How to Calculate Aliquot

How to Calculate Aliquot
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An aliquot is a factor of a whole amount, meaning that when you divide the factor into the amount, there is no remainder. In the chemical and pharmaceutical industries, the aliquot method refers to measuring out a small amount of a chemical or drug by dividing up, or diluting, a larger amount. You calculate aliquots when the dose you need is smaller than the minimum weighable quantity (MWQ) of the scale you are using, which is based the scale's sensitivity. By industry standards, pharmaceutical balances must have a minimum accuracy of 95 percent, a fact used in calculating aliquot.

    Calculate the scale's MWQ, which is equal to its sensitivity divided by its inaccuracy. For example, the MWQ for a 95 percent accurate scale that is sensitive down to 6 milligrams (mg) is 6 / (1 - 0.95), or 120 mg.

    Find the smallest multiplication factor for an individual drug dose by dividing the dose into the MWQ. For example, suppose you need to create five doses of 20 mg each. The factor for a 20 mg dose is 120/20, or 6.

    Figure how much diluent -- an inert filler such as milk powder -- to weigh by subtracting the drug dose from the MWQ and then multiplying by the smallest multiplication factor. In the example, the amount of diluent to weigh equals ((120 - 20) x 6), or 600 mg of diluent to be mixed with 120 mg of the drug. This produces six doses, but since you need only five, you would need to discard one dose.


    • You might have to waste some materials when the amount of drug you need is less than the scale's MWQ. In the example, you need only 100 mg of the drug -- five doses of 20 mg each-- but since the MWQ is 120 mg, you must make six 20-mg doses instead of five.


    • If you are using a particular scale for the first time, verify its sensitivity so that you use the correct MWQ. In the example, the scale's sensitivity is 6 mg. If you assumed an incorrect sensitivity, you would create the wrong drug doses.