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Adapted and significantly modified by JMB (2008) from C.W.Brady. ÒDilutions and dilutioncalculations,Ó Unpublished.University of Wisconsin, Whitewater
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Bacteria are often present in such huge numbers that theycan be difficult to count. Yetaccurate counts are necessary for a variety of reasons like, for instance,assessing the quality of water or the safety of food. There are a number of counting techniques but most rely onthe dilution of the sample to reduce the bacterial numbers down to a quantitythat can be counted accurately.
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Dilution requires the thorough mixing of a small,accurately measured sample with a large volume of sterile water, saline orother appropriate liquid called the diluent or a dilution blank. Accurate dilutions of a sample areobtained through the use of pipettes and the dilution blanks are preciselymeasured when prepared before the measuring experiment. For ease ofcalculation, dilutions are done in multiples of 10 or 100.
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A single dilution is calculated as follows:
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Dilution = volume of the sample
total volumeof the sample + diluent volume
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For example the dilution of 1 mL into 9 mL equals:
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1 which is the same as 1 which is written 1/10 or 10-1
1+910
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This can be called, Òa one to ten dilution.Ó
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When doing very high dilutions (like 1/10,000 or1/1,000,000), it is more accurate to do the dilution in a series of smallerdilutions rather than in one giant dilution. This is called a dilution series or a serial dilution.
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In a serial dilution, the final total dilution is aproduct of each individual dilution in the series. Thus, a series of 5, Òone toten dilutions equals Òa one to one hundred thousandÓ dilution:
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(1/10)x (1/10) x (1/10) x (1/10) x (1/10) = 1/100,000 = 10-5 dilution
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A typical dilution series is shown below:
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Practice doing the calculations in the followingexamples:
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