# Making a Phosphate Buffered Saline - (Oct/19/2012 )

I need to make 500mL of the following Phosphate Buffered Saline (normally I buy it pre-made, but we ran out of it),

0.1M Sodium Phosphate, 0.15M NaCl.

pH ~7.2

I know this is probably pretty basic, but I was hoping someone could help me out with this.

Calculate the amounts... then adjust pH.

I recommend you prepare a 10X solution so you have to prepare it only once.

If you have problems with calculations you can use this for help: http://www.graphpad.com/quickcalcs/molarityform.cfm

Ok this is what I've come up with for the amounts (making 1 Liter instead of 500 mL, a 10x solution would probably just be a waste as I only have to use this every couple of months):

I used Henderson-Hasselbalch equation for the sodium phosphate concentrations, using a pKa value of 6.865 (I'm a little confused on this as some places say it's 6.86 and some say it's 7.21)

7.2 = 6.865 + log (Disodium phosphate Dodecahydrate/Monosodium phosphate monohydrate)

Results:

24.490 grams of Disodium phosphate Dodecahydrate

4.363 grams of Monosodium phosphate monohydrate

For the NaCl I calculated:

8.766 grams of NaCl

I don't know why you are making so much calculations. There are tables to make phosphate buffers almost directly without fiddling with the HH equation

http://www.sciencega...endix/phosb.htm

Great thanks. I'd still be curious if my calculations were right if anyone wants to comment on them.