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

[Anne van Binsbergen]

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