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 the development of standard curves for quantitative colorimetric, radiometric, and enzymatic assays. Scientists perform serial dilution* to create these sets of solutions that cover a range of concentrations.
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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 determines the size and number of dilution steps required. Serial dilutions are often performed in steps of 10 or 100. They are described as ratios of the initial and final concentrations. For example, a 1:10 dilution is a mixture of one part of a solution and nine parts fresh solvent. For a 1:100 dilution, one part of the solution is mixed with 99 parts new solvent.
Mixing 100 µL 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. A 1:10 dilution is also called a 10x dilution.
The illustration above follows the relationship between the
- Volume of solvent
- Number of molecules of solute
- Concentration of a solution
Over a set of 4 dilutions. The concentration commonly reported in Molarity (M) or particles per ml (ppm). Molarity is common for chemical applications. Units like ppm are more common within microbiology when diluting bacterial cultures to low concentrations. With molar concentrations, it is safe to assume that the solute is well mixed within the solution so that the concentrations change predictably with each dilution. With particles, like bacterial cells, the solution becomes patchy at low concentrations allowing actual ppm values to diverge from expected values.
Test your understanding with the serial dilution practice problems
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