# Serial Dilution

Many procedures performed in modern biology and chemistry laboratories require sets of solutions that cover a range of concentration*s. These include quantifying the number of bacteria in a sample using plate counts and development of standard curves for quantitative colorimetric, radiometric and enzymatic assays. Sets of solutions over a range of concentrations are prepared via serial dilution.

 Container 1 2 3 4 5 Number of Particles (n) Volume (ml) Concentration (n/ml)

To perform a serial dilution, a small amount of a well-mixed solution is transferred into a new container and additional water or other solvent is added to dilute the original solution. The diluted sample is then used as the base solution to make an additional dilution. Doing this several times results in a range of concentrations.

The initial concentration and target range needed for a given assay determines the size and number of dilution steps required. Often, serial dilutions are performed in steps of 10 or 100. They are described as ratios of the original and final concentrations. For example, a 1:10 dilution is a mixture of one part of a solution and nine parts of additional solvent. To make a 1:100 dilution, one part of the solution is mixed with 99 parts of additional solvent.

Mixing 100 L of a stock solution with 900 µL of water makes a 1:10 dilution. The final volume of the diluted sample is 1000 µL (1 mL) and the concentration is 1/10 that of the original solution. This is commonly referred to as a 10x dilution.

The illustration above follows the relationship between volume of solvent, number of molecules of solute and concentration of a solution over a set of 4 dilutions. The concentration can be tracked in M which is a common unit for chemistry, or particles per ml, which is common when diluting bacterial cultures to low concentrations. With molar concentrations it is safe to assume that the solute is evenly dispersed through the solution such that the concentrations change predictably with each dilution. With particles, like bacterial cells, the solution becomes patchy at low concentrations and actual concentration can diverge from the expected concentration.

Test your understanding with the serial dilution practice problems

Video Overview

## Comments

### Dilution animation

I love that the actual number of cells in the tube is shown, as it clearly illustrates random variation.

### Solution vs suspensions

Microbiologists like it.  People who work with homogenous chemical solutions don't seem to care for it as much.