How do i make a 200ml inoculation solution?

Question

Stocks: 250mM MgCl2 and 1% Silwet

Solution needs: 10mM MgCl2 and 0.02% Silwet

Answer 1

To make 10 mM MgCl₂ from a 250 mM MgCl₂ stock, you must dilute the stock 25-fold. By doing this, you are also diluting the 1% Silwet by 25 fold, resulting in a final Silwet concentration of 0.04%. To get the Silwet to be 0.02%, you'd have to dilute the stock 50-fold. So, you can't make a single dilution of the stock and expect to end up with 10 mM MgCl₂ and 0.02% Silwet.

So, to make 200 ml inoculation solution, you would do the following:

(x ml)(250 mM MgCl₂) = (200 ml)(10 mM MgCl₂)

x = 8 mls

Take 8 mls of the stock 250 mM MgCl₂ and dilute it to a final volume of 200 ml. As stated above, the Silwet will be 0.04% (not 0.02% as desired). There is no way to get them both to be what you want in a single dilution, unless you change the stock to 250 mM MgCl₂ + 0.5% Stilwet.