Aliquot Method in Weighing and Measuring Volume

Aliquot Method for Weighing

• Preliminary Step: Determine the minimum weigh-able quantity for the desired precision on the balance.
• Step 1: Multiple Selection: Choose a multiple of the desired quantity allowing accurate weighing. For example, if 5 mg is needed and the minimum weigh-able amount is 120 mg, use a multiple of 25 (25 x 5 mg = 125 mg). Larger multiples reduce weighing errors (e.g., multiplying by 30 reduces error to 4%).
• Step 2: Dilution: Dilute the selected multiple amount with an inert substance (e.g., lactose). The total mixture weight needs to be divisible by the multiple factor used (e.g., if 125 mg is weighed as the multiple, then a total of 3000 mg will need to be prepared.
• Step 3: Aliquot Measurement: Weigh a portion (aliquot) of the drug-diluent mixture equal to 1/multiple of the total mixture weight. This portion will contain the desired amount of drug.

Aliquot Method for Measuring Volume

• Principle: Similar to the weighing method, precise volume measurement using graduated cylinders.
• Difference: Instead of "least weighable quantity," there's a "least measurable volume" for the graduated cylinder.
• Step 1: Multiple Selection: Select a multiple of the desired volume that can be accurately measured (e.g., if 0.5 mL is needed, and the smallest division is 1 mL, use a multiple of 10).
• Step 2: Dilution: Dilute the selected multiple amount with an appropriate compatible diluent (e.g., a solvent). The total volume must be evenly divisible by the chosen multiple.
• Step 3: Aliquot Measurement: Measure a portion of the diluted solution (aliquot) equal to 1/multiple of the total volume. This aliquot contains the desired volume.

Example: Hydrochloric Acid (0.5 mL)

• Method: If using a 10 mL graduated cylinder with 1 mL divisions, a 10 mL multiple would be used. Dilute 10 mL of HCl. The aliquot will be 0.1 mL of the 10 mL diluted solution, providing the desired 0.5 mL of HCl.

Example: 20 mg Drug Substance

• Needed: 20 mg drug substance with 5% accuracy.
• Balance: The balance's smallest repeatable measure (SR) is 6 mg.
• Solution: Use a multiple of 20 in relation to the minimal weight-able quantity. Determine the total mixture quantity to be weighed in multiple of the minimum weight possible in the balance (e.g., 120 mg for 6 mg SR). Weigh out a multiple that will provide 20 mg when a part of the total mixture is weighed out accurately. If a multiple of 7 is used, then 840 mg must be weighed. The aliquot will then be 120 mg in this case.

This quiz covers the aliquot method used in weighing and measuring volumes with precision. It includes steps for selecting multiples, dilution, and aliquot measurement, aimed at reducing weighing errors. Perfect for students studying pharmaceutical calculations and laboratory techniques.