Need help with dilution calculation - (Nov/12/2010 )

Having a bit of trouble getting my head around dilution calculations, I have never done them before and seem to have to do a lot of them for my final year project!

I have made up a 1 mg/ml solution (25 mg in 25 ml) and I need to use that to make a working solution of 10 ng/ml.

A friend suggested I take 1 ml of the 1 mg/ml solution and make up to 100 ml, then take 0.1 ml of that solution and make up to 100 ml again. Is this right? I dont understand how he came to this conclusion so any help would be grately appreciated.

purplepigtails on Fri Nov 12 16:50:03 2010 said:

What your friend says is right, but if you don't know how he/she got there is not much use or help really.

The rule for dilutions is the following formula: C1V1 = C2V2 where C stands for concentration and V for volume, 1 is your starting solution and 2 the one you want to make.

So, you know your initial concentration; C1=1mg/ml and you know your final concentration; C2=10ng/ml (or 10x10-6mg/ml)
All you need now is to know your required final volume. To follow your friends example lets say you want 100ml, so your V2=100ml and V1 is the volume of C1 you need.

So if C1V1=C2V2 now you have: 1mg/ml x V1 = 10x10-6mg/ml x 100ml so V1= 0.001 = 1ul

Alternatively you could do a serial dilution, which is what your friend suggested so the volumes are easier to pippete, although there should be no problem pippeting 1ul.

What your friend suggest is making a 1ug/ml dilution first: 1ml x 1mg/ml = C2 x 100ml so your new C2= 1ug/ml = 1x10-3mg/ml
if you now repeat the formula CV=CV with => C2V2=C3V3 where now C3 is 10ng/ml and V3 again is 100ml you have that
1x10-3mg/ml x V2 = 10x10-6mg/ml x 100ml V2= 1ml.

Hope I've made myself clear rather than confuse you even more

As a summary just always always remember C1V1 = C2V2 this is valid for any and every dilution you want to make.
Also I'd suggest you get used to talk in microliters rather than 0.1ml, or 0.001ml.... makes life easier specially when using micropippetes.

Thankyou so much! Your explanation has really helped; I finally understand what I'm doing!