How would you prepare 20 ml of a 1 to 400 dilution?

Answer 1

If accuracy is not a problem the most simple way to produce 20 ml of a 1:400 dilution is to take 1:400 of 20 ml (i.e. 0.05 ml) of the original solution and add 19.95 ml of liquid.

As pipettes are the most precise close to their maximum uptake volume and micro-pipettes are inherently much less precise than pipettes for larger volumes I'd do a two-step dilution.

1. step: 1.000 ml of original solution with 1 ml pipette + 4 times 4.750 ml with 5 ml pipette

2. step: 1.000 ml of solution from step 1 + 4 times 4.750 ml with 5 ml pipette.