How to Dilute a Stock Solution

How to Dilute a Stock Solution


A. Why dilute? 

Here are two situations that arise repeatedly in molecular biology labs:
1. You have a stock solution of some compound,  let’s say an antibiotic, and you want to add the
compound to growth medium, at a much smaller concentration than the stock solution.
2. You have a tube of very concentrated bacteria, perhaps a billion cells per milliliter. You want to put
a few hundred of them on a petri plate, so that the colonies that arise will be easily distinguishable.
In both cases, the way out of the problem is to dilute the original solution. If you work in a lab, you need to
know how to do this.

B. Methods of calculating dilutions 

1. DILUTION FACTOR METHOD (fast, but requires inspiration): First, figure out the factor by which the
original solution must be diluted. Second, divide the final volume of the desired solution by that
factor, yielding the volume required of the original solution.
EXAMPLE: Suppose you need to make a 3 ml solution of growth medium supplemented with 50 µM of
the antibiotic ampicillin from a stock solution of 5 mM ampicillin. The dilution factor is :
  (5 mM) / (50 µM) = (5000 µM) / 50 µM) = 100
 so you need to dilute:
   (3 ml) / 100 = (3000 µl) / 100 = 30 µl
 of the stock solution to a final volume of 3 ml.