Electrochemical Studies in Glass: I, The System NiO‐Na ₂ Si ₂ O ₅

A solid electrolyte electrochemical cell of the type Pt|Ni:NiO a =1 ∥ZrO 2 +7.5% CaO∥Ni:NiO a <1 +glass|Pt was used to measure the activities of NiO in sodium disilicate glass from 750° to 1100°C. The data indicate a solubility varying from 11 mol% (5.0 wt%) at 800° to 20 mol% (9.3 wt%) at 1100°C. From the variation in NiO activity, the activity of sodium disilicate in glass solution was estimated; from these combined data partial molar free energies and entropies of solution of NiO and Na 2 Si 2 O 5 and free energies and entropies of mixing were calculated. A partial phase diagram for the system NiO‐Na 2 Si 2 O 5 proposed from solubility data indicates a eutectic at ∼12 mol% (5.3 wt%) NiO at 830°C.