Why is pyridine used in Karl Fischer titration?
Pyridine (or, better, imidazole) prevents the solution to leave the range of pH 5,5 to 8, where the reaction is fast. If an alcohol is present, the SO2 tends to form an acid sulfuric acid monoester (SO2 is sulfurous acid anhydride!) and a base like pyridine will form a neutral salt of it not tilting the pH. Alcohol but is present in big amounts, for the iodine will come in methanolic solution. The methylsulfite anion will undergo the reaction in the same way as SO2 in theory.