Measuring Volume by Aliquot Method
This method is similar to aliquot method for weighing but employs liquid rather than solid.
Preliminary step: Determine the minimum volume that can be measured without error. For a graduated measuring cylinder, the minimum volume that can be measured without error will be 20 % of the capacity.
Step-1: Select a measureable multiple of desired volume of the substance.
Step-2: Dilute with an inert diluent (solvent, water, ethanol) to obtain a dilution.
Step-3: Measure an aliquot of the dilution that will provide desired volume of the substance.
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Solved Problem: A formula calls for 0.5 ml of acetic acid. Using 10 ml graduated cylinder, how would you obtain the required amount of acetic acid by the aliquot method (use a multiple factor of 4)? Approach: Preliminary step: Determine the minimum volume that can be measured without error. $$\%Minimum \ volume \ measured = 10 \times 0.2 = 2 ml $$ Step-1: Select a measureable multiple (in this case 4) of desired volume of the substance. The formula requires 0.5 ml of acetic acid. The minimum volume that can be measured accurately is 2 ml. choose a multiple factor of 4 and measure out 0.5 X 4 = 2 ml of drug. Step-2: Dilute with an inert diluent to obtain a dilution. 2 ml is the minimum volume that can be measured. 2 X 4 = 8 ml of dilution 8 ml dilution – 2 ml of acetic acid = 6 ml of diluent. Dilute 2 ml of acetic acid with 6 ml of diluent to obtain a 8 ml dilution. Answer: Measure ¼ of dilution, or 2 ml of dilution, which will contain 0.5 ml of acetic acid. |