Preparing a % v/v Product
When you make up a percent volume in volume solution, you measure out the required volume of the component and then MAKE THE VOLUME UP TO THE TOTAL REQUIRED. Remember, percent volume in volume is the millilitres of component in 100 millilitres of the END solution.
You cannot work out in advance what volume of the solvent (etc) you will need. This is because when some liquids mix, they contract and the overall volume is less than the sum of the two individual volumes.
To prepare 100 cm3 of 38 % v/v Ethanol.
Find what volume of Ethanol you need.
100 cm3 of 38 % v/v solution must contain 38 cm3 of Ethanol.
Measure out 38 cm3 of Absolute Ethanol. Slowly add the Ethanol to water.
Slowly add the Ethanol to water. Wait for the solution
to finish contracting and to cool to room temperature.
to finish contracting and to cool to room temperature.
Top up the solution to 100 cm3 with water.
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Mixing Ethanol and Water
The dissolution of Ethanol in water is:
- an exothermic reaction
- accompanied by a significant contraction in the total volume.
The heat produced during the exothermic reaction will affect the volume of the solution.
Preparing a solution of Ethanol in water
After you have mixed the Ethanol and water together, you must:
- wait for the solution to cool to room temperature
- wait for the solution to finish contracting.
When both of these have occurred, you can top up the solution to the volume required. You should then check that the temperature and volume remain the same.
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