Uranium determination in phosphoric acid solutions

In order to determine uranium from raw phosphoric acid solutions, resulted by the sulphuric acid attack of phosphate rocks and the strip solutions of the solvent extraction process for uranium recovery, two classes of analytical methods were established: one for low uranium content in phosphoric acid, and the other for higher uranium concentration in the same medium. The study was based on specific methods, therefore interference probability with other impurities in phosphoric acid medium is low. In the first class, X-ray fluorescence and spectrophotocolorimetric methods were used. X-ray fluorescence was applied on direct raw phosphoric acid solution and raffinate. The last one was associated with solvent extraction [di-(2-ethylhexyl) phosphate + triocylphosphine oxide] on the U(IV)-Arsenazo III complex in strip. The methods of the second class, were used for strip uranium concentrated solutions: X-ray fluorescence isotopic dilution and mass spectrometry, spectrophotocolorimetry and activation analysis associated with gamma-spectrometry. Here spectrophotocolorimetry involves two methods. The first one is based on the U(IV)-Arsenazo III complex and the other on direct U(IV)—phosphoric acid solutions measurements. A good agreement was obtained in each case for all comparative measurements involving various methods.     

Electrochemically induced processes of formation of phosphoric acid derivatives. 3. Electrosynthesis from white phosphorus in alcohol-water solutions

It has been established that the process of splitting of the P-P bonds of the white phosphorus molecules is initiated by cathode-generated nucleophiles (HO^–, RO^–), while functionalization of the P-H bond formed in phosphoric oligomers occurs under the action only of alcohol. The primary product after splitting of all the P-P bonds in phosphoric oligomers is dialkylphosphite (in alcohol-water media), or trialkylphosphite (in absolute alcohol), in the course of electrolysis being transformed into trialkylphosphate. Formation of esters of pyrophosphoric acid with reduced protogenic character of the medium was examined. It is proposed that under these conditions nucleophilic reagents of the type (P)_c-O^– form and participate in splitting of the P-P bonds.A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center of the Russian Academy of Sciences, 420083 Kazan. V. I. Ul'yanov-Lenin State University, 420008 Kazan. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 9, pp. 2033–2038, September, 1992.     

Surface modification of titanium by anodic oxidation in phosphoric acid at low potentials. Part 1. Structure,electronic properties and thickness of the anodic films

Titanium surface characteristics determine the degree of success of permanent implants. The topography, morphology of the surface in micro and nano scales, the impurities present and other characteristics are a main concern, and therefore a multi‐technique approach is required in order to evaluate modification process effects on the surface. Surface modification of titanium in the nanometrical range was performed by means of anodisation in phosphoric with the aim of improving both the biocompatibility and the corrosion resistance in the biological media. Biocompatible characteristics of the modified titanium surface, as the presence of anatase in the oxide film and the incorporation of phosphate to the surface, were determined. Moreover, the electronic properties of the surface oxide presented a carrier number adequate for biomedical applications. The increase in the film thickness from 3 to 42 nm was estimated from EIS results when anodising potentials from 0 to 30 V were applied, whereas a bi‐layer structure of the protective oxides formed was determined. Copyright © 2013 John Wiley & Sons, Ltd.     

Extraction recovery of vanadium(IV) from acid sulfate solutions with di-(2-ethylhexyl)phosphoric acid

Extraction of vanadium(IV) with di-(2-ethylhexyl)phosphoric acid from acid sulfate solutions in the presence of sodium sulfate was studied. The composition of the complex being extracted was found, and the equation of the extraction reaction was determined. The equilibrium constant of the reaction by which vanadium(IV) is extracted with di-(2-ethylhexyl)phosphoric acid was found by taking into account the complexation of vanadium(IV) in acid sulfate solutions.     

The anodic behavior of gold in phosphoric acid at elevated temperatures

Summary The anodic dissolution of gold in concentrated phosphoric acid was investigated at 145–200°C. Gold dissolves in a narrow potential region (150 mV) about 300 mV positive to the oxygen potential in the same electrolyte. At higher potentials it passivates. At very high overvoltages dissolution, which increased rapidly with temperature, was also observed in the transpassive region. The product of the dissolution is finely divided gold, probably formed by immediate decomposition of the oxydation product. At 200°C, at which temperature pyrophosphoric acid is formed, the gold dissolution also yields a clear yellow solution of gold pyrophosphate

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Zusammenfassung Die anodische Auflösung von Gold in konzentrierter Phosphorsäure wurde im Temperaturbereich von 145–200° C untersucht. Gold geht in Lösung in einem engen Potentialintervall (150 mV) etwa 300 mV positiver als das Sauerstoffpotential im gleichen Elektrolyten. Bei höheren Potentialen wird es passiv. Bei sehr hohen Überspannungen wurde auch im transpassiven Bereich Auflösung beobachtet, deren Geschwindigkeit mit der Temperatur stark ansteigt. Bei der Auflösung entsteht fein verteiltes Gold, vermutlich durch unmittelbar folgende Zersetzung des Oxydationsproduktes. Bei 200° C, wo Pyrophosphorsäure gebildet wird, ergibt die Goldauflösung auch eine klare gelbe Lösung von Goldpyrophosphat.

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Dedicated to Prof. Dr. M. v. Stackelberg on his 70th birthday.     

Aluminum alloy etching in phosphoric acid solutions

The rate of dissolution of aluminum and qualitative properties (gloss and uniformity) of its surface were examined as influenced by the concentration of H₃PO₄ and fluorine-containing compounds and their nature, as well as by the temperature, pH, and various additions (acids, surfactants, inhibitors).     

Surface modification of titanium by anodic oxidation in phosphoric acid at low potentials. Part 2. In vitro and in vivo study

Electrochemical studies in SBF solution were performed, in order to determine the best corrosion resistance condition, comparing as‐received titanium, covered with its native surface oxide, and titanium anodized in phosphoric acid. The results indicate that the anodic films obtained at a constant potential of 30 V have higher barrier effect, and the protective layer remains effective against the aggressive anions present in SBF after 30 days of immersion. Due to the promising corrosion performance in simulated biological media coupled with the biocompatible surface characteristics, anodic films on titanium obtained at 30 V were implanted on Wistar Rats to compare the osseointegration results of this modified surface with that corresponding to as‐received titanium. It was found that, after 8 weeks of implantation, although the amount of bone surrounding the implant did not differ across the two different surface implants conditions, bone formation at the implant interface was found to be more homogeneous in anodized titanium. Copyright © 2013 John Wiley & Sons, Ltd.     

Mechanism of anodic oxidation of tungsten in neutral sulphate-fluoride solutions

The anodic oxidation of tungsten has been studied in 1 M Na₂SO₄ solutions containing 0–0.25 M NaF. Steady-state currents measured in the passivation and passivity ranges increase significantly with increasing fluoride concentration, indicating enhanced dissolution of the oxide film. The electrochemical impedance response is dominated by the processes in the barrier layer and at its interface with the electrolyte. The presence of a pseudo-inductive loop in the impedance spectra at intermediate frequencies indicates point defect interaction during film growth and dissolution processes. A kinetic model including the recombination reaction between oppositely charged point defects at the film/solution interface as well as a kinetic scheme for tungsten dissolution through the film mediated by cation vacancies is proposed. It is found to reproduce satisfactorily the steady-state currents and the impedance spectra in the potential range 0.2–2 V. Such a model for the conduction mechanism in the barrier layer is believed to be an essential part of a modelling approach to the formation of a nanoporous overlayer on tungsten in fluoride-containing solutions.     

Solvent extraction of Fe(III) by di-(2-ethylhexyl)phosphoric acid from phosphoric acid solutions

The extraction of iron(III) from aqueous phosphoric acid was studied using di-(2-ethylhexyl)phosphoric acid and trioctylphosphine oxide in nonaromatic hydrocarbon diluent. Distribution ratios have been investigated as a function of concentration of iron(III), phosphoric acid concentration, extractant concentration and extraction temperature. The apparent enthalpy change for the extraction reaction has been calculated from the temperature dependence data. It was found that the extractant dependency for iron(III) is first power indicating hydrolysis of iron(III) in the aqueous phase.     

Sorption extraction of lanthanides from phosphoric acid solutions

The possibility of sorption extraction of lanthanides from nitric-phosphoric and phosphoric acid solutions with inorganic sorbents based on hydrated titanyl hydrophosphate was studied. New technological solutions were suggested for lanthanide sorption from the products which are formed in processing of the Khibiny apatite concentrate on mineral fertilizers (frozen nitric-phosphoric acid extract, a product of nitric acid decomposition of apatite, and the production phosphoric acid from the dihydrate process).     

Electrochemical degradation of buprofezin insecticide in aqueous solutions by anodic oxidation at boron-doped diamond electrode

Buprofezin (2-tert-butylimino-3-isopropyl-5-phenyl-1,3,5-thiadiazinan-4-one) is identified as a commonly used chemical with satisfactory biological activities against sucking insect pests, but its disposal causes serious environmental problems. This pesticide was treated by an electrolysis system using a boron-doped diamond (BDD) as anode and platinum as cathode. A number of experiments were run on a laboratory scale and the results are presented. The chemical oxygen demand (COD) measurement during the processing permitted the evaluation of the kinetic of organic matter decay and the instantaneous current efficiency. Different operating conditions and factors affecting the treatment process including current density, conductive electrolyte, pH, concentration of buprofezin, and time of electrolysis were studied and optimized. The best obtained conditions for COD removal on the BDD anode to degrade buprofezin solutions (COD₀ = 1,200 mg L^?1) include operating at 60 mA cm^?2 and 25 ± 3 °C. The high efficiency of this technology can be explained in terms of the direct electrooxidation at the BDD surface and the oxidation carried out by hydroxyl radicals (OH^?) and other electro-generated oxidants (Cl^?, ClO^?).     

Kinetics and mechanism of oxidation of acetanilide by quinquevalent vanadium in acid medium

Summary The kinetics of the oxidation of acetanilide with vanadium(V) in sulphuric acid medium at constant ionic strength has been studied. The reaction is first order with oxidant. The order of reaction in acetanilide varies from one to zero. The reaction follows an acid catalyzed independent path, exhibiting square dependence in H⁺. A Bunnett plot indicates that the water acts as a nucleophile. The thermodynamic parameters have been computed. A probable reaction mechanism and rate law consistent with these data are given.

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Kinetik und Mechanismus der Oxydation von Acetanilid mit fünfwertigem Vanadium in saurem Medium

Zusammenfassung Es wurden kinetische Untersuchungen der Oxydation von Acetanilid mit Vanadium(V) in schwefelsaurem Medium bei konstanter Ionenstärke durchgeführt. Gegenüber dem Oxidans ist die Reaktion erster Ordnung, die Reaktionsordnung gegenüber Acetanilid variiert zwischen 1 und 0. Die Reaktion folgt einem von der Säurekatalyse unabhängigen Weg, wobei die Abhängigkeit von H⁺ quadratisch ist. Ein Bunnett-Plot zeigt, daß das Wasser als Nucleophil wirkt. Die thermodynamischen Parameter wurden berechnet. Ein möglicher Reaktionsmechanismus und ein Geschwindigkeitsnetz, das mit diesen Daten in Einklang ist, wird angegeben.

     

The solubility of tri-n-octylamine in water and phosphoric acid solutions

The solubility of tri-n-octylamine (TOA) in water and phosphoric acid is reported. Equilibration is achieved by circulating the solutioin through PVC pieces saturated with the absorbed amine in a glass tube. The dissolved TOA is determined by capillary gas chromatography which can detect amine concentrations down to 10 μg l^?1. The solubility of TOA in water is 0.039 mg l^?1 and in 5.5 M phosphoric acid 0.82 mg l^?1; these values are much lower than those reported in the literature.     

Reactivity of the cadmium ion in concentrated phosphoric acid solutions

The solvation transfer coefficients which characterize the changes of ion reactivity with phosphoric acid concentration have been calculated for cadmium from the constants of the successive chloride complexes, and for silver and diethyldithiophosphate from potentiometric measurements. They evidence the strong desolvation of the cadmium species in concentrated phosphoric acid media, causing a remarkable increase of its reactivity. They allow the results of liquid-liquid extraction, precipitation and flotation reactions to be correctly interpreted and their changes to be foreseen when the reagents are modified.     

Kinetics of oxidation of vanadium(III) by hydroxylamine in acid medium

The kinetics of oxidation of vanadium(III) by hydroxylamine have been investigated at high acidities in the temperature range 25–30 °C. Rates decreased with increasing acidity of the medium. Both NH₂OH and NH₃OH⁺ are capable of oxidizing V(III) in parallel reactions, the order being unity each in oxidant and reductant.

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(III) 25–30 °C. . NH₂OH NH₃OH⁺ V(III) , , .

     

Kinetics of oxidation of ascorbic acid by methylene blue in acid solutions

We propose a kinetic scheme for reduction of methylene blue by ascorbic acid, characterized by the existence of two pathways: outer-sphere reduction of methylene blue according to a second-order reaction, and reduction through formation of a 11 complex with ascorbic acid.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 5, pp. 424–429, September–October, 1993.     

Kinetics of oxidation of phenylacetic acid hydrazide by vanadium(V)

The title reaction was studied in aqueous perchloric acid medium in the presence of 15% acetic acid by volume. It was observed that the reaction proceeds via an intermediate 11 oxidant-substrate complex. The reaction is inhibited by H⁺ ions. A mechanism consistent with the experimental results is proposed.

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(V) - 15% . - - 11. , .