Carlo Maccà

Effect of eluent composition and pH and chemiluminescent reagent pH on ion chromatographic selectivity and luminol-based chemiluminescence detection of Co2+, Mn2+ and Fe2+ at trace levels

The alkaline-luminol/H(2)O(2)-based chemiluminescent (CL) detection of Fe(2+), Co(2+), and Mn(2+), separated with a Dionex CS5A ion chromatographic phase was studied by means of a multi-pump flow system allowing the variation of the post-column solution composition. A perchlorate gradient at pH 1.9 (with HCl) was used to separate cations partially complexed with 5.6mM oxalate present in the eluent and necessary for the chosen separation phase. A 0.91mM luminol, 3.3mM H(2)O(2) in 0.25M carbonate buffer at pH 10.5 composition was chosen as CL reagent solution. The chosen pH value warrants signal repeatability and wider linearity range although absolute signal is not maximum. The CL signal was related to the pH of the two post-column mixing solutions. Calibration plots of Co(2+) and Fe(2+) were linear in the chosen concentration range whilst a parabolic model was the best fit for Mn(2+). Detection limits were 0.24, 0.50 and 375nM for Co(2+), Fe(2+) and Mn(2+), respectively. The method was used to determine Co(2+) at trace level in commercial copper chelates used for animal feeding. A comparison with a chromatographic method with spectrophotometric detection was made giving results comparable both in absolute values and accuracy.

Determination of potentiometric selectivity

Abstract The recommendations for reporting selectivity coefficient values of ion-selective electrodes recently issued by the Commission on Electroanalytical Chemistry of the IUPAC Analytical Chemistry Division are commented. Properties, drawbacks and advantages of the matched potential method, newly included in preference to traditional methods, are discussed; a different denomination, interference ratio, is proposed for the quantity measured with this method.

Determination of selectivity coefficients of ion-selective electrodes by means of linearized multiple standard addition techniques

Abstract The application of multiple known-addition or dilution techniques to the evaluation of potentiometric selectivity coefficients of ion-selective electrodes is proposed. The validity of the common form of the Nikolskii—Eisenman equation is assumed and experimental data are processed by using Gran-type linear functions. Six possible methods are derived which make it possible to work under bi-ionic conditions for the electrode response. Optimization of the experimental procedures is discussed.

Linearity range of Gran plots for the end-point in potentiometric titrations

Simple criteria and procedures to assess which part of a potentiometric titration curve can be used to obtain an appreciably linear Gran plot and an accurate extrapolation to the equivalence volume are discussed for one-step precipitation, complexometric, redox and acid-base titrations. Equations were derived for calculating the limiting values of the experimental parameters that prevent titration errors higher than an assigned value.

Aluminium carboxylates in aqueous solutions. Part 2. Metal speciation in the AlIII–lactate–OH––H2O system

The tris(hydroxyl–carboxyl) chelated complex Al(lact)3(lact = lactate) dissolves in water to give stable acidic (pH ca. 3) solutions, in which free and metal-co-ordinated lactate ligands are detected by IR spectra in D2O. Proton NMR spectra (90 MHz) reveal that ligand exchange between metal-bonded and free lactate ligands occurs, also at +4 °C. The separate resonances due to metal-bonded and free lactate are clearly detected at 400 MHz. When the pH of an Al(lact)3 solution (analytical concentration 0.1 mol dm–3) is adjusted to 7.5 with NaOD the resulting 1H and 13C NMR spectra are very similar to those exhibited by a 0.3 mol dm–3 Na(lact) solution. The IR spectrum of the 0.1 mol dm–3 aluminium(III) solution displays two major bands at 1585 and 1420 cm–1 due to the unco-ordinated lactate ligands. A 0.1 mol dm–3 neutralised solution is (meta)stable for months and the whole of the data at pH 7.5 can be interpreted in terms of the release of most of the lactate from the co-ordination sphere of AlIII, with concomitant formation of metastable species including possibly [Al(OH)3(H2O)3].

Gran plots and rigorous linear plots for weak acid titrations : the chemical rationale

SummaryLinear plots suitable for treatment of single monofunctional weak acid titration data are critically surveyed. The chemical principles underlying approximate and corrected Gran plots are discussed. The relationships between Gran plots and other linear plots of the types commonly employed for the determination of binding constants (like the Hofstee, Scatchard, Lineweaver-Burk and Scott plots) are shown. The main features of the different kinds of plots and the effect of measurement errors on their performance with moderately strong and very weak acids are illustrated.

A pH-stat study of the reaction of some transition metal cations with disodium ethylenedinitrilotetraacetate (EDTA) and its analytical application

The pH-stat titration technique is an autonomous and very powerful tool for performing and monitoring chelatometric titrations of metal cations with great accuracy, poorly known, however, and seldom exploited. Based on measurement of the amount of strong base required to keep the pH of the test solution at a selected value during stepwise known additions of ethylenedinitrilotetraacetate (EDTA), it requires a glass electrode as the only sensor and is easily implemented on potentiometric titrators. It was introduced a quarter of century ago on an empirical basis for a very peculiar purpose (determination of calcium in diary products), but only very recently it was generalised and its fundamentals were thoroughly examined. In this work, pH-static titrations of some transition metal cations of analytical relevance (Co(2+), Cu(2+), Mn(2+), Zn(2+)) were thoroughly investigated in the acid pH range between 2.3 and 5 or 7 (the highest pH depending on the metal hydroxide or carbonate solubility). The results at higher acidity showed unsuspected properties of such chelation reactions. At moderately acid pH (generally >/=4), indeed, pH-static titrations yield results of high precision and accuracy. On decreasing pH, however, the reaction stoichiomety deviates more and more from the 1:1 ratio between chelating agent and cation, seemingly because of formation of binuclear complexes, an occurrence very seldom mentioned in the current literature. The optimal titration conditions for each metal are defined, and directions for establishing a laboratory protocol for quantitative determinations are given.

pH-stat techniques in titrimetric analysis: IV. pH-stat monitoring of chelatometric titrations

Abstract The theory of pH-stat chelatometric titrations recently developed [Anal. Chim. Acta 456 (2002) 313] is experimentally substantiated here. The titrations of four representative doubly charged cations having different behaviour (copper, zinc, calcium, magnesium) are taken as examples. Copper is titratable between pH 3 and 5, zinc between 3 and 6, calcium between 6 and 10, and magnesium between 7 and 10. The shapes of the titration plots agree well with the theory, accounting for simultaneous equilibria involving proton exchange. The technique yields accurate and precise results, which compare favourably with those of other instrumental techniques, in particular photometric titrations.

pH-stat techniques in titrimetric analysis: Part 3. The principles of pH-stat chelatometric titrations

Abstract Chelatometric titrations can be effectively monitored by means of the pH-stat technique. During the addition of the primary titrant (the chelating agent), the pH of the titrated solution is kept at a pre-selected value by adding a strong base as the auxiliary titrant. The equivalence point is characterised by a sudden change in the ratio between the added volumes of auxiliary and primary titrant solutions, i.e. in the slope of the linearly segmented titration plot representing the volume of the base against the volume of the chelating agent. The slope of the linear segments depends on the reactions involving proton exchange, which accompany the primary complexation reaction. The dependence of the shape of the plots on the imposed pH for typical titrations (magnesium(II) and copper(II)) is examined and the conditions for the feasibility of pH-stat chelatometric titrations are discussed.

Potentiometric characterization of weak acids by multiple sample addition-I. Linear equations and intrinsic performance of the method.

The suitability of linearized multiple sample-addition for the potentiometric characterization of monoprotic weak acids is examined. Linear equations for the separate or simultaneous determination of the acidity constant and of the sample concentration by treatment of experimental data are introduced. The intrinsic performance of the method and the application range of the different equations are discussed with reference to the theoretical effect of measurement errors on the values of the quantities to be determined.