Sodium molybdate—a possible alternate additive for sodium dichromate in the electrolytic production of sodium chlorate

Sodium dichromate is commonly used in sodium chlorate production to maintain high current efficiency; however, it is also a well documented carcinogen. To reduce the environmental impact, identification of a suitable alternative with similar buffering characteristics to dichromate and without adverse effect on the electrolytic performance of sodium chlorate production is important; sodium molybdate is a good candidate. Molybdate ion and its conjugated acid work as a buffer pair at pH 5–6, a lower and slightly narrower pH window than the typical buffer region of dichromate. Nonetheless, the molybdate buffer works effectively during the electrolytic process by maintaining pH at 5.9. Although the use of molybdate buffer will lower the overpotential of hydrogen evolution reaction (HER) by 100 mV, the average off-gas oxygen content is noticeably compromised at 3.6–4.6%, measured using a pilot cell operated at 3 kA m⁻²and 80 °C during a 3-day trial. The resulting current efficiency of 91 92% is significantly lower than when dichromate is employed as the process additive (> 96%). Mixtures of different dichromate and molybdate ratio were also investigated in terms of the resulting cathode surface potential.