We use Dulbeccos PBS, Sigma, or use a PBS, pH 7.4 already in your lab,
dissolve 4 gm paraformaldehyde in 100 mls. Stirred and heated from 56C to 60C
no more than than to prevent  breakdown of PFA, use a thermometer on
a heater/magnetic stirrer, and it will go into solution within an hour.
I am very careful to not heat above 60C, my EM expert says this is a
no no!

I use Sigma PFA, high quality, which goes in quickly, cool with ice water
or cold water, check pH, you are in business.  My DPBS starts at  pH 7.4,
and only lowers to about 7.35 after PFA is in solution.  Store in refrig,
use fresh, if stored for a time, any ppt in bottom indicates solution needs
to be discarded.

Gayle Callis