# need help - (Sep/30/2009 )

If I have a 5ug stock (100ng/ul) of DNA at 50ul original volume, and I add in 50ul water, it will dilute the original stock concentration down. What is the new concentration after dilution?

Please how to solve this or what is the formula

-claritylight-

claritylight on Sep 30 2009, 09:04 AM said:

If I have a 5ug stock (100ng/ul) of DNA at 50ul original volume, and I add in 50ul water, it will dilute the original stock concentration down. What is the new concentration after dilution?

Please how to solve this or what is the formula

What exactly do you mean by "add in 50 ul of water?" If you have 50 ul of solution at 100 ng/ul, then add to it 50 ul water, you will have 100 total ul at 50 ng/ul. This is a straightforward 1:2 dilution. Another way to look at it is that you have 5 ug stock, so in 50 ul, dissolving 5 ug DNA will yield a concentration of 5 ug/50 ul= 0.1 ug/ul or 100 ng/ul. In 100 ul, 5 ug DNA will yield a concentration of 5 ug/100 ul= 0.05 ug/ul or 50 ng/ul.
If you mean to add 1 ul of your 100 ng/ul stock to 49 ul of water, this is a 1:50 dilution, so your final concentration is 100 ng/50 ul = 2 ng/ul.
Adding 1 ul to 50 ul of water will yield 1.96 ng/ul (100 ng/51 ul).

In any case, use the formula:
(concentration in ng/ul) * (volume of sample used for dilution in ul)/(new solution total volume in ul) = new concentration in ng/ul

-Dr Teeth-