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

[Jorge Tornero]

Hi Ann and All,

I know about those recipes, but I would rather prefer to be able to
calculate it by myself and solve my problem "for ever"

Thank you very much anyway


Jorge Tornero
IEO-Cádiz
Spain

2008/5/14 Anne van Binsbergen <annigyg <@t> gmail.com>:

> Hi Jorge
> google 'neutral buffered formalin' and you will have the recipe for NBF
> Anne
>
> 2008/5/14 Jorge Tornero <jorge.tornero <@t> gmail.com>:
>
> > 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
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>
>
>
> --
> Anne van Binsbergen (Hope)
> Abu Dhabi
> UAE