Potentiometric titration of diprotic systems

 The approximate nature of Gran’s method of linearization of potentiometric titration curves applied to diprotic systems has been thoroughly investigated. A function involving pH and titrant volume V (which is linearly dependent on V and becomes zero at the first equivalent point of titration of a weak diprotic base with a strong monoprotic acid) has been critically examined and validated by means of widely extended simulated experiments. Accurate experimental confirmation of the theoretical expressions has been obtained by performing many real titrations of the primary standard Na₂CO₃ with HCl. This particular application, analytically significant itself, can be a prototype of analogous applications. Received: 25 March 1996/Revised: 15 November 1996/Accepted: 19 November 1996     

Catalytic determination of ultra trace amounts of vanadium with detection by linear sweep voltammetry

Vanadium has a very strong catalytic effect on the bromate oxidation of gallocyanine at pH of 4.0. The oxidation product of gallocyanine exhibit a voltammetric wave at + 0.10 V vs. Ag/AgCl reference electrode in 0.10 mol/l KNO₃ medium. The linear scan voltammetric behaviour of the reaction products at HMDE has been studied and selected as the indicator component for the reaction. A detection limit of 0.05 ng/ml and calibration graph from 0.30–200.00 ng/ml vanadium were obtained. Vanadium in water and gasoline samples was determined by this method, with satisfactory results. Received: 26 September 1996 / Revised: 6 November 1996 / Accepted: 15 November 1996     

Characterization of di(2-ethylhexyl)thiophosphoric acid by potentiometric titration and capillary zone electrophoresis

 The acidity constant of di(2-ethylhexyl) thiophosphoric acid (DEHTPA) was investigated using potentiometric measurements with a pH glass electrode. Owing to the low solubility of DEHTPA in pure water, its concentration and acidity constants were determined by linearised pH-metric titration in 16.7% ethanol. A K_a value of (2.39±0.08)×10^-4 was obtained. Quantitative determination of DEHTPA was performed by capillary zone electrophoresis (CZE) with carbonate buffer (pH 11.0) as a background electrolyte, operating at −20 kV with direct UV detection at 210 nm. The migration time was 8.97 min and the detection limit was 5 ppm. Received: 2 September 1996/Revised: 19 November 1996/Accepted: 21 November 1996     

Inductively coupled plasma mass spectrometry for the sequential determination of trace metals in sea water after electrothermal vaporization of their dithiocarbamate complexes in methyl isobutyl ketone

 A sensitive method has been developed for the sequential determination of V, Mn, Fe, Co, Ni, Cu, Zn, Mo and Sb in sea water using inductively coupled plasma mass spectrometry (ICP-MS) after electrothermal vaporization of their dithiocarbamate complexes in methyl isobutyl ketone (MIBK). After complexion with sodium diethyldithiocarbamate (NaDDTC), all trace analyte elements were simultaneously separated from sea water matrix and concentrated 20 fold in a single extract of MIBK, followed by introduction of 10 μL of the extract into argon plasma using a pyrolytic graphite rod electrothermal vaporizer (ETV). Sensitivity enhancement due to chemical modification using a mixed modifier of Pd(NO₃)₂-Mg(NO₃)₂ was observed for all the elements. The limits of detection ranged from 2 ng/L for Co to 329 ng/L for V. For replicate determinations of the elements in sea water, the repeatability was within 10% (as a coefficient variation), except for V (12.8%). The recovery test performed on a sea water sample resulted in a range value from 87% for Sb to 119% for V. The method has been successfully applied to sea water samples collected from the surface to the depth of 5000 m at a sampling station in the northwest Pacific Ocean. Received: 1 July 1996/Revised: 24 September 1996/Accepted: 25 September 1996     

Inductively coupled plasma mass spectrometry for the sequential determination of trace metals in sea water after electrothermal vaporization of their dithiocarbamate complexes in methyl isobutyl ketone

 A sensitive method has been developed for the sequential determination of V, Mn, Fe, Co, Ni, Cu, Zn, Mo and Sb in sea water using inductively coupled plasma mass spectrometry (ICP-MS) after electrothermal vaporization of their dithiocarbamate complexes in methyl isobutyl ketone (MIBK). After complexion with sodium diethyldithiocarbamate (NaDDTC), all trace analyte elements were simultaneously separated from sea water matrix and concentrated 20 fold in a single extract of MIBK, followed by introduction of 10 μL of the extract into argon plasma using a pyrolytic graphite rod electrothermal vaporizer (ETV). Sensitivity enhancement due to chemical modification using a mixed modifier of Pd(NO₃)₂-Mg(NO₃)₂ was observed for all the elements. The limits of detection ranged from 2 ng/L for Co to 329 ng/L for V. For replicate determinations of the elements in sea water, the repeatability was within 10% (as a coefficient variation), except for V (12.8%). The recovery test performed on a sea water sample resulted in a range value from 87% for Sb to 119% for V. The method has been successfully applied to sea water samples collected from the surface to the depth of 5000 m at a sampling station in the northwest Pacific Ocean. Received: 1 July 1996/Revised: 24 September 1996/Accepted: 25 September 1996     

Cathodic stripping voltammetric determination of selenium(IV) at a thin-film mercury electrode in a thiocyanate-containing electrolyte

The well-known method for the determination of selenium(IV), which is based on the cathodic stripping voltammetry of copper(I) selenide, has been adapted for application at the thin-film mercury electrode on glassy carbon (TFME). Insufficient reproducibility and sensitivity have been overcome by using a 0.1 mol/L HClO₄ electrolyte solution containing 0.02 mol/L thiocyanate ions. Thiocyanate ions have been found to increase the peak height of the selenium response and shift it to more positive potentials. This behaviour is explained by an adsorption of SCN^– at the interface glassy carbon/Cu₂Se and its action as an electron transfer catalyst between glassy carbon and copper(I) selenide. A 3σ-detection limit of 75 ng/L Se^(IV) has been achieved. The relative standard deviation is 5.2% at 5 μg/L selenium(IV). The influence of cadmium(II), arsenic(III), zinc(II), iron(III) and lead(II) ions on the selenium response has been studied. In case of lead ions, a new signal occurred at more negative potentials than the reduction of Cu₂Se. This signal, which is probably due to the reduction of PbSe, can also be used for the determination of selenium(IV). Received: 13 November 1996 / Revised: 19 December 1996 / Accepted: 24 December 1996     

Graphite furnace atomic absorption spectrometric (GFAAS) determination of Cu, Cd, Cr, Mn, Ni and Pb in tellurium metal using precipitation and ion-exchange procedures

 Two independent procedures have been developed for the determination of Cd, Cu, Ni, Mn, Cr and Pb in high-purity tellurium by GFAAS. In the first, tellurium is precipitated as TeO₂ in the presence of EDTA at pH 4.5±0.2 to remove of 99.4% of tellurium as tellurium dioxide. The supernatant is analysed for impurities. In the second procedure, the trace impurities are preconcentrated on a Chelex-100 resin column at pH 9.4±0.2, and eluted by two column volumes of 2 mol/L HNO₃. In this case a matrix separation coefficient (k_s) of 1500 is achieved. The analyses are carried out by GFAAS. The relative standard deviations of the analyte concentrations in the samples are 4–8%. Separation of the matrix and recovery of the analytes are essentially complete as confirmed by standard addition of the analytes to the matrix. Received: 30 July 1996/Revised: 7 November 1996/Accepted: 13 November 1996     

Evaluation of potentiometric acid-base titrations by partial-least-squares calibration

Partial-least-squares calibration is used for the evaluation of acid-base titration curves. The method is advantageous because no physical model is assumed and only information from the titration curves is used. In the procedure, a model is developed from samples of known composition (calibration set) and is then used to predict unknowns (test set). The only assumption is that the two sets are similar. The method was evaluated first by titrating mixtures of two monoprotic acids with a small difference in pK_a values and then by titrating samples containing four or five diprotic acids. The results show that it is possible to determine monoprotic acids having a difference in pK_a of only 0.1, or up to five diprotic acids having pK_a values in the range 1.8–7.2, simultaneously.     

Interaction of photosurfactants, 2-[4-(4-alkylphenylazo)phenoxy] ethyltrimethyl-antmonium bromides with γ-cyclodextrin and thermodynamics of complexation of photosurfactants with cyclodextrins

 The stoichiometry, stability, and structure of the inclusion complex between the photosurfactants, 2-[4-(4-alkylphenylazo) phenoxy]ethyltrimethylammonium bromides (alkyl=ethyl, and butyl, where the abbreviations are EZ and BZ, and ZT for both, respectively), and γ-cyclodextrins (γ-CDx) in aqueous solution have been studied by the induced circular dichroism (ICD) and UV/VIS spectra as well as the potentiometric titration method. It is shown that the trans-ZT/γ-CDx systems form not only 1:1 (EZ) or 2:2 (BZ) but also 2:1 (ZT:γ-CDx) inclusion complexes. On the contrary, the steric hindrance of cis-ZT destabilizes the inclusion so that there is only 1:1 complex formation with γ-CDx. The thermodynamic discussion suggests that the stability of ZT complex is governed by the hydrophobic and the van der Waals interaction between the photosurfactants and cyclodextrins. However, the van der Waals interaction is more predominant in the trans-ZT/α-CDx system than in the β- and γ-CDx’s except for (BZ/γ-CDx)₂ system. Moreover, the alkylchain-length dependence of the free energy changes of complex formation of ZT and CDx are less than those of micellization of ZT because the cavity depth of CDx is limited to incorporate all hydrophobic parts of ZT. Finally, the iso-equibrium temperature is estimated by the compensation plot of thermo-dynamic parameters. Received: 29 August 1996 Accepted: 13 November 1996     

A study of HPLC separation and spectrophotometric and voltammetric detection of 4′-substituted derivatives of 3-carboxy-4-hydroxy-6-acetylaminoazobenzene

 Optimised conditions have been found for the separation of 3-carboxy-4-hydroxy-6-acetylaminoazo-benzene derivatives substituted in the position 4′ (4′-R-CHAAB, where R=–H, –CH₃, –OCH₃, –Cl, –COCH₃, –NO₂ and –NHCOCH₃) using reversed phase HPLC with a C18 chemically bonded stationary phase. Suitable mobile phases are mixtures of 0.01 mol/L NaH₂PO₄ at pH 4 with methanol (1+1), 0.01 mol/L NaH₂PO₄ at pH 2 with acetonitrile (1+1) or 1% aqueous acetic acid with methanol (4+6). UV photometry is the most universal detection technique and yields limits of detection around 10^-6 mol/L. Direct anodic voltammetry on a glassy carbon fibre array detector yields lower limits of detection for –COCH₃ derivatives and higher limits of detection for –NO₂ and –NHCOCH₃ derivatives. When the analytes are chemically reduced using zinc powder in acetic acid, the voltammetric detection has limits of detection one order of magnitude lower than those obtained UV photometrically. Received: 27 June 1996/Revised: 25 October 1996/Accepted: 3 November 1996     

Coulometric determination of the flow rate of small masses of substances II. Application to on-line measurement of the EOX-parameter

 Today’s microprocessor-controlled coulometric systems can generate complex functions obtained from the response of the microcoulometric cell. A previously developed procedure for determining the flow-rate (mass per unit time of detected species) has been accomplished by integrating, in the software, the full time dependence of the reaction of the coulometric cell and their electrodes, as well as of the titration curve. This leads to a time dependence of the measured signal, which has a fast response time without any delay. The titration curves are determined by means of various procedures. The applicability of the method is demonstrated for the case of the continuous analysis of the EOX-parameter (Extractable Organic Halogens X=Cl, Br, I) by coupling the coulometric detection with a FIA system. Results of continuous measurements of aqueous samples containing different chlorinated compounds as analytes are presented by taking into account different flow velocities in the FIA system as well as different measuring times. Recovery yields of the FIA system better than 70% have been obtained. Received: 7 November 1995/Revised: 15 May 1996/Accepted: 18 May 1996     

Determination of hemoglobin in plasma and serum by linear-sweep polarography

A new method for the determination of hemoglobin (Hb) is described. It is based on the amplification of the Hb enzyme-catalyzed reaction between O-phenyldiamine (OPD) and H₂O₂ and polarographic measurement of the enzyme-generated product 2,2′-diaminoazobenzene (DAA). A linear calibration plot has been obtained within the range of 1∼20 μg/ml (r = 0.987) Hb. The assay is performed on the microtitre plate to avoid the influence of serum protein and to obtain good reproducibility. The method is practical, reliable and convenient, and has been applied to determine Hb in plasma and serum. Received: 13 November 1995 / Accepted: 26 January 1996     

Micellization of triblock copoly(ethyleneoxide/tetrahydrofuran/ ethylene oxide)

 The association behaviour of triblock copoly(ethylene oxide/tetrahydrofuran/ethylene oxide), in particular E₁₀₀T₂₇E₁₀₀, in aqueous solutions has been investigated by means of static and dynamic light scattering, nuclear magnetic reso-nance (NMR) and surface tension techniques. On raising the polymer concentration at room temperature, the copolymer aggregates to form micelles with an aggregation number of about 105 (R _G, mic≈15 nm and R _H, mic≈13 nm, as revealed by light scattering and FT-PGSE NMR measurements, respectively). The micelles are kinetically quite stable, the micellar lifetime is shown to be more than 1 h. The residence time of a single unimer in a micelle is more than 140 ms. The apparent radius of gyration R _G, mic is fairly independent of concentration, but large effects are observed on varying the temperature. Raising the temperature initially results in an increase of the apparent micellar size, followed by a maximum at an intermediate temperature (≈45 °C). At higher temperatures a contraction of the micelles is observed. The shape of the micelles also appear to vary in this temperature interval. The interactions responsible for these phenomena are discussed in terms of, e.g., the temperature-dependent solubility of the alkylene oxide segments in water and polydispersity effects. Received: 29 January 1996 acccepted : 4 November 1996     

Sensitive determination of nucleotides and polynucleotides based on the fluorescence quenching of the Tb3+-Tiron complex

 A sensitive method using fluorescence quenching for the determination of nucleotides (ATP, ADP, AMP, CTP, UTP) and polynucleotides[poly(A), poly(I), poly(U)] is proposed. It is based on the ability of nucleotides and polynucleotides to inhibit the formation of a strongly fluorescent complex of Tb³⁺ ion with Tiron. The possibilities of spectrofluorimetric measurements of these systems were studied under optimal conditions (pH 6.9 in hexamethylene tetramine-HCl buffer, 1.2×10^-6 mol/L of Tb³⁺, 4.0×10^-6 mol/L of Tiron, λ_ex=317 nm, λ_em=546 nm). The results showed that the Tb³⁺-Tiron complex could be used as a fluorescence test for the phosphate moieties of nucleotides and polynucleotides. The detection limits are 0.3, 1.2, 3.7, 0.2, 0.3, 1.1, 0.6 and 0.9 ng/mL for ATP, ADP, AMP, CTP, UTP, poly(A), poly(I), and poly(U), respectively. The relative standard deviations (6 replicates) are within 4.0% in the middle of the linear range. The fluorescence quenching mechanism of these systems is also discussed. Received: 16 July 1996 / Revised: 13 November 1996 / Accepted: 13 November 1996     

Determination of Hf, Sc and Y in geological samples together with the rare-earth elements

 A method is described for the determination of Hf, Sc and Y simultaneously with the REE in geological materials. An earlier method for REE separation from major elements was studied with the aim to apply it also to the determination of Hf, Sc and Y. Sample decomposition was carried out by melting with LiBO₂. The method involves separation and concentration stages, using the cation-exchange resin DOWEX AG 50W-X8. Matrix elements were eluted with 2 mol/l HCl, whereas 6 mol/l HNO₃ with oxalic acid and 8 mol/l HNO₃ were used to elute the elements to be determined. Some of the matrix elements could not be completely removed. This effect as well as the recovery rates of the determined elements were investigated. The measurements were performed by ICP-AES. Spectral interferences were also tested. Received: 8 November 1995/Revised: 12 March 1996/Accepted: 14 March 1996     

Determination of trace inorganic selenium in organoselenium (selenosugar) oral nutrition liquids by graphite furnace atomic absorption spectrometry with hydride generation

 A method has been proposed for the determination of trace levels of inorganic selenium in organoselenium (selenosugar) oral nutrition liquids using hydride generation-graphite furnace atomic absorption spectrometry (HG-GFAAS), taking advantage of the fact that this organic selenium compound did not generate volatile hydride upon reduction. K₂S₂O₈ was selected for the decomposition of the compound in a boiling water bath. Selenium was found to give a sharp analytical signal upon reduction with NaBH₄ in 1.0 mol L^-1HCl medium. The characteristic mass giving an integrated absorbance of 0.0044 s was 21 pg. An absolute detection limit (3s) of 36 pg was obtained. The recovery was in the range of 94.2–102.1%. Less than parts per million levels of inorganic Se in the presence of organic selenium can be determined. Received: 7 November 1996/Revised: 13 January 1997/Accepted: 29 January 1997     

Determination of trace inorganic selenium in organoselenium (selenosugar) oral nutrition liquids by graphite furnace atomic absorption spectrometry with hydride generation

 A method has been proposed for the determination of trace levels of inorganic selenium in organoselenium (selenosugar) oral nutrition liquids using hydride generation-graphite furnace atomic absorption spectrometry (HG-GFAAS), taking advantage of the fact that this organic selenium compound did not generate volatile hydride upon reduction. K₂S₂O₈ was selected for the decomposition of the compound in a boiling water bath. Selenium was found to give a sharp analytical signal upon reduction with NaBH₄ in 1.0 mol L^-1HCl medium. The characteristic mass giving an integrated absorbance of 0.0044 s was 21 pg. An absolute detection limit (3s) of 36 pg was obtained. The recovery was in the range of 94.2–102.1%. Less than parts per million levels of inorganic Se in the presence of organic selenium can be determined. Received: 7 November 1996/Revised: 13 January 1997/Accepted: 29 January 1997     

Theoretical study of thermal decomposition of azomethane

Summary.  Thermal one- and two-bond dissociation processes of cis- and trans-azomethane were studied by ab initio computation with DZP and TZ2P basis sets, using the d(N–C) bond lengths as the reaction coordinates. The geometries were optimized at the MP2 level, and the dissociation energies obtained exploiting a single-point, fourth-order M?ller–Plesset calculations [MP4SDTQ/TZ2P]. At this level of theory including zero-point energies, the trans-isomer is by 9.3 kcal/mol more stable than the cis-isomer. The results show that the energetically more favourable one-bond cleavage proceeds without transition state with the predicted bond dissociation energy D ₀ of 47.8 kcal/mol for trans-azomethane and 38.5 kcal/mol for cis-azomethane. With calculated barrier heights the unimolecular dissociation rate constants have been determined by means of the RRKM theory. The second-order saddle points localized for synchronous decomposition pathways lie 13 (trans)-23(cis) kcal/mol above the one-bond dissociation energies [MP2/DZP]. Received May 28, 1996/Final version received November 1, 1996 / Accepted November 1, 1996     

Effect of Ar/H2 and Kr/H2 mixtures as discharge gas on the ion intensity in dc glow discharge mass spectrometry

 In dc glow discharge mass spectrometry, the addition of small amounts of H₂ to pure Ar as discharge gas has greatly increased the ion intensities of elements compared with the conventional method using pure Ar. This phenomenon was also observed for the addition of H₂ to pure Kr. The reason for the increase of the ion intensities of elements was studied by using a Kr gas mixture containing 0.2% (v/v) H₂. The ion intensities of the elements P, Se and As (whose first ionization potentials are higher than the energy levels of the excited state of Kr) did not increase even if the Kr/H₂ gas mixture was used. The results show that the addition of H₂ significantly contributed to increasing the number of metastable argon or krypton atoms (Penning ionization). Received: 4 November 1995/Revised: 5 January 1996/Accepted: 10 January 1996     

Effect of Ar/H2 and Kr/H2 mixtures as discharge gas on the ion intensity in dc glow discharge mass spectrometry

 In dc glow discharge mass spectrometry, the addition of small amounts of H₂ to pure Ar as discharge gas has greatly increased the ion intensities of elements compared with the conventional method using pure Ar. This phenomenon was also observed for the addition of H₂ to pure Kr. The reason for the increase of the ion intensities of elements was studied by using a Kr gas mixture containing 0.2% (v/v) H₂. The ion intensities of the elements P, Se and As (whose first ionization potentials are higher than the energy levels of the excited state of Kr) did not increase even if the Kr/H₂ gas mixture was used. The results show that the addition of H₂ significantly contributed to increasing the number of metastable argon or krypton atoms (Penning ionization). Received: 4 November 1995/Revised: 5 January 1996/Accepted: 10 January 1996