Phase Stability of Chromium(III) Oxide Hydroxide in Alkaline Sodium Phosphate Solutions.

Ziemniak, S. E. ; Opalka, E. P.

Technical Information Center Oak Ridge Tennessee

关键词: Phase stability;    Hydroxides;    Chromium oxides;    Precipitation;    Corrosion;   

RP-ID  :  DE2004821873

学科分类：工程和技术（综合）

美国|英语

来源: National Technical Reports Library

【 摘 要 】

Grimaldiite ((alpha)-CrOOH) is shown to transform to a sodium-chromium (III)-hydroxyphosphate compound (SCHP) in alkaline sodiumphosphate solutions at elevated temperatures via CrOOH(s)+ 4Na(sup+)+ 2HPO(sub 4)(sup 2-)= Na(sub 4)Cr(OH)(PO(sub 4))(sub 2)(s)+ H(sub 2)O. X-ray diffraction analyses indicate that SCHP possesses an orthorhombic lattice having the same space group symmetry (Ibam, No.72) as sodium ferric hydroxyphosphate. A structurally-consistent designation for SCHP is Na(sub 3)Cr(PO(sub 4))(sub 2)(center-dot) NaOH; the molar volume of SCHP is estimated to be 1552 cm(sup 3). The thermodynamic equilibrium for the above reaction was defined in the system Na(sub 2)O-P(sub 2)O(sub 5)-Cr(sub 2)O(sub 3)-H(sub 2)O for Na/P molar ratios between 2.0and 2.4. On the basis of observed reaction threshold values for sodium phosphate concentration and temperature, the standard molar entropy (S(sup o)), heat capacity (C(sub p)(sup o)) and free energy of formation((Delta)G(sub f)(sup o)) for SCHP were calculated to be 690 J/(mol-K), 622 J/(mol-K) and -3509.97 kJ/mol, respectively.