Determination of trace concentrations of citrate in aqueous systems

A fluorimetric method for the determination of citrate is described. Through a series of reactions, citrate is converted into citrazinic acid. The ammonium salt of this compound exhibits a fluorescence maximum at 430 nm when an excitation source of 340 nm is used. The method does not suffer from the normal interferences. The limit of detection is 0.01 μg ml^-, and the best working range is 0.1–10 μg ml^-1.     

Differential drop calorimetry for the determination of enthalpy of fusion

Drop calorimetry was modified in order to adapt it to the measurement of the enthalpy of fusion of materials with above-ambient fusion temperatures, T_F, such as salt hydrates suitable as phase change materials. In the modified form, differential drop calorimetry, the energy delivered to the molten sample at temperatures above T_F and by the crystalline sample at temperatures below T_F are extrapolated to T_F to give the enthalpy of freezing. The calorimetric system was calibrated electrically and with acetamide (T_F=353.3 K) and applied to the congruently melting magnesium nitrate hexahydrate, cadmium nitrate tertrahydrate, ammonium alum, and several hydrated salt mixtures.     

The quantitative separation and determination of tantalum and niobium using anion exchange chromatographY

A method is described for separating .and determining niobium and tantalum in mixtures of the two. A solution of the two elements in 3M hydrochloric.0.1M hydrofluoric acid is put on a column of Deacidite FF, the niobium is rapidly eluted with 3M hydrochloric.0.1M hydrofluoric acid and the tantalum is recovered by elution with 4M ammonium chloride-M ammonium fluoride. A complete separation is obtained and the two elements are recovered as their oxides after precipitation. The effecth of some other elements have been examined.     

Studies on Am(III) separation from simulated high-level waste using cobalt bis(dicarbollide) (1? ) ion derivative covalently bound to N,N′-di-n-octyl diglycol diamide as extractant and DTPA as stripping agent

The separation of minor actinides from high level liquid waste (HLLW) belongs to the principal challenges in current nuclear treatment. A derivative based on two cobalt bis(dicarbollide) (1^?) ions covalently bound to the N,N??-di-n-octyl diglycolyl amide platform via diethyleneglycol chain with the formula {[(N,N??-(8-(OCH₂ CH₂)₂-1,2-C₂B₉H₁₀)(1??,2??-C₂B₉H₁₁)-3,3??-Co)(N,N??-n-C₈H₁₇)NCOCH₂]₂O}Na₂ (TODGA-COSAN), dissolved in low polar mixture of hexyl methyl ketone and n-dodecane, was used as an extractant for efficient Am(III)/Eu(III) separation from PUREX HLLW. Am(III) could be selectively stripped from loaded organic phase by using a stripping agent composed from 0.05?M DTPA and 1?M citric acid as a buffer and 1?M NaNO₃ at pH?3.0. Separation factor between europium and americium of 13 was achieved. The europium remaining in the organic phase could be consecutively effectively stripped by using solution of ammonium citrate or ammonium citrate with ammonium DTPA at pH~7.     

Anion exchange studies of zirconium(iv) in citrate solution separation from mixtures

The anion exchange behaviour of zirconium(IV) in a citrate system is described. Nitric acid, hydrochloric acid, perchloric acid and ammonium chloride were tested as eluants on Dowex ₂₁K column. Zirconium is separated from fission product elements e.g., barium, strontium, cadmium, caesium, molybdate and also from lead.     

Solid solutions of Ni and Co molybdates in silica-dispersed and bulk catalysts prepared by sol-gel and citrate methods

Silica-dispersed catalysts based on cobalt, nickel and mixed Ni-Co molybdates of various compositions are prepared by a sol-gel procedure from silicon alkoxides, metal nitrates and ammonium heptamolybdate. For comparative purposes, the corresponding bulk molybdates are obtained by the citrate route. The polymorphism and the textural properties of these catalysts are investigated by means of X-ray diffraction, UV-visible diffuse reflectance spectroscopy and Raman spectroscopy. Their surface and textural characteristics are investigated by X-ray photoelectron spectroscopy and nitrogen or krypton physisorption. In the sol-gel prepared catalysts, the β-phase of Ni_1−xCo_xMoO₄, which is the most active phase in the oxidative dehydrogenation of light alkanes, is stabilized over the whole composition range. In the analogous bulk catalysts prepared by the citrate method, this phase is stabilized only at high Co content (x?0.55). Dispersion of the active phase in silica is shown to improve the propene productivity in comparison with bulk catalysts obtained by citrate or coprecipitation methods.     

Improved detection limit for ammonium/ammonia achieved by Berthelot's reaction by use of solid-phase extraction coupled to diffuse reflectance spectroscopy

The proposed procedure is based on the extraction of the indothylmol blue into C₁₈ solid-phase extraction (SPE) membranes and direct quantification on the membrane surface by diffuse reflectance spectroscopy. The analytical performance of the proposed method has been evaluated for standard solutions of ammonium using reflectance values, R, as well as the Kubelka-Munk function, F(R). The results have been compared with those obtained by the conventional method, which uses UV-vis absorption spectroscopy with a sensor-based method. The described methodology provided satisfactory linearity and reproducibility within the ammonium concentration intervals 25-250 μg L⁻¹ and 25-500 μg L⁻¹ when using R and F(R), respectively. The limit of detection was around 10 μg L⁻¹, which is markedly lower than that of the classical procedure and than those provided by Nessler and OPA/thiol fluorimetric methods. For air samples the linear interval expressed as μg of ammonia is 0.24-2.4 or 0.24-4.7 employing R or F(R), respectively. The effect of potential interferences such as metals and aliphatic amines has also been evaluated. Finally, the proposed methodology has been adapted to the determination of ammonia in air and water samples. The method can be also used as a detector support for visual estimation.     

Determination of chromium(VI) in dialysis fluids by alternating current and differential pulse voltammetry

Alternating current (ACV) and differential pulse voltammetry (DPV) are employed for the determination of Cr(VI) in dialysis fluids, using 0.1 mol/1 dibasic ammonium citrate as supporting electrolyte (pH 5.9). A three-electrode cell was used: The working electrode was a long-lasting sessile-drop mercury electrode (LLSDME) with a drop time of 240 to 300 s. Precision, expressed as relative standard deviation (s _r%), and accuracy, expressed as relative recovery (R%) are also reported.     

Photometric determination of quaternary ammonium compounds with hexanitrodiphenylamine. II

In strongly alkaline medium (more than 0.1 N NaOH) hexanitrodiphenylamine anion reacts with base. The reaction is rather slow and is partly reversible.Quaternary ammonium ions can be determined by extraction of their hexanitrodiphenylamine complexes with chloroform or methylene chloride at pH 11 or with methylene chloride from 3N NaOH. Only the latter method is applicable to compounds containing several quaternary ammonium groups. The compounds extracted have high molar extinction and good stability. Quaternary ammonium ions can be determined in the presence of amines, and in some cases determination of quaternary ammonium ions in admixture is possible.     

Synthesis and photoluminescence properties of π-extended fluorene derivatives: the first example of a fluorescent solvatochromic nitro-group-containing dye with a high fluorescence quantum yield

Functional π-extended fluorene derivatives, 2,7-di(p-substituted-phenyl)fluorenes containing different functional groups such as hydrogen, trimethylsilyl (TMS), methoxycarbonyl, cyano, and nitro groups, were synthesized. Except for the nitro group, the resulting compounds exhibited extremely high fluorescence quantum yields (Φ_F >0.85 in chloroform). The diphenylfluorene containing nitro groups have higher fluorescence quantum yield (Φ_F = 0.31 in N,N’-dimethyl-formamide) than other nitro-group-containing fluorophores which were previously reported (Φ_F <0.1). Furthermore, this compound exhibited large Stokes’ shift with green to orange emission and unique on-off behavior of the emission by solvents.     

Mechanochemical synthesis,structure and properties of lead containing alkaline earth metal fluoride solid solutions MxPb1-xF2 (M = Ca,Sr, Ba)

The paper deals with the mechanochemical synthesis of lead containing alkaline earth metal fluoride solid solutions M_xPb_1-xF₂ (M = Ca, Sr, Ba) by high-energy ball milling. Several metal precursors and fluorinating agents were tested for synthesizing M_0.5Pb_0.5F₂. Metal acetates and ammonium fluoride as precursors show the most promising results and were therefore used for the formation of M_xPb_1-xF₂ with different metal cationic ratios. The characterization of the local fluorine coordination and the crystal structure was performed by ¹⁹F MAS NMR spectroscopy and X-ray diffraction. Additional calculations of ¹⁹F chemical shifts using the superposition model allow a deeper insight into the local structure of the compounds. The fluoride ion conductivity was followed by temperature dependent DC conductivity measurements. Significantly higher conductivities were found in comparison with those of the corresponding binary fluorides. The highest values were observed for samples with high lead content M_0.25Pb_0.75F₂, bearing in mind the much higher conductivity of PbF₂ compared to MF₂.     

The enthalpy of precipitation of univalent cations with tetraphenylborate by titration calorimetry

The enthalpy of precipitation of six common univalent cations with the tetraphenylborate anion has been measured at 25.0°C. Reaction entropies range from −8.24kcal/mole for ammonium to −19.7 kcal/mole for silver(I). The entropy of reaction was computed from the above measurements and the corresponding Gibbs free energy changes. A linear relationship exists between ΔF°′ and ΔH°′. The entropy of precipitation of ammonium ions was +5.0 e.u., an unusually large positive value.     

Copper salicylaldoxime and its use in the heterometric microdeterminatton of copper in the presence of foreign metals

1. The initial precipitation of copper-salicylaldoxime occurs at pH~1.5.Between pH 1.5 and 3.0, the precipitation is incomplete. A complete precipitation occurs in the pH-region of 3–10. Between pH 10.1 and 10.7 the precipitate redissolves At higher pH's a clear solution is obtained. 2. The solubility of copper salicylaldoximate is about ten times greater in 0.01M sulfuric acid (~0.5·10^-4 mol) than in 0.1M acetic acid. 3. In 10 ml of 0.1M acetic acid solution in the presence of ~99% foreign bivalent metals, one mg copper could be determined heterometrically with an error of 1–2%. 4. In the presence of ~95% alumin um or chromium, either citrate or tartrate (0.2M) were used as masking agents. In the presence of iron (~ 95%), clear end-points were obtained only with citrate. One mg of copper could be determined with an error of 0–3%. 5. Whichever pH is used, copper is not precipitated in the presence of ethylenediaminetetraacetate.     

Facile one-pot preparation of gold nanoparticles in the presence of fluoroalkyl end-capped oligomers,fluoroalkyl end-capped oligomers/silica nanocomposites,and fluoroalkyl end-capped oligomers/polyaniline nanocomposites

A variety of fluoroalkyl end-capped oligomers, such as fluoroalkyl end-capped acrylic acid oligomer [R_F-(ACA) _n -R_F], acryloylmorpholine oligomer [R_F-(ACMO) _n -R_F], 2-acrylamido-2-methylpropanesulfonic acid oligomer [R_F-(AMPS) _n -R_F], 2-(methacryloyloxy)ethanesulfonic acid oligomer [R_F-(MES) _n -R_F], and N,N-dimethylacrylamide oligomer [R_F-(DMAA) _n -R_F], were applied to the autoreduction of gold ions to give the corresponding oligomers/gold nanocomposites, of whose sharp plasmon absorption bands are observed around 535 nm. In these fluorinated oligomers, R_F-(ACA) _n -R_F oligomer and R_F-(ACMO) _n -R_F were effective for the one-pot preparation of the gold nanoparticles under very mild conditions; although the other fluorinated oligomers and the corresponding non-fluorinated–(ACMO) _n -oligomer were unable to afford the gold nanoparticles. R_F-(ACA) _n -R_F/SiO₂ nanocomposites and R_F-(ACMO) _n -R_F/SiO₂ nanocomposites, which were prepared by the sol–gel reactions of tetraethoxysilane in the presence of silica nanoparticles and the corresponding oligomers under alkaline conditions, were also applied to the encapsulation of gold nanoparticles into these fluorinated nanocomposite cores through the autoreduction of gold ions at room temperature. Interestingly, these fluorinated oligomers/silica nanocomposite-encapsulated gold nanocomposites before and after calcination at 800 °C were found to exhibit the same plasmon absorption band around 525 nm. R_F-(MES) _n -R_F oligomer and R_F-(AMPS) _n -R_F oligomer are not suitable for the autoreduction of gold ions; however, R_F-(MES)_n-R_F[or R_F-(AMPS) _n -R_F]/polyaniline [PAn] nanocomposites, which were prepared by the polymerization of aniline initiated by ammonium persulfate in the presence of the corresponding oligomer, enabled the formation of gold nanoparticles through the oxidation of PAn in the composites at room temperature. The reversible conformational change of PAn in the nanocomposites from the polyemeraldine salt to the oxidized pernigraniline base was observed during such oxidation process. Graphical abstract

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Ni-based catalysts obtained from perovskites oxides for ethanol steam reforming

Perovskites as host structures of cations were used in order to generate in situ active and stable catalysts for ethanol steam reforming. For this purpose, La_1-xMg_xAl_1-yNi_yO₃ (x = 0.1; y= 0, 0.1, 0.2, 0.3) perovskites were synthetized by the citrate method. Ni segregation is evident for a substitution level higher than 0.2. The segregation of Ni as NiO generated species interacts with different metal-support after the reduction step. The y= 0.1 catalyst presents the highest H₂ yield value about 85% during reaction time, with low mean values of CH₄ and CO selectivities of 3.4% and 11%, respectively and a low carbon formation. The better performance of y= 0.1 catalyst could be attributed to the minor proportion of segregated phases, thus a controlled expulsion of Ni is successfully reached.     

Mechanochemical synthesis,structure, and properties of solid solutions of alkaline earth metal fluorides: Ma1−xMbxF2 (M: Ca,Sr, Ba)

The capability of mechanochemical synthesis for the formation of solid solutions of alkaline earth metal fluorides M^a_1−xM^b_xF₂ (M: Ca, Sr, Ba) was tested by fluorination of metal acetates and metal hydroxides with ammonium fluoride directly at milling. Evidence was found for a mutual substitution of cations on their lattice positions in Ca_1−xSr_xF₂ and Ba_1−xSr_xF₂ samples. For the Ba/Ca-system this synthesis route is only partially successful. X-ray diffraction and ¹⁹F MAS NMR spectroscopy were used to characterize all samples concerning their crystal structure and local fluorine coordination. Calculations of ¹⁹F chemical shifts with the superposition model along with probability calculations for the intensity of the individual ¹⁹F lines, performed in dependence on the molar composition of the samples, perfectly agree with the experimental findings. The fluoride ion conductivity of as-prepared samples, determined by temperature dependent DC conductivity measurements, is significantly higher than those of crystalline binary fluorides. Moreover, a higher F⁻ ion conductivity is observed for samples with higher mixing grade in the Ca/Sr-and the Ba/Sr-systems.     

Study on the (R)-oxynitrilase activity of Pouteria sapota

Mamey (Pouteria sapota) defatted meal was used to catalyze the enantioselective addition of HCN to α,β-unsaturated aldehydes. Using a biphasic system of diisopropyl ether and citrate buffer (0.1 M, pH 5.0, 10% v/v), the (R)-cyanohydrins obtained showed good conversion (from 54 to 98%) and enantiomeric excess (from 74 to 99%).     

Revision of iron(III)–citrate speciation in aqueous solution. Voltammetric and spectrophotometric studies

A detailed study of iron (III)–citrate speciation in aqueous solution (θ = 25 °C, I_c = 0.7 mol L⁻¹) was carried out by voltammetric and UV–vis spectrophotometric measurements and the obtained data were used for reconciled characterization of iron (III)–citrate complexes. Four different redox processes were registered in the voltammograms: at 0.1 V (pH = 5.5) which corresponded to the reduction of iron(III)–monocitrate species (Fe:cit = 1:1), at about −0.1 V (pH = 5.5) that was related to the reduction of FeL₂⁵⁻, FeL₂H⁴⁻ and FeL₂H₂³⁻ complexes, at −0.28 V (pH = 5.5) which corresponded to the reduction of polynuclear iron(III)–citrate complex(es), and at −0.4 V (pH = 7.5) which was probably a consequence of Fe(cit)₂(OH)_x species reduction. Reversible redox process at −0.1 V allowed for the determination of iron(III)–citrate species and their stability constants by analyzing E_p vs. pH and E_p vs. [L⁴⁻] dependence. The UV–vis spectra recorded at varied pH revealed four different spectrally active species: FeLH (log β = 25.69), FeL₂H₂³⁻ (log β = 48.06), FeL₂H⁴⁻ (log β = 44.60), and FeL₂⁵⁻ (log β = 38.85). The stability constants obtained by spectrophotometry were in agreement with those determined electrochemically. The UV–vis spectra recorded at various citrate concentrations (pH = 2.0) supported the results of spectrophotometric–potentiometric titration.     

The determination of traces of ammonium-nitrogen in aqueous solution by optical emission spectrometry with a high-frequency inductively coupled argon plasma source

A method for the determination of low levels of ammonium ion in solution by optical emission spectrometry with an inductively coupled argon plasma source operated at 27 MHz is presented. The ammonium ion is oxidized with sodium hypobromite in alkaline medium, the evolved nitrogen is passed into the argon plasma, and the NH emission intensity produced in the plasma at 336.0 nm is monitored. A practical detection limit of 0.1 μg N ml^-1 for 5-ml aqueous sample solutions has been obtained. The method has been applied to the determination of the exchangeable ammonium content of soil samples.     

Separation and electrodeposition of226Ra

The chemical behavior of calcium, barium and radium in the ion exchange resins Dowex 50W-X8, AG 50W-X8 and Merk I in the presence of ammonium tartrate, EDTA, and citrate has been studied. No differences were observed in results while using any one of the three resins. Calcium, barium and radium were fixed to the exchange column at pH 4.8 EDTA solution. Calcium was eluted in an EDTA solution at pH 5.3, barium and radium between pH 8–11. Elution in citrate media for calcium was achieved at pH 6.1 and for radium at pH 10. In ammonium tartrate, calcium was eluted at pH 6, barium and radium at pH 11.5. Radium was also eluted from the ion exchange resins with a 2M nitric acid solution. The radium free of calcium was electrodeposited onto a stainless steel disc cathode using a 0.1 M potassium fluoride solution, pH 12–14, with a yield of >50%. The energies of²²⁶Ra were analyzed through high resolution -spectra. The²²⁶Ra utilized for these experiments was separated from Mexican carnotite.