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Appendix 7. Dilutions

There are situations when practicing the methods in this manual when you will be required to prepare a final volume of a diluted solution from a concentrated stock solution. This will require calculating the volume of the stock solution and the volume of diluent to prepare the diluted solution. These volumes can be calculated from the ratio of the concentration of the stock solution and the concentration of the diluted solution.

A useful formula for calculating dilution volumes is C1V1 = C2V2

C1 = Concentration of stock solution

V1 = Volume of stock solution

C2 = Concentration of diluted solution

V2 = Volume of diluted solution

Make sure you recognize the units involved.

For C1 and C2 the units of concentration (amount/volume) must be the same.

For V1 and V2 the units of volume must be the same.

This formula can be applied to all situations requiring dilution of a stock solution.

Worked example

This example is taken from a recent I-2 Newcastle disease vaccine trial carried out at the John Francis Virology Laboratory.

Aliquots of I-2 ND vaccine stock were stored at -70°C. One aliquot had been removed and titrated and had an Infectivity Titre of 109.3 EID50 per mL.

On the day of vaccination, a second aliquot of the vaccine stock was removed and thawed to prepare diluted vaccine. Calculate the volume of vaccine stock and diluent required to prepare 5 mL of diluted vaccine to give 106 EID50 in a volume of 30 µL (0.03 mL) to be delivered by eye-drop.

C1 = 109.3 EID50/mL

V1 = ?

C2 = 106 EID50/0.03 mL

V2 = 5 mL

C1V1 = C2V2

V1 = (C2V2) ÷ C1

V1 = (106 EID50/0.03 mL × 5 mL) ÷ 109.3 EID50/mL = 0.084 mL = 84 µL

To prepare the diluted vaccine, measure 5 mL of diluent. Use a micropipette to remove 84 µL of diluent and discard. Add 84 µL of undiluted vaccine to the diluent. Mix well.

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