A sodium sensor to measure a concentration of sodium methylate in methanol. The sensor assembly includes a solid alkali ion conducting membrane, a reference electrode, and a measurement electrode. The solid alkali ion conducting membrane transports ions between two alkali-containing solutions, including an aqueous solution and a non-aqueous solution. The reference electrode is at least partially within an alkali halide solution of a known alkali concentration on a first side of the solid alkali ion conducting membrane. The measurement electrode is on a second side of the solid alkali ion conducting membrane. The measurement electrode exhibits a measurable electrical characteristic corresponding to a measured alkali concentration within the non-aqueous solution, to which the measurement electrode is exposed.