The transformation of ferrihydrite into goethite or hematite, revisited

International audienceDuring the oxidation of iron, poorly crystallized phases are firstly formed: 2- and 6-line ferrihydrite, which presents for the last phase, a similarity with wustite FeO but also with hematite α-Fe2O3. Crystallization increases with time and the solid phase obtained is dependent on temperature and pH. Obviously, high temperature favours the formation of the oxide hematite α-Fe2O3. As for the pH factor, it is more complicated. Low and high values of pH (2–5 and 10–14) favour the formation of goethite α-FeOOH, while obtaining hematite is favoured at neutral pH (values around 7). Goethite or hematite are obtained either through a dissolution–crystallization process or in the solid state, through a topotactic transformation.Given the structural relationships observed between ferrihydrite and wustite and hematite, it is allowed to think that a structural continuity could exist between wustite Fe(1−x)O and hematite via ferrihydrite

Two new aluminum hydroxychromates Al13(OH)11(CrO4)14·xH2O (x = 41 and 51)

International audienceThe study of the ternary system: Al2O3, CrO3, H2O at 70°C and 20°C reveals two new hydroxychromates; the formulae of which are: Al13(OH)11(CrO4)14 , xH2O, x being equal to 41±1 and 51±1. Al2(CrO4)3,5H2O, the compound announced by Raychoudhury [l] is not observed. Experimental techniques are the same as those used for the ternary system at 110°C and 150°C, and described in a precedent publication, where a detailed analysis of previous work on aluminum chromates is given

Étude cinétique de l'oxydation de l'ammoniac en présence d'ions cuivriques

International audienceOxidation of ammonia by dioxygen, to give rise to nitrite ions, at room temperatureand normal pressure, is connected with the ratio of divalent copper ions present in the concentratedsolution of ammonia. It is performed in two steps corresponding to the formation of the complexes:Cu(NH3)4(NO2)+ and Cu(NH3)4(NO2)2. This phenomenon is a consequence of the Jahn-Tellereffect, very important in the case of the divalent copper.L'oxydation de l'ammoniaque par le dioxygène pour donner naissance à des ions nitrite, à température et pression ambiantes, est corrélée à la teneur en ions cuivriques de la solution d'ammoniaque concentrée. Elle s'effectue en deux étapes aboutissant à la formation des complexes Cu(NH3)4(NO2) + et Cu(NH3)4(NO2)2. Ce phénomène est une conséquence de l'effet Jahn-Teller, important dans le cas du cuivre divalent

Étude cinétique de l'oxydation de l'ammoniac en présence d'ions cuivriques

International audienceOxidation of ammonia by dioxygen, to give rise to nitrite ions, at room temperatureand normal pressure, is connected with the ratio of divalent copper ions present in the concentratedsolution of ammonia. It is performed in two steps corresponding to the formation of the complexes:Cu(NH3)4(NO2)+ and Cu(NH3)4(NO2)2. This phenomenon is a consequence of the Jahn-Tellereffect, very important in the case of the divalent copper.L'oxydation de l'ammoniaque par le dioxygène pour donner naissance à des ions nitrite, à température et pression ambiantes, est corrélée à la teneur en ions cuivriques de la solution d'ammoniaque concentrée. Elle s'effectue en deux étapes aboutissant à la formation des complexes Cu(NH3)4(NO2) + et Cu(NH3)4(NO2)2. Ce phénomène est une conséquence de l'effet Jahn-Teller, important dans le cas du cuivre divalent

Synthesis and studies of copper hydroxynitrites

International audienceA new route for the preparation of copper ammine hydroxynitrites and copper hydroxynitrites by soft chemistry, at room temperafure and normal pressure, is exposed. Well crystallized solid phases were obtained by this way : Cu 5 (OH) 6 (NO 2) 4 (NH 3) 4 and Cu 2 (OH) 3 NO 2. Crystallographic studies show that the first compound is monoclinic, space group C2/m probably, a(Å) = 12,488(4), b(Å) = 6,841(3), c(Å) = 9,784(4), beta° = 93,69(3), z = 2, p exp. = 2,7, p calc. = 2,70. The second is triclinic space group P-1 : a(Å) = 5,6240(14), b(Å) = 6,052(3), c(Å) = 6,643(2), alpha°= 90,01(3), beta° = 95,37 (3), gamma° = 89,84(3), z = 2, p exp .= 3,3, p calc. = 3,31. A thermic study shows strong exothermic peaks when ammonia and nitrites coexist. In Cu 2 (OH) 3 NO 2 , the stability of brucite type layers is strengthened by the presence of the nitrite anion

Etudes cristallographiques et infrarouges d'hydroxychromates de fer et d'aluminium de structure alunite

International audienceSummary. - Crystallographic and infrared spectrophotometric studies have been carried out on iron and aluminum hydroxychromates : M(I)M(III)3(OH)6(CrO4)2 ( M(I) = Na+ , K+ , NH4 + ; M(III) = Fe, Al ). KFe3(OH)6(CrO4)2 crystallizes in the space group R-3m: a(hex) = 7,430(5) Å c(hex) = 17,440(10) Å The crystal structure was studied with a single crystal, on the basis of an isomorphism with alunite: KAl3(OH)6(SO4)2. The refinement of the atomic and isotropic thermal parameters, reduced the R factor to 0,057. NaAl3(OH)6(CrO4)2, is also rhomboedric (R-3m): a(hex) = 7,060(3) Å c(hex) = 17,250(14) Å The crystal structure was determined from powder pattern data (R: 0,062). Infrared spectrophotometric studies conclude to an isomorphism with alunite for the three others hydroxychromates NaFe3(OH)6(CrO4)2, NH4Fe3(OH)6(CrO4)2 and KAl3(OH)6(CrO4)2. It is to be noted that the present work confirms the choice of the R-3m space group made for alunite and extends to chromate compounds the structural type of alunite which was only observed for sulfate compound

Synthesis and studies of copper hydroxynitrites

International audienceA new route for the preparation of copper ammine hydroxynitrites and copper hydroxynitrites by soft chemistry, at room temperafure and normal pressure, is exposed. Well crystallized solid phases were obtained by this way : Cu 5 (OH) 6 (NO 2) 4 (NH 3) 4 and Cu 2 (OH) 3 NO 2. Crystallographic studies show that the first compound is monoclinic, space group C2/m probably, a(Å) = 12,488(4), b(Å) = 6,841(3), c(Å) = 9,784(4), beta° = 93,69(3), z = 2, p exp. = 2,7, p calc. = 2,70. The second is triclinic space group P-1 : a(Å) = 5,6240(14), b(Å) = 6,052(3), c(Å) = 6,643(2), alpha°= 90,01(3), beta° = 95,37 (3), gamma° = 89,84(3), z = 2, p exp .= 3,3, p calc. = 3,31. A thermic study shows strong exothermic peaks when ammonia and nitrites coexist. In Cu 2 (OH) 3 NO 2 , the stability of brucite type layers is strengthened by the presence of the nitrite anion