut-off filters and by taking discontinuous measurements with suitable integration times. All these possibilities, which are of general application, are discussed.

Methods for analysis of the mononuclear and binuclear manganese(III) cyano-complexes.

Simple titrimetric methods are described for the analysis of potassium hexacyanomanganate(III) and of heptapotassium mu-oxo-bis[pentacyanomanganate(III)] cyanide. Mn(2)O(CN)(6-)(10) is determined by potentiometric titration with hexacyanoferrate(III). Cyanide is determined in both complexes by back-titration with Hg(II) and SCN(-), and manganese(III) is determined by back-titration with Fe(II) and MnO(-)(4). The absorption spectra of both cyano-complexes in cyanide and in acidic solutions are also described, and their molar absorptivities are reported.