ISSN:
1572-8927
Keywords:
Phase equilibrium: solubility
;
carbonates
;
magnesium
;
eitelite
;
Pitzer model
;
carbonic acid
;
carbonato complex formation
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
Notes:
Abstract The stoichiometric solubility constant of eitelite (NaMg 0.5 CO 3 +2H+ ⇄ Na++0.5Mg 2+ +CO 2 (g)+H 2 O, log*K pso I =14.67±0.03 was determined at I=3 m (mol kg−1) (NaClO 4 ) and 25°C. The stability of magnesium (hydrogen-)carbonato complexes in this ionic medium was explicitely taken into account. Consequently, trace activity coefficients of free ionic species, calculated from the Pitzer model with ion-interaction parameters from the literature, were sufficient for an evaluation of the thermodynamic solubility constants and Gibbs energies of formation for eitelite (−1039.88±0.60), magnesite (−1033.60±0.40), hydromagnesite (−1174.30±0.50), nesquehonite (−1724.67±0.40), and brucite (−835.90±0.80 kJ-mol −1 ). The increasing solubilities of nesquehonite and eitelite at higher sodium carbonate molalities were explained by invoking a magnesium dicarbonato complex (Mg2++2CO 3 2− ⇄ Mg(CO3) 2 2− , log βz = 3.90 ± 0.08). A set of ion-interaction parameters was obtained from solubility and dissociation constants for carbonic acid in 1 to 3.5 m NaClO 4 media $$(\theta _{HCO_3^ - ,ClO_4^ - } = 0.081, \theta _{CO_3^{2 - } ,ClO_4^ - } = 0.071, \psi _{{\rm N}a^ + , HCO_3^ - , ClO_4^ - } = - 0.019,\psi _{{\rm N}a^ + , CO_3^{2 - } , ClO_4^ - } = - 0.006,\lambda _{ClO_4^ - ,CO_2 } = - 0.076)$$ which reproduce these constants to 0.02 units in log K. The following Pitzer parameters are consistent with the previously studied formation of magnesium (hydrogen-)carbonato complexes in 3m NaClO 4 $$(\psi _{Mg^{2 + } , HCO_3^ - , ClO_4^ - } = - 0.36, \lambda _{ClO_4^ - ,MgCO_3 } = 0.081)$$ . The model and Gibbs functions of solid phases derived here reproduce original solubility data (−log [H+], [Mg 2+ ] tot ) measured in perchlorate medium within experimental uncertainty.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00667217
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