A serial dilution is a step-wise series of dilutions, where the dilution factor stays the same for each step. The purpose of a serial dilution is to estimate the initial concentration of a sample, or to obtain the desired concentration of a reagent, chemical or compound.

Serial dilutions are routinely used in microbiology to estimate the number of microorganisms in a sample with an unknown concentration. Imagine that you have a bacterial culture tube, and want to know how many bacterial cells are growing in it. As every milliliter of media could contain a billion bacteria, it is impossible to count them in such high concentrations.

The solution is to perform a serial dilution. After diluting 1 milliliter of the sample with 1 billion bacterial cells per milliliter in 9 milliliters of diluent, you will get a concentration of 100 million bacterial cells per milliliter. Diluting 1 milliliter of this dilution in 9 milliliters of diluent will allow you to obtain a bacterial concentration of 10 million cells per milliliter, and so on. This means that you will get to a manageable concentration of 100 bacterial cells per milliliter in 7 dilution steps.

Here's how performing a serial dilution works:

First of all, you need to choose the proper diluent for the substance that you want to dilute. Many compounds can be mixed with distilled water, but for bacteria or cells, for example, a fresh culture medium should be used.

Once you have determined the diluent, fill the tubes that you will use to set up the serial dilution with the desired amount of diluent, for example, 9 ml.

Ensure that the sample, reagent, chemical or compound that you want to dilute is well mixed, then transfer a defined volume, for example, 1 ml, to the first tube.

Thoroughly mix the first dilution, then aspirate the same transfer volume that you used in the previous step and dispense it into the second tube.

Extend the process of mixing the most recently obtained dilution and transferring some of it to the next tube, until you get to the end of your serial dilution experiment.

Your last tube will contain more liquid than all the previous dilutions. For some applications, this is not an issue at all. In some cases, however, the liquid volume in every tube needs to be equal. In this case, aspirate the transfer volume from the last tube and discard it.

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