If you’re new to a lab environment, you may quickly notice that certain solutions aren’t described in terms of the absolute concentrations of their components. Rather, they might have something like 50X, 10X, 2X, etc written on them. What does that mean and why do people do it?
Well, this nomenclature is just a shorthand that tells you that the solution is X times more concentrated than the final solution you should use for whatever procedure you are doing. In other words, any volume of the solution you use will have all its components at 50 times the amount they should be. Why use this over the absolute concentration? Because then you don’t have to remember what the absolute concentration of your final solution should be and you don’t have to calculate how much to dilute it by. All this information is contained in this concentration factor. Let’s look at an example of this.
Say we have a 50X TAE stock solution and we want to make 1X TAE solution to use for running our agarose DNA gel. How much water do we need to add to make it 1X? It’s all right there with the number 50. It’s currently 50 times more concentrated than it should be so how do we bring it down to 1X? We increase the volume by 50 times (by adding water) and thus dilute the components in the TAE by a factor of 50. So, for every 1 mL of 50X TAE, we would add 49 mL of water to make 50 mL of the final 1X TAE.
Another example would be antibodies that typically come as small aliquots of 100 uL or less. A general rule of thumb is that for western blotting, these stock antibodies come at roughly 1000x. So for 10 mL of antibody staining solution you would add only 10 uL of antibody stock solution.
Another benefit of describing solutions in this way it’s very easy to gauge how much final solution you can make from a given stock. If I have 1 liter of a 50X solution, I know right away I can ultimately make 50 times more (50 liters) of the final 1x solution.
