The electrochemical determination of extremely low sodium activities may be affected by electronic transference through the electrolyte as far as potentiometric measurements using a cell with a sodium ion conducting solid electrolyte are concerned. Until now, this source of error has been ignored in the literature. Therefore, a new experimental approach has been developed which is based on the reproducible transformation of the sodium activity into an oxygen potential. The latter is measured by means of an oxygen concentration cell under conditions that definitely exclude any impact of electronic transference. The feasibility of the measuring technique has been demonstrated for the heterogeneous phase system NaSiyO2y+0.5/SiO2 ( y = 1.5). As a result, the standard Gibbs energy of formation of NaSi1.5O3.5 has been obtained to be (350- 800degreesC) Delta(f)Gdegrees(NaSi1.5O3.5) ( kJ/ mol) = 1697.787 +/- 24.16 + (339.512 +/- 26.69) x 10(-3) T/ K which appears reasonable from a comparison with data known in the literature on Na2SiO3. (C) 2004 The Electrochemical Society.