Acetate-acetic Buffer Research Articles

Impacts of chemical pre-treatments on the hydrogen isotope composition of clay minerals: Determining potential effects in the absence of impurities

The structural hydroxyl hydrogen (H)-isotope compositions of clay minerals are frequently used as tracers of alteration or change in paleoenvironmental and material science studies. Dissolution pre-treatments are commonly used to remove cements and H-bearing impurities prior to isotopic analysis. These pre-treatments commonly include reactions respectively with: HCl or acetic-acetate buffer, H2O2 or NaClO, and a Na-dithionite-Na-citrate-bicarbonate (DCB) solution in order to remove carbonates, organic matter, and iron (oxy)hydroxides. However, there has been inconsistent data regarding to what extent the structural hydroxyl H-isotope composition of a clay mineral is altered during exposure to these chemical pre-treatments.This study examined the “most extreme” impacts of each pre-treatment on the structural H-isotope composition of reference nontronite, montmorillonite, saponite, and kaolinite samples obtained from the Clay Minerals Society Source Clay Repository. All chemical reagents were isotopically labeled to a δ2H value ≈ +200 or +570‰, to sharply contrast the average clay starting material δ2H value ≈ –120‰. H-isotope compositions were measured before and after pre-treatments by High Temperature Elemental Conversion (TCEA) coupled with an Isotope Ratio Mass Spectrometer (IRMS). TCEA-IRMS measurements were done on potassium (K)-exchanged samples pre-dried under active vacuum at 220 °C to mitigate isotopic contamination by adsorbed water vapor.The results showed that only the H-isotope composition of kaolinite was unaffected by all pre-treatments. NaClO and DCB treatments affected the H-isotope compositions of saponite and nontronite, respectively, with the remaining pre-treatments not causing measurable changes for these two clay minerals. The HCl and H2O2 pre-treatments caused massive alteration to the H-isotope compositions for montmorillonite and are considered unadvisable for use with this clay mineral, with respect to H-isotope analysis. The study confirms the possibility of altering the H-isotope composition of clays when using common selective dissolution procedures. The choice of applied pre-treatments before H-isotope measurements for clay-bearing materials should be based on the specific properties and mineralogy of the investigated samples and the amount and type of impurities they contain.

Flow-injection spectrophotometric determination of nicotinic acid in micellar medium of N-cetylpyridinium chloride

A simple flow-injection (FI) method with spectrophotometric detection at 440 nm is proposed for the determination of nicotinic acid, which is based on the formation of a polymethine dye at room temperature. In the FI system, nicotinic acid is hydrolyzed with 0.2 M cyanogen bromide at pH 5 (acetic-acetate buffer) in a non-micellar medium. The glutaconic aldehyde formed is injected into a stream containing 0.1 M aniline at pH 7, in a 0.1 M micellar medium of the cationic surfactant N-cetylpyridinium chloride (NCPC), to produce the polymethine. The performance of NCPC is compared with that of other surfactants: cetyltrimethylammonium bromide (CTAB, cationic), sodium dodecyl sulphate (anionic), and Triton X-100 (non-ionic). In the presence of NCPC and CTAB, the coupling reaction increases its sensitivity in ×3.3 and ×2.6 factors, respectively. This effect is explained by the higher concentration of glutaconic aldehyde and aniline on the micelle surface due to electrostatic attraction and hydrophobic association to the cationic surfactant. The dispersion coefficient of the FI system was D=2.4, and the sampling rate, 95 samples per hour. The limit of detection was 1.2×10 −6 M and the coefficient of variation 1.0 and 0.8% for 1×10 −5 and 1×10 −4 M nicotic acid, respectively. The optimized manifold was applied to the determination of commercial pharmaceuticals containing nicotinic acid, with recoveries in the 96.7–101% range.

Kinetic-potentiometric determination of copper in real samples with a new antimony(V) ion-selective electrode

A sensitive and selective kinetic method for the determination of copper is proposed. It is based on the catalytic effect of copper on the reaction between hexachloroantimonate(V) and hydroxylamine at pH 4.4 (acetic-acetate buffer). The rate of the reaction is potentiometrically monitored with a new antimony(V) ion-selective electrode using 1,2,4,6-tetraphenyl-pyridinium hexachloroantimonate(V) as electroactive material in a poly-(vinyl chloride) membrane. The proposed method allows the determination of copper(II) concentrations in the 5–510 ng ml−1 range, with a variation coefficient of 3.5%. The method has satisfactorily been applied to a variety of real samples.

Trace determination of molybdenum using a catalytic current in differential-pulse polarography

The differential-pulse polarographic determination of molybdenum(VI) coupled with a catalytic homogeneous reaction was carried out in a system containing the molybdenum-ferron complex and an oxidant in acetic-acetate buffer. On addition of the powerful oxidant, the peak current for the reduction of the molybdenum(VI) complex was increased significantly. The calibration graph was linear in the concentration range 1 × 10 −6 −1 × 10 −7 M and the detection limit was 9.5 ng ml −1.

Pyrocatechol-1-aldehyde salicyloylhydrazone as reagent for the spectrofluorimetric determination of zinc

A procedure is developed for the Spectrofluorimetric determination of 10–800 ppb of zinc with pyrocatechol-1-aldehyde salicyloylhydrazone in a 60% (v/v) ethanol-water medium at apparent pH 6.7 (acetic-acetate buffer). λex=400 nm, λem=530 nm. Interferences have been evaluated and the method applied to the determination of zinc in alloys and lubricating oils.