The type of Base (R'N) and its concentration affect teh overall reaction rate. Traditionally, pyridine was used as the base. However, because of its weak basicity, pyridine can not completely neutralize the alkylsulphurous acid intermediate. As a result, reaction (ROH + SO2 + R'N - [R'NH]SO3R) does not go to completion, the reaction is slow and the end point is not stable. Because of this lack of stability, the repeatability of the results is often very poor. In addition, pyridine has a noxious odor.