sodium/potassium phosphate solubility

[WonTolla]

Hello chromatographers,

I have been working with RP-HPLC for more than ten years, and always assuming as matter of faith something that one of my supervisors told me long time ago: potassium salts are more soluble than sodium salts, and they precipitate less when mixed with organic solvents.

I´ve found such statement in other sources, but never in a scientific reference.

Do you agree? Nowadays I am wondering if there is any scientific reason that justifies this...

Regards

[DSP007]

You agree. And have a steeper dependence of solubility on temperature.
This is due to the ionic radii, more accurately can not say I'm not a physicist

[WonTolla]

But sodium is smaller...

you mean that the bigger the radium the higher the solubility?

[zokitano]

This article might be helpful for you (i hope):
http://chromatographyonline.findanalyti ... rticle.pdf

Regards

[WonTolla]

Thank you zokitano!

Indeed I find it useful BUT they did not compare Na vs. K, only K(n)PO4 vs. NH4Ac. In fact, they assumed: "We expect that buffer solubility should follow the same trend as the solubility of cations in water (NH4+ > K+ > Na+)."

I can find the solubility of phosphate salts in water (they are even in Wikipedia), but still haven´t found what I´m looking for (as U2 said).

I wil say in other words: if I have to develop a new method for RP-HPLC with buffered (pH<3) mobile phase and DAD, which phosphate buffer would you use? and at pH 7? Which is the maximum concentration you would use?

I know that John Dolan recommends 20 mM K, but as far as I know, there is no explanation for it

Thanks!

Regards

[zokitano]

I wil say in other words: if I have to develop a new method for RP-HPLC with buffered (pH<3) mobile phase and DAD, which phosphate buffer would you use? and at pH 7? Which is the maximum concentration you would use?

I know that John Dolan recommends 20 mM K, but as far as I know, there is no explanation for it

Thanks!

Regards

Is it so important to know the "absolute" maximum concentration for using a phosphate buffer? You should use the lowest buffer concentration which will give you a reproducible chromatography and decent peak shapes or column efficiency. Also using high buffer concentration is not recommended not only from the standpoint of possible solubility issues but also from standpoint of column performance deterioration (when using high buffer concentrations).
Also, there is an evidence (a paper in J. Chromatogr. A I think published in the 90's) which shows that the usage the phosphate buffers at high pH (around 7) will promote the dissolution of silica support (for silica based columns), which normally will be more pronounced under higher phosphate buffer concentrations and/or higher content of water in the mobile phase.

Regards