Sodium methylate reacts with humidity, yielding methanol and sodium hydroxide. Hence, in case you are using a water-spray scrubber to dissolve evaporated methanol, it would be of upmost importance to avoid any possible water vapour backflow into the storage tank head-space. It should help to constantly flush the storage tank head space with dry nitrogen while maintaining enough back pressure to avoid inert gas reflux. A suitable gas moisture absorber could possibly be inserted before the scrubber for the same purpose. Alternatively, you may consider to substitute the scrubber by a condenser, perhaps of the cold-finger or cold-trap kind. Keeping the tank cold might help to reduce evaporation. Also to be considered, is if constantly venting the tank head-space (ultimately to the atmosphere) can be somehow avoided; yet without compromising safety and compliance with mandatory safety regulations. Being that the case, a pressure relief safety device should be considered. If the sodium methylate is kept dissolved in methanol, the methanol should be anhydrous. Besides avoiding moisture, admission of carbon dioxide, oxygen, or air, should be also prevented. The storage tank should be kept protected from heat sources, and any possible ignition sources, including electrostatic charging, in dry vented area. Materials contacting the liquid solution or its vapour should be properly selected for corrosion resistance to sodium methylate, methanol, and sodium hydroxide. Generic safety guidelines for the storage and handling of corrosive and flammable solutions should be considered.