Church hyb solution questions

David Micklem drm21 at mole.bio.cam.ac.uk
Wed Mar 8 07:16:41 EST 1995


In article <3jk1ai$bqt at elna.ethz.ch>, bckraev at wawona wrote:

> In the original paper it is suggested to titrate Na2HPO4 with phosphoric
> acid to pH 7.2. The only difficulty you may get here is that this chemical
> is available with 2H2O and 10H2O, so watch the MW on the bottle. And, 
> although it is kind of textbook reminder, "1 M Na2HPO4" is 1M with respect
> to sodium and 0.5 M in phosphate, so in practice you divide the MW by 2 and
> weigh that out to be dissolved in 1 liter. It is impossible to dissolve one
> mole of this salt in 1 liter at r.t., but I saw many people hopelessly waiting
> by the stirrer for it! Cheers,

Oh No! Now I'm REALLY confused....  I always thought that a 1M solution
was one mole of the material dissolved in 1 liter of water (at standard T
and P...)

That would make 1M Na2HPO4  _2 molar_ wrt sodium and _1 molar_ in
phosphate.  When preparing phosphate buffers, this is exactly what I want
(I think!). 
The relative insolubility of this salt is just one of those little things
that eg Sambrook et al fail to tell you when giving directions for making
different pH Na phosphate buffers :-)

Cheers,

David

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D.R.Micklem,
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