Roles of Sodium Carbonate when Extracting Caffeine from Tea Leaves

A student failed to add sodium carbonate (Na₂CO₃) when extracting caffeine from tea leaves with hot water. Though, high yield of caffeine was produced. The crystals melted at temperatures between 202 and 214 degree Celsius. Explain.

This happened because sodium carbonate did not interfere with the caffeine content of the tea leaves. Sodium carbonate (which is a base) deprotonates the phenolic hydroxyl group (-OH) (acidic) of tannins rendering them anionic. Hence, making them highly soluble in water, but parsimoniously soluble in methylene chloride. So, if sodium carbonate is not added when extracting caffeine with hot water, the extracted caffeine would also contain tannins.

Additionally, sodium carbonate is added during the extraction to keep caffeine (an alkaloid which is an organic base)  in a free base form. It prevents caffeine from reacting with available acid available, which may lead to the formation cation. It reacts with a phenolic hydroxyl group of tannins thus maintaining them in the aqueous phase when the extraction (liquid-liquid extraction) is performed.