Why, when I titrate with sodium hydroxide (NaOH), do I sometimes see two equivalence points?
Sodium hydroxide (and potassium hydroxide) has a tendency to absorb carbon dioxide forming sodium carbonate, which also then reacts with any acid in the sample. This results in this double inflection point effect. Even freshly prepared sodium hydroxide solutions often show this effect. The reason for this is that the solid sodium hydroxide (often pellets) also absorbs carbon dioxide, forming a thin layer of carbonate on the surface. The only way to prepare fresh carbonate free sodium hydroxide solutions is to prewash the pellets or use fresh pellets and to use freshly boiled deionized (or distilled) water. Once such a solution has been prepared it should be protected from carbon dioxide by fitting an absorption tube filled with sodium hydroxide on a carrier. Whenever this double inflection point is apparent when titrating with sodium hydroxide (or potassium hydroxide) it is recommended that the solution be discarded and fresh titrant prepared.
