Determination of traces of uranium and thorium in (Ba, Sr) TiO3 ferroelectrics by inductively coupled plasma mass spectrometry

 Traces of uranium and thorium in barium(II), strontium(II) titanate ((Ba, Sr)TiO₃) ferroelectric materials were determined by inductively coupled plasma mass spectrometry (ICP-MS). Samples were completely dissolved by a mixture of 1.4% H₂O₂ and 1.0 mol⋅l^-1 HNO₃. For a complete separation of the analytes from the matrix elements, a two step separation technique involving leaching and anion-exchange was applied. By the leaching step with HNO₃ more than 90% of the matrix can be removed whereas the analytes completely remained in the solution. The anion-exchange step was carried out on a BIO⋅RAD AG1-X8 column with a mixture of 1.0 mol⋅l^-1 HF and 0.5 mol⋅l^-1 HNO₃ as eluent. The content of uranium and thorium was subsequently measured by ICP-MS. The detection limits (D.L.) obtained were 0.043 ng g^-1 and 0.035 ng g^-1 for U and Th, respectively. The reproducibility was satisfactory with a relative standard deviation of less than 3% (at the 1 ng g^-1 level, n=5). The matrix concentrations in the final solution were reduced to the sub-μg ml^-1 level which is in the range of the detection limits of USN-ICP-AES (ultrasonic nebulization-ICP-atomic emission spectroscopy). The method was successfully applied to the determination of uranium and thorium in three synthetic (Ba, Sr)TiO₃ samples spiked with the analytes at levels of 1, 5 and 10 ng g^-1 and three (Ba, Sr)TiO₃ ferroelectric samples containing sub-ng g^-1 levels of the analytes. Received: 26 February 1996/Revised: 28 May 1996/Accepted: 5 June 1996     

Determination of trace elements in AR grade alkali salts after preconcentration by column solid-phase extraction on TiO2 (anatase)

 Column solid-phase extraction using TiO₂ (anatase) as a solid sorbent was applied to preconcentrate traces of Cd, Co, Cu, Fe, Mn, Ni and Pb from AR grade alkali salts prior to their measurements by atomic absorption spectrometry (AAS). Multi-element preconcentration was achieved from NaCl, KCl, KNO₃, NaNO₃, CH₃COONa, NaHCO₃ and Na₂CO₃ solutions, whereas the sorption of trace elements from phosphates and sulfates is not quantitative. Optimal conditions (recoveries of the analytes >95%) for solid-phase co-extraction of the most common heavy metal ions are proposed. The conditions for quantitative and reproducible elution and subsequent AAS are established. A method of determination of trace elements in different salts is proposed. It is characterized by precision, reproducibility and a high preconcentration factor. The solid-phase extraction by TiO₂, combined with ETAAS allows the determination of 0.1 ng g^-1 Cd, 2 ng g^-1 Co, 1 ng g^-1 Cu and Ni, 0.5 ng g^-1 Mn and 0.4 ng g^-1 Pb. Received: 1 April 1996/Revised: 24 June 1996/Accepted: 9 July 1996     

Evaluation of on-line coupling size exclusion chromatography electrothermal atomic absorption spectrometry for selenium speciation

 Chromatographic effluents were on-line analyzed by Zeeman-ETAAS, using a flow-through cell placed in a graphite furnace autosampler as interface. To obtain high sampling rates, the use of fast graphite furnace programmes was studied. Conventional programmes of 96 s were reduced to 18 s by using hot injection (120 °C) and reducing the charring step to 2 s. The increase of the injection volume from 20 to 60 μl lengthened the programme to 46 s. Nickel had to be added to get a comparable response for both inorganic and organic selenium species (selenite and selenomethionine) and to reduce the interferent effect produced in presence of the chromatographic eluent (TRIS 0.01 mol l^-1, NH₄NO₃ 0.1 mol l^-1, pH 7). The optimized conditions were applied to the speciation of selenium in human erythrocyte lysates by size exclusion LC-ETAAS. Using a high performance size exclusion column selenium could be assigned to proteins of 100 and 35 kDa. Detection limits in the range of 1 ng (2 μg l^-1 for 500 μl injection volume) were obtained for the combined technique. Received: 9 October 1996/Revised: 8 July 1996/Accepted: 14 July 1996     

Comparison of Pyrolysis and Microwave Acid Digestion Techniques for the Determination of Mercury in Biological and Environmental Materials

 Microwave digestion reduction-aeration and pyrolysis combined with cold vapour atomic absorption and cold vapour atomic fluorescence are compared for the determination of total mercury in several biological and environmental matrices. The biological samples were digested in a mixture of HNO₃/H₂O₂, the environmental samples in a mixture of HNO₃/HClO₄. After reduction with SnCl₂, the mercury was collected by two-stage gold amalgamation. After microwave digestion reduction-aeration, detection limits of 1.4 ng g⁻¹ and 0.6 ng g⁻¹ were obtained for cold vapour atomic absorption spectrometry (CVAAS) and cold vapour atomic fluorescence spectrometry (CVAFS), respectively, for 250 mg of environmental samples. For biological samples (500 mg) the detection limits were 0.7 ng g⁻¹ (CVAAS) and 0.4 ng g⁻¹ (CVAFS). After pyrolysis, detection limits of 3.5 ng g⁻¹ and 1.6 ng g⁻¹ for CVAAS and CVAFS, respectively, were obtained for a 10 mg sample. Pyrolysis can only be applied when the organic content of the sample is not too high. Accurate results were obtained for 8 certified reference materials of both environmental and biological origin. In addition, a real sludge sample was analysed. Author for correspondence. E-mail: richard.dams@rug.ac.be Received September 18, 2002; accepted December 3, 2002 Published online May 5, 2003     

Buffer Standards for pH Measurement of N-(2-Hydroxyethyl)piperazine-N′-2-ethanesulfonic Acid (HEPES) for I=0.16 mol⋅kg−1 from 5 to 55 °C

The values of the second dissociation constant, pK ₂, of N-(2-hydroxyethyl) piperazine-N′-2-ethanesulfonic acid (HEPES) have been reported at twelve temperatures over the temperature range 5 to 55 °C, including 37 °C. This paper reports the results for the pa _H of eight isotonic saline buffer solutions with an I=0.16 mol⋅kg⁻¹ including compositions: (a) HEPES (0.01 mol⋅kg⁻¹) + NaHEPES (0.01 mol⋅kg⁻¹) + NaCl (0.15 mol⋅kg⁻¹); (b) HEPES (0.02 mol⋅kg⁻¹) + NaHEPES (0.02 mol⋅kg⁻¹) + NaCl (0.14 mol⋅kg⁻¹); (c) HEPES (0.03 mol⋅kg⁻¹) + NaHEPES (0.03 mol⋅kg⁻¹) + NaCl (0.13 mol⋅kg⁻¹); (d) HEPES (0.04 mol⋅kg⁻¹) + NaHEPES (0.04 mol⋅kg⁻¹) + NaCl (0.12 mol⋅kg⁻¹); (e) HEPES (0.05 mol⋅kg⁻¹) + NaHEPES (0.05 mol⋅kg⁻¹) + NaCl (0.11 mol⋅kg⁻¹); (f) HEPES (0.06 mol⋅kg⁻¹) + NaHEPES (0.06 mol⋅kg⁻¹) + NaCl (0.10 mol⋅kg⁻¹); (g) HEPES (0.07 mol⋅kg⁻¹) + NaHEPES (0.07 mol⋅kg⁻¹) + NaCl (0.09 mol⋅kg⁻¹); and (h) HEPES (0.08 mol⋅kg⁻¹) + NaHEPES (0.08 mol⋅kg⁻¹) + NaCl (0.08 mol⋅kg⁻¹). Conventional pa _H values, for all eight buffer solutions from 5 to 55 °C, have been calculated. The operational pH values with liquid junction corrections, at 25 and 37 °C have been determined based on the NBS/NIST standard between the physiological phosphate standard and four buffer solutions. These are recommended as pH standards for physiological fluids in the range of pH = 7.3 to 7.5 at I=0.16 mol⋅kg⁻¹.     

Application of chitosan functionalized with 3,4-dihydroxy benzoic acid moiety for on-line preconcentration and determination of trace elements in water samples

Chitosan resin functionalized with 3,4-dihydroxy benzoic acid (CCTS-DHBA resin) was used as a packing material for flow injection (FI) on-line mini-column preconcentration in combination with inductively coupled plasma-atomic emission spectrometry (ICP-AES) for the determination of trace elements such as silver, bismuth, copper, gallium, indium, molybdenum, nickel, uranium, and vanadium in environmental waters. A 5-mL aliquot of sample (pH 5.5) was introduced to the minicolumn for the adsorption/preconcentration of the metal ions, and the collected analytes on the mini-column were eluted with 2 M HNO₃, and the eluates was subsequently transported via direct injection to the nebulizer of ICP-AES for quantification. The parameters affecting on the sensitivity, such as sample pH, sample flow rate, eluent concentration, and eluent flow rate, were carefully examined. Alkali and alkaline earth metal ions commonly existing in river water and seawater did not affect the analysis of metals. Under the optimum conditions, the method allowed the determination of metal ions with detection limits of 0.08 ng mL⁻¹ (Ag), 0.9 ng mL⁻¹ (Bi), 0.07 ng mL⁻¹ (Cu), 0.9 ng mL⁻¹ (Ga), 0.9 ng mL⁻¹ (In), 0.08 ng mL⁻¹ (Mo), 0.09 ng mL⁻¹ (Ni), 0.9 ng mL⁻¹ (U), and 0.08 ng mL⁻¹ (V). By using 5 mL of sample solution, the enrichment factor and collection efficiency were 8–12 fold and 96–102%, respectively, whereas the sample throughput was 7 samples/hour. The method was validated by determining metal ions in certified reference material of river water (SLRS-4) and nearshore seawater (CASS-4), and its applicability was further demonstrated to river water and seawater samples.     

Polarographic Catalytic Wave of Prednisone in the Presence of Persulfate and its Application

 A polarographic catalytic wave of prednisone in the presence of K₂S₂O₈ was observed. The polarographic catalytic wave of prednisone as catalyst was attributed to such chemical reaction parallel to electrode reaction as oxidized the free radical from one electron reduction of the Δ^1,4-3 keto group of prednisone to regenerate the original keto group. The catalytic wave can be used for the determination of prednisone with the help of conventional polarographic equipment, such as linear-potential scan polarograph. In 0.12 mol l⁻¹ HAc-0.08 mol l⁻¹ NaAc-0.014 mol l⁻¹ K₂S₂O₈ (pH 4.6) supporting electrolyte, the second-order derivative peak current of the catalytic wave was rectilinear to prednisone concentration in the range of 3.2 × 10⁻⁷∼1.6 × 10⁻⁵ mol l⁻¹. The detection limit was 8.0 × 10⁻⁸ mol l⁻¹. Received August 6, 2001; accepted April 17, 2002; published online July 22, 2002     

Solid-phase spectrophotometric microdetermination of iron with ascorbic acid and ferrozine

 A very sensitive and selective method for the determination of trace amounts of iron has been developed, based on the reduction of Fe(III) to Fe(II) by ascorbic acid, followed by chromogenic chelation of Fe(II) with ferrozine. The complex Fe(II)-ferrozine is easily sorbed on a dextran-type anion-exchange gel packed in a 1 mm cell, and the absorbance of the gel is measured directly at 569 and 800 nm. The extended linear range of the determination is 0.5–10 ng ml^-1 of iron (apparent molar absorptivity=4.4×10⁷ l mol^-1 cm^-1) and the precision (RSD) 1.3% for a concentration of 5 ng ml^-1 of iron (n=10). The detection limit for a sample volume of 1000 ml, using 0.040 g of anion-exchanger, corresponds to 0.12 ng ml^-1. The method has been successfully applied to the determination of iron in natural and waste waters, wine, soil extract and previously digested vegetal tissues, drugs and human hair. Received: 20 November 1995/Revised: 23 January 1996/Accepted: 26 January 1996     

Ammonium pyrrolidinedithiocarbamate-modified activated carbon micro-column extraction for the determination of As(III) in water by graphite furnace atomic absorption spectrometry

A simple and selective method using ammonium pyrrolidinedithiocarbamate modified activated carbon (APDC-AC) as solid phase extractant has been developed for speciation of As(III) in water samples. At pH 1.8–3.0, As(III) could be adsorbed quantitatively by APDC-AC, and then eluted completely with 2.0 mL of 0.1 mol L⁻¹ HNO₃, while As(V) could almost not be retained at pH 1–7. Effects of acidity, sample flow rate, concentration of elution solution and interfering ions on the recovery of As(III) have been systematically investigated. Under the optimal conditions, the adsorption capacity of APDC-AC for As(III) is 7.3 mg g⁻¹. The detection limit (3σ) of As(III) is 0.05 ng mL⁻¹ for graphite furnace atomic absorption spectrometry (GFAAS) with enrichment factor of 50, and the relative standard deviation (RSD) is 4.1% (n = 9, C = 5 ng mL⁻¹). The method has been applied to the determination of trace As(III) in water, and the recoveries of As(III) are 100 ± 10%. Correspondence: Yiwei Wu, Department of Chemistry and Environmental Engineering, Hubei Normal University, Huangshi 435002, P.R. China     

Improvement of an old spectrophotometric method for the microdetermination of ascorbic acid by the use of a micellar medium

₃]²⁺. Whereas the initial method is carried out in an aqueous solution, the improved one employs an aqueous micellar medium formed by the cationic surfactant cetylpyridinium bromide (CPBR). The λmax in both methods is the same, i.e. 510 nm. The mean apparent molar absorptivity (ɛ) and Sandell’s sensitivity (Ss) were calculated as ɛ=2.10×10⁴ Lmol^-1 cm^-1 and Ss=8.37 ng cm^-2 for the initial procedure, and ɛ=2.62×10⁴ Lmol^-1 cm^-1 and Ss=6.72 ng cm^-2 for the improved one. The regression line equation for the improved method was: A=1.487×10^-1C −1.415×10^-2 (r=0.9998). The accuracy and the precision of the improved method were investigated and the conclusions were satisfactory. The results obtained for ASCA by both the described method and an official one, were statistically compared by means of the Student’s t-test and by the variance ratio F-test; and no significant difference was observed. Received: 15 January 1996/Revised: 7 March 1996/Accepted: 13 March 1996     

Determination of uranium and thorium in complex samples using chromatographic separation, ICP-MS and spectrophotometric detection

The paper describes a research of possible application of UTEVA and TRU resins and anion exchanger AMBERLITE CG-400 in nitrate form for the isolation of uranium and thorium from natural samples. The results of determination of distribution coefficient have shown that uranium and thorium bind on TRU and UTEVA resins from the solutions of nitric and hydrochloric acids, and binding strength increases proportionally to increase the concentration of acids. Uranium and thorium bind rather strongly to TRU resin from the nitric acid in concentration ranging from 0.5 to 5 mol L⁻¹, while large quantities of other ions present in the sample do not influence on the binding strength. Due to the difference in binding strength in HCl and HNO₃ respectively, uranium and thorium can be easily separated from each other on the columns filled with TRU resin. Furthermore, thorium binds to anion exchanger in nitrate form from alcohol solutions of nitric acid very strongly, while uranium does not, so they can be easily separated. Based on these results, we have created the procedures of preconcentration and separation of uranium and thorium from the soil, drinking water and seawater samples by using TRU and UTEVA resins and strong base anion exchangers in nitrate form. In one of the procedures, uranium and thorium bind directly from the samples of drinking water and seawater on the column filled with TRU resin from 0.5 mol L⁻¹ HNO₃ in a water sample. After binding, thorium is separated from uranium with 0.5 mol L⁻¹ HCl, and uranium is eluted with deionised water. By applying the described procedure, it is possible to achieve the concentration factor of over 1000 for the column filled with 1 g of resin and splashed with 2 L of the sample. Spectrophotometric determination with Arsenazo III, with this concentration factor results in detection limits below 1 μg L⁻¹ for uranium and thorium. In the second procedure, uranium and thorium are isolated from the soil samples with TRU resin, while they are separated from each other on the column filled with anion exchanger in alcohol solutions. Anion exchanger combined with alcohol solutions enables isolation of thorium from soil samples and its separation from a wide range of elements, as well as spectrophotometric determination, ICP-MS determination, and other determination techniques.     

13C- and 1H-NMR and FTIR spectroscopic evidence for aggregate formation of organosilanes in toluene

 Changes in the ¹³C and ¹H NMR chemical shifts of the silane coupling agent (3-aminopropyltri-ethoxysilane, APTS) in toluene, which were detected as the concentration of APTS increased, have been interpreted in terms of the formation of micelles and the presence of a critical micelle concentration (CMC) equal to ca. 0.47 mol 1^-1. For the protons of the n-propyl segment, ¹H NMR splitting patterns have been analyzed and conformations of the propyl segment have been discussed. Plots of relative absorbance of the two NH₂ stretch IR bands at 3324 and 3384 cm^-1 against concentration provided an inflection point (corresponding to the CMC) at a value equal to 0.46 mol 1^-1. Thus, consideration of the interaction of APTS with a surface must take into account the presence of both APTS aggregates as well as APTS monomers. Received: 16 August 1996 Accepted: 26 September 1996     

Microwave assisted volatilization of silicon as fluoride for the trace impurity determination in silicon nitride by dynamic reaction cell inductively coupled plasma-mass spectrometry

A low pressure microwave assisted vapor phase dissolution procedure for silicon nitride and volatilization of in situ generated SiF₄ has been developed using H₂SO₄, HF and HNO₃ for the determination of trace impurities present in silicon nitride. Sample was taken in minimum amount (0.5 mL for 100 mg) of H₂SO₄ and treated with vapors generated from HF and HNO₃ mixture in presence of microwaves in a closed container. An 80 psi pressure with ramp and hold times of 30 min and 60 min respectively, operated twice, resulted in 99.9% volatilization of Si. Matrix free solutions were analyzed for impurities using DRC-ICP-MS. The recoveries of Cr, Mn, Fe, Ni, Co, Cu, Zn, Sr, Y, Cd, Ba and Pb were between 80 and 100% after volatilization of Si. The blanks were in lower ng g⁻¹ with method detection limits in lower ng g⁻¹ to sub ng g⁻¹ range. The method was applied for the analysis of two silicon nitride samples.     

Simultaneous extraction and preconcentration of uranium and thorium in aqueous samples by new modified mesoporous silica prior to inductively coupled plasma optical emission spectrometry determination

A new synthesized modified mesoporous silica (MCM-41) using 5-nitro-2-furaldehyde (fural) was applied as an effective sorbent for the solid phase extraction of uranium(VI) and thorium(IV) ions from aqueous solution for the measurement by inductively coupled plasma optical emission spectrometry (ICP OES). The influences of some analytical parameters on the quantitative recoveries of the analyte ions were investigated in batch method. Under optimal conditions, the analyte ions were sorbed by the sorbent at pH 5.5 and then eluted with 1.0 mL of 1.0 mol L⁻¹ HNO₃. The preconcentration factor was 100 for a 100 mL sample volume. The limits of detection (LOD) obtained for uranium(VI) and thorium(IV) were 0.3 μg L⁻¹. The maximum sorption capacity of the modified MCM-41 was found to be 47 and 49 mg g⁻¹ for uranium(VI) and thorium(IV), respectively. The sorbent exhibited good stability, reusability, high adsorption capacity and fast rate of equilibrium for sorption/desorption of uranium and thorium ions. The applicability of the synthesized sorbent was examined using CRM and real water samples.     

A Spectrophotometric Study on Uranyl Nitrate Complexation to 150 °C

The formation constant of the mononitratouranyl complex was studied spectrophotometrically at temperatures of 25, 40, 55, 70, 100 and 150 °C (298, 313, 328, 343, 373 and 423 K). The uranyl ion concentration was fixed at approximately 0.008 mol⋅kg⁻¹ and the ligand concentration was varied from 0.05 to 3.14 mol⋅kg⁻¹. The uranyl nitrate complex, UO₂NO₃⁺, is weak at 298 K but its equilibrium constant (at zero ionic strength) increases with temperature from log ₁₀ β ₁=−0.19±0.02 (298 K) to 0.78±0.04 (423 K).     

Cathodic Adsorptive Stripping Voltammetry for Estimation of the Forest Area Pollution with Nickel and Cobalt

 Necessary conditions were established for simultaneous nickel and cobalt determination in environmental samples, such as oak wood and soil, based on cathodic adsorptive stripping voltammetry. Ni(II) and Co(II), complexed with dimethylglyoxime, were determined using a hanging mercury drop electrode. Optimum conditions were found to be: accumulation time 90 s, accumulation potential −0.80 V vs. SCE, supporting electrolyte 0.2 mol dm⁻³ ammonia-ammonium chloride buffer (pH = 9.4) + 0.05 mol dm⁻³ NaNO₂ and dimethylglyoxime 2 × 10⁻⁴ mol dm⁻³. A linear current-concentration relationship was observed up to 7.51×10 ⁻⁷ mol dm⁻³ for Ni(II) and 7.0 × 10⁻⁷ mol dm⁻³ for Co(II). Excess amounts of zinc(II) interfering with cobalt peaks were masked by complexation with EDTA. Wood and soils were mineralized by applying a microwave digestion system, using the mixtures H₂O₂ + HNO₃ or HNO₃ + HF, respectively. The developed procedure was tested by analysing international reference materials (BCR 62 Olive Leaves and GBW 08302 Tibet Soil). The developed procedure was used to determine pollution of oak stand with nickel and cobalt in different regions of Poland. Received August 10, 2000. Revision May 22, 2001.     

Buffer Standards for the Physiological pH of the Zwitterionic Compound, TAPS, From 5 to 55 ∘C

The values of the second dissociation constant, pK ₂, and related thermodynamic quantities of N-[tris(hydroxymethyl)methyl-3-amino]propanesulfonic acid (TAPS) have already been reported at 12 temperatures over the temperature range 5–55 ^∘C, including 37 ^∘C. This paper reports the results for the pH of five equimolal buffer solutions with compositions: (a) TAPS (0.03 mol⋅kg⁻¹) + NaTAPS (0.03 mol⋅kg⁻¹); (b) TAPS (0.04 mol⋅ kg⁻¹) + NaTAPS (0.04 mol⋅kg⁻¹); (c) TAPS (0.05 mol⋅kg⁻¹) + NaTAPS (0.05 mol⋅kg⁻¹); (d) TAPS (0.06 mol⋅kg⁻¹) + NaTAPS (0.06 mol⋅kg⁻¹); and (d) TAPS (0.08 mol⋅kg⁻¹) + NaTAPS (0.08 mol⋅kg⁻¹). The remaining eight buffer solutions consist of saline media of the ionic strength I = 0.16 mol⋅kg⁻¹, matching closely to that of the physiological sample. The compositions are: (f) TAPS (0.04 mol-kg⁻¹) + NaTAPS (0.02 mol-kg⁻¹) + NaCl (0.14 mol⋅kg⁻¹); (g) TAPS (0.05 mol⋅kg⁻¹) + NaTAPS (0.04 mol⋅kg⁻¹) + NaCl (0.12 mol⋅kg⁻¹); (h) TAPS (0.6 mol⋅kg⁻¹) + NaTAPS (0.04 mol⋅kg⁻¹) + NaCl (0.12 mol⋅kg⁻¹); (i) TAPS (0.08 mol⋅kg⁻¹) + NaTAPS (0.06 mol⋅kg⁻¹) + NaCl (0.10 mol⋅kg⁻¹); (j) TAPS (0.04 mol⋅ kg⁻¹) + NaTAPS (0.04 mol⋅kg⁻¹) + NaCl (0.12 mol⋅kg⁻¹); (k) TAPS (0.05 mol⋅kg⁻¹) + NaTAPS (0.05 mol⋅kg⁻¹) + NaCl (0.11 mol⋅kg⁻¹); (l) TAPS (0.06 mol⋅kg⁻¹) + NaTAPS (0.06 mol⋅kg⁻¹) + NaCl (0.10 mol⋅kg⁻¹); and (m) TAPS (0.08 mol⋅kg⁻¹) + NaTAPS (0.08 mol⋅kg⁻¹) + NaCl (0.08 mol⋅kg⁻¹). These buffers are recommended as a pH standard for clinical measurements in the range of physiological application. Conventional pH values, designated as pH(s), for all 13 buffer solutions from 5 to 55 ^∘C have been calculated. The operational pH values with liquid junction corrections, at 25 and 37 ^∘C for buffer solutions, designated above as (b), (c), (d), (e), (j), (l), and (m); have been determined based on the difference in the values of the liquid junction potentials between the accepted phosphate standard and the buffer solutions under investigation.     

Modeling of Protonation Constants of Linear Aliphatic Dicarboxylates Containing -S-Groups in Aqueous Chloride Salt Solutions,at Different Ionic Strengths,Using the SIT and Pitzer Equations and Empirical Relationships

The protonation constants of ethylenedithiodiacetic, dithiodipropionic and dithiodibutyric acids were obtained from potentiometric measurements in NaCl(aq) (I≤5 mol⋅L⁻¹) and (CH₃)₄NCl(aq) (I≤3 mol⋅L⁻¹) at t=25 °C. Their dependences on ionic strength were modeled by the SIT and Pitzer approaches. The activity coefficients of the neutral species were obtained by solubility measurements. The literature values of the protonation constants of (HOOC)-(CH₂) _n -S-(CH₂) _n -(COOH) (n=1 to 3) and (HOOC)-(CH₂)-S-(CH₂) _n -S-(CH₂)-(COOH) (n=0 to 5) in NaCl(aq) and KCl(aq) (I≤3 mol⋅L⁻¹) at 18 °C were also analyzed using the above approaches. Both the log ₁₀ K _i ^H and interaction parameter values follow simple linear trends as a function of certain structural characteristics of the ligands. Examples of modeling these trends are reported. Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users.     

Thermodynamic Protonation Parameters of some Sulfur-Containing Anions in NaCl<Subscript>aq</Subscript> and (CH<Subscript>3</Subscript>)<Subscript>4</Subscript>NCl<Subscript>aq</Subscript> at <Emphasis Type="Italic">t</Emphasis>=25 °C

Protonation constants of one thiocarboxylate (thioacetate) and four sulfur-containing carboxylates (2-methylthioacetate, thiolactate, thiomalate, 3-mercaptopropionate) were determined by potentiometric measurements in a wide ionic strength range [0≤I≤5 mol⋅L⁻¹ in NaCl and 0 ≤I≤3 mol⋅L⁻¹ in (CH₃)₄NCl] at t=25 °C. For two of these ligands (2-methylthioacetate and thiolactate), the protonation enthalpies were also determined by calorimetric measurements in NaCl ionic medium [0 ≤I≤5 mol⋅L⁻¹] at t=25 °C. Individual UV spectra of the protonated and unprotonated 3-mercaptopropionate species, together with values of the protonation constants, were obtained by spectrophotometric titrations. Results were analyzed in terms of their dependence on the ionic medium by using different thermodynamic models [Debye-Hückel type, SIT (Specific ion Interaction Theory) and Pitzer’s equations]. Differences among protonation constants obtained in different media were also interpreted in terms of weak complex formation.     

Comparison Between Two Sample Emulsification Procedures for the Determination of Mo, Cr, V and Ti in Diesel and Fuel Oil by ICP OES Along with Factorial Design

Inductively coupled plasma optical emission spectrometry (ICP OES) was used to determine Mo, Cr, V and Ti, in diesel and in used fuel oil. Samples were introduced into the ICP as emulsions to reduce interferences and allow the use of inorganic standards for quantification. A comparative study between one Triton X-100 emulsion and one detergentless emulsion was made. A 2³ factorial design was applied to elucidate and establish the relationship between three experimental variables: presence of HNO₃, amount of diesel fuel oil (between 5 and 25%) and the presence or O₂ into the Ar plasma gas flow rate. Results indicated that best performance were achieved using 10% sample (w/w) together with concentrated HNO₃ (0.5 mL) and using O₂ as auxiliary gas (0.047 L min⁻¹). The use of O₂ minimized both carbon deposits at the injector tip and plasma background. The addition of HNO₃ resulted in good correlation between inorganic standards used for calibration, and metallo-organic standards used for sample enrichment. Analyte enriched diesel and SRM 1634b were analyzed using the optimized conditions. Recoveries from 90.1 to 106.5% were achieved, with better results for detergent emulsions which enabled limits of detection at the ng g⁻¹ range for Mo, Cr, V and Ti and at smaller background.