Dilution series

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A dilution series is the set of solutions that have been prepared from a concentrated starting solution by dilution for a specific purpose. The solutions referred to as dilution levels differ in their content (for example in the molar concentration or the mass concentration ).

In the dilution series, a distinction is made between those in which the individual dilution stages are produced by diluting the previous dilution stage (continued dilution, serial dilution), and those in which all dilution stages are produced directly from the starting solution (parallel dilution).

Applications

  • Dilution series are often required for the calibration of measuring instruments, for example in chemical analysis . These are called the standard series. Occasionally the term calibration series was also used, but the calibration is reserved for the responsible offices and test centers.
  • In the case of categorical detection methods (test with yes or no as a result), dilution series are used to determine the limit dilution (last dilution above the detection limit ) and to derive the concentration , e.g. B. a titer .
  • Dilution series are also used in microbiology to count particles such as cells or viruses, e.g. B. in plate counting . Their concentration, i.e. their number in a solution, is reduced by the dilution series. This makes it easier to count them in higher dilution levels, and the result is then calculated back to the original concentration.
  • By dilution series, particles such as B. cells or viruses isolated. In the course of a limiting dilution cloning , clonal cultures are made possible in a subsequent propagation .

Dilution series through continued dilution (geometric dilution)

Example of a dilution series. Concentration factor of the amount of substance after n steps:

For example, if you take 10 ml of a starting solution and mix it with solvent (for example water ) so that 100 ml are produced, this 1st dilution stage has only a tenth of the concentration of the starting solution. If you take out 10 ml of the first dilution stage and dilute again to 100 ml, the second dilution stage has only one hundredth of the concentration of the starting solution. If this solution is diluted in the same way, a third dilution stage is created with a thousandth of the concentration of the starting solution. If the molar concentration of the starting solution has, for example, 2 mol / L, the concentrations of the solutions in the dilution series are hereby 0.2 mol / L (1st dilution stage); 0.02 mol / l (2nd dilution stage); 0.002 mol / L (3rd dilution stage). If the concentrations of the individual dilution levels differ by a factor of 10, as in this case, one also speaks of a decimal dilution series. In addition to the decimal dilution series that are frequently encountered, there are also dilution series with other dilution factors.

Dilution series with uniform distribution of the dilution levels (arithmetic dilution)

With this type of dilution series, the contents of the dilution levels are evenly distributed over a content range. For example, the contents of a dilution series with a total of 5 dilution levels can evenly cover the mass concentration range between 0 g / L and 1 g / L. The mass concentrations of the individual dilution levels are then 0.2 g / L, 0.4 g / L, 0.6 g / L, 0.8 g / L and 1.0 g / L. With such dilution series, all dilutions are usually made from one and the same starting solution and not by continued dilution.

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