Conversion of Bicarbonate to Carbonate as a Function of Time and Temperature

Understanding the carbonate equilibrium and how it behaves in solid form at different temperatures is important as it occurs throughout nature in the formation of minerals; maintaining proper pH in the human body, cycling carbon dioxide in the atmosphere to the hydrosphere and other instances.  Sodium bicarbonate and sodium carbonate are also common chemicals studied in teaching laboratories for acid/base titrations and mono- versus poly-protic systems. Preparing these materials for use as primary standards often requires baking them at 105°C to remove atmospheric water.  The sodium carbonate and/or sodium bicarbonate are then either kept in a drying oven at 105°C or stored in desiccators, to preserve their anhydrous forms.  However, experimental evidence has shown that sodium bicarbonate slowly converts to sodium carbonate when kept at 105°C over time.  The goals of these experiments are to determine the minimum amount of time required to initiate the conversion, the minimum amount of time required for complete conversion, and the effect of decreasing temperature on conversion.  Using a box furnace, data is collected at a variety of different temperatures and times at standard atmospheric pressure.  The mass of carbonate and bicarbonate is derived by automated titration with standardized hydrochloric acid from the first equivalence point and second equivalence point, respectively.  The minimum amount of time required to initiate the conversion of bicarbonate to carbonate, held at 105°C, is found to be 30 minutes with the formation of 8.15% of carbonate. The minimum amount of time required  for almost complete conversion at 105°C is seven hours with 99.56% carbonate formed.  Decreasing the temperature slowed and eventually stopped the rate of conversion.  The maximum temperature at which there is negligible conversion over an extended period of time is 65°C, with only 1.05% difference between heated and unheated samples.  Knowing these conversation rates and temperatures will aid in the understanding of the carbonate/bicarbonate system, its contributions to nature and new experiments in teaching laboratories.