Explain why sodium hydroxide solution should not be left standing in a glass burette.
The concentration of a solution of sodium hydroxide was found by titration with a standard solution of hydrochloric acid.
Sodium hydroxide (NaOH) is a strong base that will react with many other elements and compounds.
Carbon dioxide (CO2) is partially soluble in water and when dissolved some of it combines with water to form carbonic acid.
CO2 + H2O --> H2CO3.
The sodium hydroxide will react with this carbonic acid to form sodium carbonate. You can see this by blowing through a straw into a solution of sodium hydroxide.
2 NaOH + H2CO3 --> Na2CO3 + 2 HOH
The net result is that over time the strength of the NaOH solution will decrease and no longer be able to accurately titrate an acid.