Reactive Recrystallization of Sodium Bicarbonate

The thermal decomposition of sodium bicarbonate, NaHCO3, in mixed solvents composed of ethylene glycol and water was found to occur solution mediated; i.e., bicarbonate ions (HCO3-) convert in solution to carbonate ions (CO3 2-), carbon dioxide, and water. The equilibrium of this reaction shifts with increasing temperature toward the bicarbonate decomposition. Above 90 (degree) C, the decomposition of the bicarbonate ions in solution results in the conversion (recrystallization) of solid NaHCO3(s) (nahcolite) in contact with the solution. This recrystallization of NaHCO3(s) at temperatures above 90 (degree) C is independent of the water content for the tested mixed solvents. Thus, slurries of solid sodium bicarbonate (NaHCO3(s)) above 90 (degree) C undergo a solution-mediated recrystallization via wegscheiderite (Na2CO3·3NaHCO3(s))-as an intermediately formed solid phase-to sodium carbonate anhydrate (Na2CO3(s)). The decomposition rates were measured and the kinetics fitted to the most probable mechanism, i.e., first order in concentration of the bicarbonate ion in solution. Additionally, the solubility of nahcolite (NaHCO3(s)) was determined for a range of mixed solvent compositions and temperatures.