Making up a standard solution

A standard solution is a solution where know the exact concentration of it. Standard solutions are required to be used in titrations (volumetric analysis).

You will be required to make up a primary standard by accurately weighing the correct mass of a pure substance. You then make this up, by dissolving it in distilled water, to an accurately known and planned volume. You will then calculate the concentration of this primary standard.

Substances used as primary standards must:

• be available in a pure form

• not absorb water from the air

• be unreactive/stable when stored, and

• have a high molar mass so weighing errors are less significant.

Once you have made up your primary standard solution you can then titrate this against a secondary standard solution to accurately determine its concentration by titration.

Example - preparing 250mL standard solution of 0.200 molL^-1 sodium carbonate solution

5.57 g of anhydrous sodium carbonate, Na₂CO₃, was weighed on a 2 decimal place electronic balance using a weighing boat. The 5.57g was carefully transferred to a clean and dry 250 mL glass beaker. Approximately 150 mL of distilled (deionised) water was added to the solid in the beaker and it was stirred with a glass stirring rod to dissolve the solid. The dissolved solution was poured into a clean and dry 250mL volumetric flask using a clean and dry funnel. A small amount of distilled water was used to rinse out the beaker and the funnel into the volumetric flask. The funnel was removed and the solution was very carefully made up to the 250 mL mark on the volumetric flask using distilled water, remembering to read the volume from the bottom of the meniscus.

The concentration of the sodium carbonate was calculated as shown below: