The Pharmaceutics and Compounding Laboratory
Triturations and Aliquots

Liquid Aliquot Method

This technique is similar to the trituration method but employs liquid rather than solid diluents. It is used only when the product to be dispensed is a liquid. A weighable quantity of drug is dissolved in a calculated volume of a suitable solvent, usually water or ethanol. Then, an aliquot of the solution containing the desired amount of drug is removed. Both the total volume of solution and the aliquot volume should be such that they are easily and accurately measured (preferably whole number multiples of 5 ml).

(A) Weight of drug in volume = (C) Weight of drug in aliquot (B) Volume of solution (D) Aliquot volume

e.g. Prepare 100 ml of a solution containing 0.2 mg/ml clonidine.

(Need 100 ml x 0.2 mg/ml = 20 mg clonidine)

1. Select a suitable solvent (based on drug solubility and stability, route of administration, and potential toxicity of the solvent).

    Select water (or possibly ethanol)

2. Select the aliquot volume (D) in which the desired amount of drug(C) will be contained. This establishes the concentration of the solution to be prepared. Volume must be large enough that concentration does not exceed the solubility of the drug, but small enough that it does not exceed the total volume of the prescription.

(Hint: check Remington's or Merck Index for solubility data, then pick a concentration that is less than this solubility)

    Select 5 ml; then concentration = 20 mg/5 ml = 4 mg/ml

3. Select weight of drug (120 mg) to prepare the solution. Whenever practical, use the least weighable quantity so as to minimize waste. (This is A in the proportion)

    Select 120 mg

4. Calculate the total volume (B) of solution to be prepared.

    120 mg clonidine = 20 mg clonidine x ml solution 5 ml aliquot x = 30 ml

5. Remove aliquot of solution and q.s. to total Rx volume with the appropriate vehicle.

Steps 1-4 are simply the technique for weighing the drug. Step 5 completes the prescription compounding. Hence, we would:

  • Weigh 120 mg clonidine
  • Dissolve the clonidine in sufficient water to make 30 ml (Why not dissolve in 30 ml H2O?)
  • Remove 5 ml aliquot
  • q.s. with H2O to 100 ml to complete the prescription