According to medical dictionaries, an aliquot is any portion that bears a known quantitative relationship to a whole, or to other portions of the same whole.
Therefore, an aliquot can be defined as a dose of medication that is derived as a portion of a larger dose. It can be used to measure out a more accurate dose than trying to achieve a quarter or eighth of a tablet.
The smallest size tablet of metoprolol is 50 mg. If you need to administer metoprolol 12.5 mg, dose accuracy could become an issue if you tried to break the tablet into 4 quarters. A more accurate method would be to make an aliquot. This can be done by crushing the metoprolol 50mg tablet and dissolving it in 10 mL of water, making a solution of 5 mg/mL solution. Thus, 12.5 mg can be given as an aliquot of 12.5 mg/2.5 mL.
Rifampin is supplied as a 300 mg capsule. You wish to administer 150 mg per NG. Empty the contents of the capsule into 10 mL of water to provide a 3 mg/mL solution. Thus, you have created an aliquot of 150 mg/5 mL.
Remember to try and stir the solution well so that you draw up a true mixture. The remaining solutions should be discarded, as it will have an unknown stability compared to the original dosage form.