A process for electrochemical preparation of sodium alkoxide is performed in an electrolysis cell having three chambers, wherein the middle chamber is separated from the cathode chamber by a solid-state electrolyte permeable to sodium ions, and from the anode chamber by a diffusion barrier. The geometry of the electrolysis cell protects the solid-state electrolyte permeable to sodium ions from acidic destruction by the pH of the anolyte that falls in the course of electrolysis. The anolyte used in the process is a brine also comprising carbonates and/or hydrogencarbonates, as well as NaCl. The process solves the problem that CO2 from these carbonates and/or hydrogencarbonates forms in the electrolysis cell during the electrolysis of this brine obtained from pretreatment. The process prevents the formation of a gas bubble in the electrolysis cell that disrupts electrolysis and reduces the contamination of the chlorine with CO2.