How do you make a serial dilution?

a serial dilution is nothing more than a "series" of dilutions.  The defining feature of this is that you're decreasing the concentration by a consistant order of magnitude with each dilution.. such as a 1000mM solution to a 100mM to a 10mM etc. etc.

its really easy when you're decreasing by a factor of 10.  Consider of this.. you need to make up 5ul of a 10mM solution from a 1000mM stock solution.  Take 1ul of the solution you're diluting and add 9ul of water (notice how the total volume is 10.. 1 of your solution into a TOTAL volume of 10 equals a 1/10 dilution.. so the final concentration is now 100mM.  OK now lets do the same thing:  take 1ul of your new 100mM solution and add 9ul H20 to it.. so now this gets diluted further to our desired 10mM solution.  pipette out 5ul of it (we have a total volume of 10ul again) and you're good to go.

the key to serial dilutions is to make a dilution.. then use this diluted tube to make a new dilution.. then use the new diluted tube to make another.  Its crucial when you're working with small volumes.

you're question up top asks you to make a quick 1/2 dilution.  This can be made a million ways depending on what volume of it you need.  Add 10mL of your 3.5% solution into 10mL of h20 and now you have 20mL of a 1.75% solution.  OR add 5ul of your 3.5% into 5ul of H20.. now you have 10ul of a 1.75% solution.  equal parts is the key to decrease a concentration by a factor of 2.

hope this helps.  always reason through these questions.

1ml [say] 3.5% solution + 1ml diluent will give you, after mixing, a 1.75% solution.
If you wish to make a serial dilution you would need to prepare a row of tubes labelled [say] 1 to 4.
Into tube 1 put 2mls neat stock soln. [3.5%].
Into tubes 3, 4 & 5 put 1 ml diluent.
Use pipette and transfer carefully 1 ml from tube 1 to tube 2. Mix.
New pipette [or tip] same procedure, tube 2 to 3 & 3 to 4.
The final 1 ml goes to waste.
This is a double-dilution & you will have:
Tube 1 NEAT 3.5%.
Tube 2 [1:2 ]     1.75%.
Tube 3 [1:4 ]     0.875%.
Tube 4 [1:8 ]       0.4375%.
[answer].