Cation exchange separation of51Cr-labelled species in aqueous Cr(VI)−Cr(III) solution

A convenient and precise separation procedure for aqueous⁵¹Cr(VI),⁵¹Cr(III)-monomer,⁵¹Cr(III)-dimer and⁵¹Cr(III)-polymer species, using a cation exchange resin, is described. Standard deviations of 0.3% for each component may be routinely obtained.     

Speciation of the reaction products of51Cr/VI/ with formic acid

The chemical behaviour of Cr/VI/ at low concentrations /10^–6–10^–7 mol.1^–1/ in several aqueous formic acid solutions was studied using high specific activity⁵¹Cr/VI/. A simple and efficient radiochromatographic method using paired cation and anion exchange analyses was used to quantify the⁵¹Cr/VI/ remaining in solution after different time periods as well as the several⁵¹Cr/III/ species formed. The product distribution of monomeric⁵¹Cr/III/ species includes those with 0, +1, +2 and +3 charges. Very little dimeric or higher polymeric species are formed. The rate of reaction of⁵¹Cr/VI/ with formic acid depends on the acid concentration, the presence or absence of sodium chloride and wheter, or not, the samples are exposed to light.     

Gamma irradiation effects of51Cr/III/ Isotope exchange in doped magnesium chromate — zinc chromate mixtures

Gamma irradiation effects of chromium isotope exchange in magnesium chromate — zinc chromate mixtures doped with⁵¹Cr/III/ were investigated. It was found that irradiation has an oxidation effect and the percentage of⁵¹Cr/VI/ exchanged increases with increasing -ray dose. The data were explained in terms of mechanistic model involving metal and ligand vacancies exchange and substitution reactions.     

Determination of small quantities of51Cr(III) in the presence of large quantities of51Cr(VI) by coprecipitation with zinc hydroxide

A procedure is described for the determination of⁵¹Cr(III) in⁵¹Cr(VI) samples having a⁵¹Cr(VI) to⁵¹Cr(III) ratio of 10⁵, by means of a single zinc hydroxide coprecipitation at apH of ca. 13. Acceptable precision (standard deviation of 10–15%) is possible with solid⁵¹Cr(VI) samples containing 10 ppm of⁵¹Cr as⁵¹Cr(III).     

Instant capture of recoil51Cr(III) from neutron activated K2CrO4

We have recently developed a new method of measuring the initial⁵¹Cr(III) produced from nuclear recoil of K₂CrO₄. In our method, K₂CrO₄ was mixed with MgO in the presence of a small amount of water, and the mixture was irradiated in a nuclear reactor. After irradiation, the mixture was dissolved in water, and MgO precipitate was separated from the solution. The yield of recoil⁵¹Cr(III) could be calculated from the⁵¹Cr activity in the precipitate measured. On the other hand, the yield of retention of⁵¹Cr as chromate could be calcualted from the activity found in the supernatant. The⁵¹Cr(III) yield thus obtained is almost a factor of 2 higher than observed in pure K₂CrO₄ without mixing with MgO, irradiated under the same condition. Another important observation is that the⁵¹Cr(III) yield is independent of irradiation time in the presence of MgO. Without MgO, the observed⁵¹Cr(III) yield decreases with increasing irradiation time, suggesting possible oxidation of Cr(III) to chromate during irradiation. This variation is not observed in the system of K₂CrO₄ containing MgO, indicating that the initial Cr(III) is adsorbed immediately after nuclear recoil by MgO and is protected from oxidation by gamma radiation.     

Effect of pressure on transfer annealing in51Cr/III/-doped potassium chromate

The effect of compression on⁵¹Cr/III/-doped potassium chromate, pre-irradiated, or not, with -radiation, has been studied. Compression appeared to have no effect on the initial⁵¹Cr/VI/ yield, while it was found to retard considerably the transfer annealing process.     

Austausch von Chrom(III) mit Chrom(II) und Szilard—Chalmers-Reaktion von Kaliumchromat

Zusammenfassung Durch Versuche mit⁵¹Cr wird gezeigt, daß Cr³⁺ mit Cr²⁺ austauscht, wenn man ein Chrom(III)-Salz in wässeriger Cr²⁺-Lösung auflöst, d. h. die Hydratation zum reaktionsträgen [Cr(H₂O)₆]³⁺ verläuft relativ langsam. Die Bedeutung dieses Austausches für dieSzilard—Chalmers-Reaktion von Chromat wird diskutiert.

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Experiments with⁵¹Cr prove the exchange of Cr³⁺ with Cr²⁺ when a Cr(III)-salt is dissolved in an aqueous solution of Cr²⁺. This indicates that the hydration to the inert [Cr(H₂O)₆]³⁺ is relatively slow. The importance of this exchange for theSzilard—Chalmers reaction of chromate is discussed.

     

Speciation of the products from acid reduction of51Cr(VI)

Chromium speciation implies the quantitative determination of Cr(III) and Cr(VI). However, the presence of hydrolytic forms of Cr(III) and the instability of tracer level Cr(VI) in acid media complicates this speciation. The present work describes the stability of several monomeric Cr(III) species formed in the acid reduction of⁵¹Cr(VI). The distribution of Cr(VI) and Cr(X)_n(H₂O) _6–n ^(3–n)+ as a function of time was followed by paired cationic and anionic exchange analyses. The distributions and their time dependences are functions of the initial concentrations of both Cr(VI) and acid. The Cr(III) species eventually level to the hexaaquo form.     

Column chromatographic speciation of chromium for Cr(VI) and several species of Cr(III)

Summary Chromium can be present in aqueous solution as Cr(VI) or in monomeric, dimeric, trimeric and higher polymeric forms of Cr(III). Many monomeric forms of Cr(III) are possible, with the water molecules of Cr(H₂O) ₆ ³⁺ substituted by anionic or neutral species. This proliferation of Cr(III) species makes the complete speciation of chromium a continuing challenge to the analyst. A simple and effective cation exchange procedure for the separation of various of these species uses a small glass column containing 1 mL of pre-treated cation exchange resin (Na⁺ form). Stepwise elution with solutions of perchloric acid, Ca²⁺ (pH=2) and La³⁺ (pH=2) separates Cr(VI) and seven Cr(III) species from CrX₃ to tetramer. Radiometric (Cr-51), spectrophotometric and other detection methods can be employed; the use of radiochromium gives the lowest detection limit.     

Recoil implantation reaction in geometrical isomers of tris/benzoylacetonato/chromium/III/

Hot atom chemical reaction by⁵⁰Cr/n, /⁵¹Cr and⁵²Cr/, n/⁵¹Cr reactions, and recoil implantation reaction by⁵¹V/p, n/⁵¹Cr reaction were investigated using geometrical isomers /mer and fac/ of tris/benzoylacetonato/ chromium/III/ /Cr/ba/₃/. The production of counter isomer was observed for both mer- and fac-targets, although the yield of labelled parent isomer was larger. The observed mer/fac yield ratio suggests that the main formation mechanism of⁵¹Cr/ba/₃ is the reaction of ba^– and Cr/ba/ ₂ ⁺ which has the same geometrical configuration of target complex, and the substitution reaction of central metal atom by recoil⁵¹Cr. Furthermore, implantation gave rise to a much higher yield of labelled Cr/ba/₃ compared to the case of in situ nuclear recoils.     

Oscillatory transfer annealing in radiodoped chemical compounds

Doping of crystalline /CoEDTA/₂ Ba and /Co(en)₃/(NO₃)₃ by⁶⁰Co²⁺ creates a thermodynamically dissipative structure which gives rise to an oscillatory exchange of the radiocobalt with the inactive Co/III/ of the complex ions. The same compounds in which energetic⁶⁰Co atoms have been produced by neutron irradiation show kinetically the same oscillatory parameters. The Volterra-Lotka model is applied for the oscillations.     

Thermal effects in51Cr(III)-doped and in neutron irradiated and recrystallized potassium chromate

The effect of thermal treatment, in air and in vacuum, on⁵¹Cr(III)-doped potassium chromate and on post-irradiation-recrystallized potassium chromate has been studied. The⁵¹Cr(VI) yield was found to increase and the M yield to decrease in all samples, while the DP yield remained constant up to 180 °C and decreased at higher temperatures. The results were not affected by the presence of oxygen.     

Chromium (III) removal by microporous ion exchanger Amberlite.120 (Na+): kinetic, equilibrium and thermodynamic studies

Cr (III) sorption on microporous strong cation exchanger Amberlite.120 (Na⁺) is studied as a function of time and temperature. The pH changes show the co-sorption of H⁺ ions along with the chromium. The rate constant values for Cr (III) sorption are calculated for both film and particle diffusion processes. However, the particle diffusion is found to be more dominant than the film diffusion. The temperature is found to have a positive effect on both the diffusional processes. The low values of energy of activation also confirm the diffusional nature of the process. Equilibrium data are explained with the help of Langmuir equation. Various thermodynamic parameters (??H, ??S and ??G) for Cr (III) exchange on the resin are calculated. The ??G values are found to be negative, while both the ??H and ??S values obtained are positive.     

Studies on bio-accumulation of <Superscript>51</Superscript>Cr by <Emphasis Type="Italic">Piper nigrum</Emphasis>

The present study is performed to examine the accumulation efficiency of ⁵¹Cr(III) and ⁵¹Cr(VI) by the alkaloid piperine, derived from the fruits of Piper nigrum (Family Piperaceae) as well as using the fruit commonly known as black pepper by radiometric technique. The pH dependence and the effect of the concentration of chromium on the accumulation have also been examined. The maximum accumulation (52%) of Cr(III) is found by black pepper at pH 4 whereas piperine shows slight accumulation at this condition. Accumulation of Cr(VI) by black pepper is always negligible. It has also been observed that some other constituents of the black pepper like gum, terpenoid, etc., besides piperine is responsible for the accumulation of chromium.     

On achieving measurement quality in air pollution studies employing nuclear techniques

In order to study the metabolism of physiological amounts of⁵¹Cr (10 μg/100 g of body wt.) intragastrically administered in rats, the activable enriched stable isotope Cr-50 compound Cr₂O₃ was used as a tracer. The absorption and distribution of⁵¹Cr(III) in rats with time were studied. Significant⁵¹Cr contents were found in all the organs and tissues of interest. The kidney, liver and bone contain higher amounts of⁵¹Cr than others. The fact that specific activities of⁵¹Cr are notably high in kidney, bone, spleen and pancreas and decrease gradually with time suggests that there are tighter binding of chromium in these organs. The excretion of⁵¹Cr at various time intervals was also studied. Almost totally intragastrically administered dose was excreted in the feces. The increased urinary excretion of⁵¹Cr with time indicates that the urine-chromium is the metabolic derivative of organism. In view of the tissues distribution and excretion, it can be concluded that no more that 1% of the dose was absorbed from the gastrointestinal tract.     

Solubility and Phase Behavior of Cr(III) Oxides in Alkaline Media at Elevated Temperatures

A platinum-lined, flowing autoclave facility is used to investigate the solubility behavior of Cr₂O₃ and FeCr₂O₄ in alkaline sodium phosphate, sodium hydroxide, and ammonium hydroxide solutions between 21 and 288°C. Baseline Cr(III) ion solubilities were found to be on the order of 0.1 nmolal, which were enhanced by the formation of anionic hydroxo and phosphato complexes. At temperatures below 51°C, the activity of Cr(III) ions in aqueous solution is controlled by a Cr(OH)₃·3H₂O solid phase rather than Cr₂O₃; above 51°C the saturating solid phase is -CrOOH. Measured chromium solubilities were interpreted via a Cr(III) ion hydrolysis/complexing model and thermodynamic functions for the hydrolysis/complexing reaction equilibria were obtained from least-squares analyses of the data. The existence of four new Cr(III) ion complexes is reported: Cr(OH)₃(H₂PO₄)^–, Cr(OH)₃(HPO₄)^2–, Cr(OH)₃(PO₄)^3–, and Cr(OH)₄(HPO₄)-(H₂PO₄)^4–. The last species is the dominant Cr(III) ion complex in concentrated, alkaline phosphate solutions at elevated temperatures.     

Radiochemical solvent extraction of Cr(III) using51Cr tracer by 8-hydroxyquinoline and acetylacetone as mixed ligands in chloroform

A radiochemical solvent extraction method has been developed for the determination of Cr(III) using⁵¹Cr tracer. It is based on the complexation of Cr(III) with 8-hydroxyquinoline (oxine) and acetylacetone as mixed ligands at pH 3.8 and extraction in chloroform. Effect of various parameters such as pH, time of equilibration, nature of solvent, quantitative nature, effect of diverse ions has been studied. The method can be used up to 200 ng of Cr.     

Amorphous aluminosilicates containing trivalent chromium in a non-octahedral coordination environment

Amorphous aluminosilicates containing up to 5.8 wt.% chromium have been prepared by cation exchange on an amorphous sodium aluminosilicate using Cr(III) salts. Electronic spectroscopy has shown that the ligand arrangement around the Cr(III) sites does not correspond to the octahedral geometry. No isomorphous substitution of Al³⁺ by Cr³⁺ in the aluminosilicate occurs, and it is not possible to exchange back chromium by Na⁺ ions. The amorphous chromium-substituted aluminosilicates (abbr. ACSAS) are slightly acidic and when heated in air at 800 °C no oxidation of Cr(III) takes place. The chromium species in the ACSAS undergoes ligand replacement reactions.

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Amorphe Alumosilicate mit dreiwertigem chrom in nicht-oktaedrischer Koordinationsumgebung

Zusammenfassung Bis zu 5,8 Gew.% Chrom enthaltende amorphe Alumosilicate wurden durch Kationenaustausch am amorphen Natriumalumosilicat unter Anwendung von Cr(III)-Salzen hergestellt. Elektronenspektroskopische Untersuchungen zeigten, daß die Ligandanordnung um die Cr(III)-Lagen nicht der oktaedrischen Geometrie entspricht. Im Alumosilicat erfolgt keine isomorphe Substitution von Al³⁺ durch Cr³⁺ und es ist nicht möglich, Chrom durch Na⁺ zurückzuersetzen. Die amorphen durch Chrom ersetzten Alumosilicate (abgekürzt ACSAS) sind schwach sauer und wenn sie in der Luft auf 800 °C aufgeheizt werden, erfolgt keine Oxydation von Cr(III). Das in den ACSAS vorliegende Chrom unterliegt Ligandaustauschreaktionen.

     

Small-defect clusters in chromia-doped rutiles

Bright-field phase contrast electron microscope images of chromia-doped rutiles have been used to study the size and distribution of small defect clusters in specimens of TiO₂, (Ti,Cr)O_1.995, and (Ti,Cr)O_1.985. Comparisons of observed and measured spot contrast densities, and contrast calculations using computer simulations of the defect structures, lead to the conclusion that the smallest clusters observed contained ～32 Cr³⁺ cations. Larger clusters occurred with increasing frequency for higher dopant levels, which is consistent with increased interaction and aggregation of traditional or reconstructed small defects (e.g., Cr³⁺ interstitials or charge-compensated oxygen vacancies).     

Mass spectrometric investigations of the kinetic stability of chromium and copper complexes with humic substances by isotope-labelling experiments

Isotope-labelling exchange experiments were carried out to investigate the kinetic stability of Cr(III) complexes with humic substances (HS). To compare the results with those of an ion, not expected to form kinetically stable HS complexes with respect to its electron configuration, Cu(II) was investigated under the same conditions. HS solutions of different origin were therefore spiked with ⁵³Cr(III) or ⁶⁵Cu(II) after saturation of HS with chromium and copper of natural isotopic composition. In fractions of metal/HS complexes with different molecular weight, obtained by ultrafiltration and HPLC/ICP-MS using size exclusion chromatography (SEC), respectively, the isotope ratios of chromium and copper were determined by ICP and thermal ionisation mass spectrometry. Distinct differences in the isotopic composition of chromium were found in the permeate of the ultrafiltration compared with the corresponding unseparated solution, which indicates kinetically stable Cr(III)/HS complexes. On the other hand, the copper isotopic composition was identical in the permeate and the unseparated solution, which shows that a total exchange of Cu²⁺ ions took place between free and HS complexed copper ions. The SEC/ ICP-MS experiments also resulted in a different isotopic distribution of chromium in the chromatographically separated complexes whereas the copper complexes, separated by SEC, showed identical isotopic composition. The kinetic stability of Cr(III)/HS complexes could be explained by the d³ electron configuration of Cr³⁺ ions, a fact which is well known from classical Cr(III) complexes, and influences substantially the mobility of this heavy metal in the environment.