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
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).
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)
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)
4. Calculate the total volume (B) of solution to be prepared.
5. Remove aliquot of solution and q.s. to total Rx volume with the appropriate
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