Serial dilutions

Serial dilutions are useful when we want to obtain a very diluted solution from very concentrated stock solutions. For example, if we want to obtain 10 ml of 1 mM solution from a 1M stock solution

By direct dilution:

M₁V₁ = M₂V₂

V₁ = M₂V₂ / M₁

V₁ = (10 ml * 1/1000 M)/1 M

V₁ = 0.01 ml

Water to add: 10 ml - 0.01 ml = 9.99 ml

In these cases, direct dilution is not very helpful because it would be difficult to pipette exactly 9.99 ml.

By serial dilution

Whereas if it was done by serial dilution, we first look at the final volume and concentrations needed, then determine the dilution factor.

So if we need 10 ml of a 1 mM solution, our dilution factor is:

Dilution factor = 1 / (final concentration / initial concentration) = 1 / (0.001M / 1M) = 1000X

We can split up 1000X into multiple factors: 10 X 10 X 10. This means we can set up 3 tubes to perform the serial dilution. When it is a 10X dilution, it means that it is 1 part of the solution, 9 parts of the diluent (usually it is water). Similarly, for a 15X dilution, it will be 1 part of the solution, 14 parts of the diluent.

Figure 1: Serial dilution. 1 mL of stock is transferred to the second tube that contains 9 mL of diluent (e.g water). Then 1 mL (aliquot) of the second tube is taken and transferred to the third tube. The process of transferring aliquot is repeated until the desired dilution concentration is achieved.

Although serial dilution requires more preparation, it will give more accurate dilutions, provided that your technique is precise.