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.

The best way of doing this is to use a primary standard, which is a solid acid at room temperature that can be weighed. A good primary standard should not absorb anything from the air. It should have a high molar mass, so you can weigh out a reasonable quantity of it without getting too many moles so your titration does not take too long. In this lab, you will calibrate the molarity of a sodium hydroxide solution with an acid called potassium hydrogen phthalate (KHP). Potassium hydrogen phthalate chemical formula is KHC8H4O4. KHP is ideal because it is solid at room temperature, it is stable or non-reactive in air, it has a single acidic hydrogen and has a molar mass of 204.22 g/mol.

Technique: Performing a Titration

Be sure to also review the titration technique page.

Animations & Simulations to Practice for Lab

Simulated Titration to Determine the Molarity of a Sodium Hydroxide Solution - produced by Glencoe Publishing