[Histonet] About phosphate buffer - simple (silly?) question

[Jorge Tornero]

Hi,

I have to prepair phosphate buffered formaline to preserve anchovy gonads. I
am pretty new in this bussines, and unfortunately my knowledge on chemistry
is far from ideal.

When I started my work some time ago, I "inherited" some formulas about the
matter, but soon I've realized that those formulas are imprecise and vague:
for instance, they just say "mix x grams of dibasic salt with y grams of
monobasic salt", and those x's and y's vary from one formula to other. Of
course, I can't tell how many hydration water molecules are considered for
the chemical species in the formulas, and of course there is nothing about
the purity of the salts used. So I've decided to try to make the
calculations by myself, thus solving the problem for every time I need to
make this buffer or other.

So I have been looking around and found the Henderson-Hasselbalch equation
and I've employed it to make calculations, and everything is ok, BUT...

The dissociation constant for phosphate, pKa, seems to be in most cases 7.2,
but I've found, in several places, to be 6.86. They say it is a apparent pKa
(they call it pKa') due to the ionic strenght of the solution. The problem
is: Which pKa should I use? I guess the correct is 6.86, but I don't know
the reasons for it. I've read that pKa'=pKa+correction factor, calculations
involving ionic strenght but I am not able to find the tables where the
correction factor for a given ionic strenght are tabulated.

So my quesion is: Which pKa should I use? Why? How to calculate that pKa?

I hope my bad english is not a problem for you to understand me.

Best Regards,

Jorge Tornero
IEO-Cádiz
Spain