Oxide-Sulfate equilibria in the Mg, Ni, and Mn systems measured by a solid K 2 SO 4 concentration cell

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I.

INTRODUCTION

THE use

of solid-state electrolytes for determining the thermochemical properties of gases and materials of metallurgical interest is well established. ~,,2,z For example, the study of oxygen-bearing systems with the aid of zirconia-based electrolytes has seen a continuous growth in interest and applications since the original work was published in the fifties. 4 Recently other solid-state oxyanion electrolytes such as alkali metal sulfates, nitrates, and carbonates have been successfully employed to measure partial pressures of SOs, NO2, and CO2. ~b,5,7For example, the following cell based upon the solid electrolyte K2SO4 has been used to measure the equilibrium partial pressure of SOs 5 in an SO2-O2-SO3 gas mixture:

Pt[SO3(Pso~),O2(Po2)lK2SOalSO~(e~os),O~(P'o)lPtCell I (working)

(reference)

The half-cell reactions at the electrodes can be written as: 9 1

SOs(g) + ~O2(g) + 2e- = SOZ4-(electrolyte) 1

t

SO4Z-(electrolyte) = SO~(g) + ~ O 2 ( g ) + 2e-

[1]

[21

which leads to the virtual cell reaction: 1

1

,

SOs(g) + ~O/(g) = SO~(g) + ~02(g)

[3]

It has been indirectly demonstrated 5 that the ionic transport number, tio., is unity. Hence, the Nernst equation for the cell electromotive force is: RT P s o 3 " Po21/2 E = - - In t 2F

[4]

Pso3 "Pro21/2

MICHEL GAUTHIER is Project Leader, Hydro-Qutbec Research Institute (IREQ), 1800 Montee Sainte Julie, Varennes, Qutbec, Canada J0L 2P0. CHRISTOPHER W. BALE is Associate Professor, l~cole Polytechnique (University of Montreal), Box 6079, Montrtal, Qutbec, Canada H3C 3A7. Manuscript submitted February 18, 1981. METALLURGICAL TRANSACTIONS B

By defining P~o3 and Po2 at the reference electrodes it is possible to deduce the value of Pso3 at the working electrode from the measurements of the cell potential if Po2 is also known. If the oxygen potentials at both electrodes are fixed at the same value, Eq. [4] simplifies to: RT Pso3 E = - - In 2F P~os

[5]

The gaseous equilibrium may be replaced by a solid-gas equilibrium. 7 For example, a metal oxide-sulfate-gas electrode can be incorporated into the cell design as follows: PtISOs(Pso),MSO4,MOIK2SO4[SO~(Pgo),O~(P62)IPt (working) (reference) Cell II where the solid sulfate-oxide mixture forms a univariant equilibria within the range of Po: under investigation. The following chemical equilibrium is then attained at this electrode: MSO4(s) = MO(s) + SOs(g)

[6]

If MO and MSO4 are pure solids, the equilibrium constant of Eq. [6], KI, is the value of Pso~. IfPo2 is known, or ifPoz and P~: have the same value, the equilibrium constant K~ can thus be calculated from the emf measurements. In the present study, cell H has been used to measure emf's from which the thermodynamic equilibria of solid MgO/MgSO4, NiO/NiSO4, and MnzOs/MnSO4 mixtures over the temperature range 873 to 1173 K have been calculated. In each case the oxide is that which is the most stable in air (Po2 = 0.21 atm).

II.

EXPERIMENTAL

The sulfate electrochemical cell used in the present investigation is schematically illustrated in Figure