A simple way to make a dilution series
Robert L. Metzenberg
Department of Biology, California State University-Northridge, Northridge CA 91330
Fungal Genetics Newsletter 53: Pagination Pending
Often one needs to determine a suitable concentration of a previously untested nutrient or inhibitor to use in subsequent experiments. A series with relative concentrations of 100, 30, 10, 3, 1... gives a good estimate. However, making such a series by diluting from one tube to its neighbor requires different amounts of diluent for different tubes, as well as resetting the pipetting device at each transfer -- a tedious and mistake-prone process. Alternatively, one can make a series of three-fold dilutions. This is easy, but gives daunting relative numerical values for concentration: 100, 33.3, 11.1, 3.70, 1.23,.... I find it useful to make successive dilutions of 3.16-fold (the square root of 10) so that relative concentrations are 100, 31.6, 10, 3.16, 1,.... I prepare 3.16 ml. of the most concentrated solution, set up successive tubes with 2.16 ml. of diluent, and pipet 1.0 ml. from one tube to the next . I discard 1 ml. from the penultimate tube, saving the final tube for the zero-concentration control.
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