What is a Standard Solution?

Standard solutions or secondary standard solutions are solutions with a known concentration.

They are used in titrations to help determine the unknown concentration of an acid or base.

A standard solution is prepared from dissolving a substance called a primary standard.

A secondary standard solution is a solution whose concentration has been previously determined via a separate titration.

Primary Standards

A primary standard is a solid acid or base which meets the following four criteria:

1. Readily available in its purest form, with a precise formula.

2. Does not react with its surroundings (e.g. hygroscopic).

3. Has a relatively high molar mass.

4. Soluble in water.

Two examples of primary standards, which satisfy all four criteria include anhydrous sodium carbonate and oxalic acid dihydrate:

If a substance which does not meet all four criteria is used, its mass will not accurately represent the number of moles it contains. This will therefore affect the concentration of the standard solution.

Two substances which do not meet all four criteria, and are therefore not primary standards include sodium carbonate and sodium hydroxide:

Determining the Concentration of a Standard Solution

Preparing a standard solution involves measuring a specific amount of primary standard and dissolving it in water in a volumetric flask. The following calculations can be used to determine the concentration of a standard solution: