Why is sodium hydroxide not a primary standard in titrations?
Solid sodium hydroxide absorbs water quite quickly when exposed to air – it is hygroscopic. When you try to weigh it accurately, you will find that its mass slowly increases with time. The solid starts to glisten with moisture after a short while. Sodium hydroxide also reacts with carbon dioxide in the air, to form sodium carbonate. Thus it is difficult to obtain pure, and once made up, the solutions still absorb carbon dioxide. This can interfere with the endpoint in a titration. You may have noticed the white crust of sodium carbonate around the stoppers of some bottles of sodium hydroxide solution in your school labs.
