BIOL 1406

PreLab 2.5

When do I use the serial dilution technique instead of the parallel dilution technique?

Another way to make dilutions is to use some of your existing stock solution to make a dilute solution, then use some of the dilute solution to make an even more dilute solution, then use some of that solution to make an even more dilute solution, and so on.  This procedure is called the serial dilution technique.

There are two situations where serial dilutions should be used rather than parallel dilutions:

FIRST: Use a serial dilution when you need several solutions of the same solute and there is a constant dilution factor.  For example, suppose you have a 2 M stock solution of KMnO4 and you want to make 15 mL of each of the following concentrations of KMnO4: 0.2 M, 20 mM, 2 mM, and 0.2 mM.  Notice that the concentration of each solution is 1/10th the concentration of the previous solution in the series.  The factor by which each solution is diluted compared to the previous one is called the dilution factor.

To calculate the dilution factor for each dilution, divide the concentration of the starting solution by the concentration of the diluted solution.  For example, for the first dilution 2 M divided by 0.2 M equals 10.  For the second dilution, 0.2 M divided by 20 mM equals 10.  For the third dilution, 20 mM divided by
2 mM equals 10.  And for the fourth dilution 2 mM divided by 0.2 mM equals 10.  Therefore, this series has a constant dilution factor of 10.

SECOND: Also use a serial dilution when the dilution factor is so large that the amount of stock solution needed to make the dilution in one step (using the formula C1V1 = C2V2) is too small to measure accurately. Remember that the smallest volume you can measure with the micropipettors is 2 μL.