| Summary: | Chalcogenide glasses are widely used as sensitive membranes in the chemical sensors for heavy metal ions detection. The lack of research work on sodium ion-selective electrodes (Na<sup>+</sup>-ISEs) based on chalcogenide glasses is due to the high hygroscopicity of alkali dopes chalcogenides. However, sodium halide doped Ga<sub>2</sub>S<sub>3</sub>-GeS<sub>2</sub> glasses are more chemically stable in water and could be used as Na<sup>+</sup>-sensitive membranes for the ISEs. In this work we have studied the physico-chemical properties of mixed cation (AgI)<i><sub>x</sub></i>(NaI)<sub>30-<i>x</i></sub>(Ga<sub>2</sub>S<sub>3</sub>)<sub>26</sub>(GeS<sub>2</sub>)<sub>44</sub> chalcogenide glasses (where <i>x</i> = 0, 7.5, 15, 22.5 and 30 mol.% AgI) using density, DSC, and conductivity measurements. The mixed cation effect with shallow conductivity and glass transition temperature minimum was found for silver fraction r = Ag/(Na + Ag) ≈ 0.5. Silver addition decreases the moisture resistance of the glasses. Only (AgI)<sub>22.5</sub>(NaI)<sub>7.5</sub>(Ga<sub>2</sub>S<sub>3</sub>)<sub>26</sub>(GeS<sub>2</sub>)<sub>44</sub> composition was suitable for chemical sensors application, contrary to the single cation sodium halide doped Ga<sub>2</sub>S<sub>3</sub>-GeS<sub>2</sub> glasses, where 15 mol.% sodium-halide-containing vitreous alloys are stable in water solutions. The analytical parameters of (NaCl)<sub>15</sub>(Ga<sub>2</sub>S<sub>3</sub>)<sub>23</sub>(GeS<sub>2</sub>)<sub>62</sub>; (NaI)<sub>15</sub>(Ga<sub>2</sub>S<sub>3</sub>)<sub>23</sub>(GeS<sub>2</sub>)<sub>62</sub> and (AgI)<sub>22.5</sub>(NaI)<sub>7.5</sub>(Ga<sub>2</sub>S<sub>3</sub>)<sub>26</sub>(GeS<sub>2</sub>)<sub>44</sub> glass compositions as active membranes in Na<sup>+</sup>-ISEs were investigated, including detection limit, sensitivity, linearity, ionic selectivity (in the presence of K<sup>+</sup>, Mg<sup>2+</sup>, Ca<sup>2+</sup>, Ba<sup>2+</sup>, and Zn<sup>2+</sup> interfering cations), reproducibility and optimal pH-range.
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