Dissociation pressure and thermodynamic properties of Na2SiF6

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Abstract

The standard Gibbs energy for the dissociation of sodium fluosilicate (Na2SiF6) on the basis of the measured SiF4 pressure is given by the following relations (where ΔG° is in calories per mole): Na2SiF6(α) → 2NaF(s) + SiF4(g)Δ° = 38645−74.237T + 4.75TinT 298 K < T< 833 K and Na2SiF6(β) → 2NaF(s) + SiF4(g)ΔG° = 27 950 − 61.398T + 4.75TInT 833 K < T < 968 K The Gibbs energy change for the polymorphic transition from a to β phase is consequently ΔG° = 10 695 − 12.839Tcalmol−1 The dissociation pressure at 968 K, the eutectic temperature for the pseudobinary NaF-Na2SiF6 system, is 0.88 atm. The pressure increases rapidly as the temperature is increased above 968 K for compositions in the Na2SiF6(β)-liquid two-phase region; the calculated value is 78 atm at 1123 K (850°C). In the NaF-liquid two-phase region the calculated pressure is 1.3 atm at 1123 K and decreases with increase in temperature as the liquidus composition approaches pure NaF at 1269 K.

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Cited by (11)

  • Thermodynamic optimization of the (Na<inf>2</inf>O + SiO<inf>2</inf> + NaF + SiF<inf>4</inf>) reciprocal system using the Modified Quasichemical Model in the Quadruplet Approximation

    2011, Journal of Chemical Thermodynamics
    Citation Excerpt :

    The lack of data is due to the experimental challenges related to the very high partial pressure of SiF4 in the liquid-Na2SiF6 two-phase region (NaF-rich side). Chiotti [114] reported a total pressure of 7.903 MPa at 1123 K in this region. The (NaF + Na2SiF6) pseudo-binary system is reported to be a simple eutectic at 968 K and 0.244 mole fraction SiF4 [115].

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