STANDARDIZATION OF SODIUM HYDROXIDE SOLUTION

Introduction & Background Information

In this lab you will precisely determine the concentration of a sodium hydroxide solution. Unknown concentrations of acids may be determined by titration with sodium hydroxide. Before those titrations with sodium hydroxide are attempted, the exact concentration of sodium hydroxide solution will need to be determined.  This will be done by conducting another titration, called standardization will be done. 

Standardization of NaOH

In titrations, sodium hydroxide (NaOH) is commonly used to determine the concentration of an unknown acid.  We say the NaOH solution is a standard solution.  Standardized solution are solutions where the exact concentration is known. Preparing a standardized solution of a particular concentration is actually somewhat problematic for two reasons:  First, solid NaOH absorbs a significant amount of water from the air.  Solutions that do this are said to be hygroscopic.  When making a NaOH solution and weighing solid hydroxide pellets it is very difficult to determine the exact mass of NaOH because the pellets absorb water from the air.  Secondly,  solutions of NaOH react with carbon dioxide from the air.  The overall reaction is:

CO2 (g) + 2 NaOH (aq)  ->  Na2CO3 + H2O

This reaction decreases the concentration of the OH- ions in the solution.  Unless the bottle is tightly sealed, the concentration of the OH- in solution will change on a daily basis. In order to have accurate concentrations of NaOH solutions, you must standardize the NaOH solution before you use it as a standard.