How to make NaxPO4 buffer?

I am currently trying to make a buffer for protein expression. According to [the article](http://cshprotocols.cshlp.org/content/2006/1/pdb.rec8303), I am to use a buffer with $\pu{50 mM}$ $\ce{Na\_{x}PO4}$, $\pu{pH 8}$ (and $\ce{0.5 M NaCl}$). How do I make the $\ce{Na\_{x}PO4}$ buffer?


I have previously made phosphate buffers from cold spring harbor protocols
(Mixing aliquots of $\ce{1 M NaH2PO4}$ (monobasic) and $\ce{1 M Na2HPO4}$ (dibasic) aliquots to get the desired pH and concentration. Is this the same buffer, just with another name? Or what to use to make $\ce{Na\_{x}PO4}$?




The Cold Spring Harbor protocol is probably fine. There is another recipe that uses a mixture of potassium and sodium salts (for better solubility).



> 
> 50 mM NaxPO4, pH 8 (and 0.5 M NaCl)
> 
> 
> 


This is a weird notation, and the x should be a subscript:


[![enter image description here](https://i.stack.imgur.com/Pwqdp.png)](https://i.stack.imgur.com/Pwqdp.png)


Source: <https://www.nature.com/articles/s41467-020-18907-6.pdf?proof=t>


If you are concerned about correct stoichiometry, you have to include some hydrogen as well


$$\ce{Na\_xH\_{3-x}PO4}$$


At pH 8, you would expect some hydrogenphosphate ($\ce{HPO4^2-}$) and some dihydrogenphospate ($\ce{H2PO4-}$) ions. There is no simple way to write this combination as a formula, though.


[![enter image description here](https://i.stack.imgur.com/O8FMY.png)](https://i.stack.imgur.com/O8FMY.png)


Source: <https://commons.wikimedia.org/wiki/File:PiSpeciation.svg>


 2

$\ce{Na\_xPO4}$ is a non-official way of defining any mixture of $\ce{Na2HPO4}$ and $\ce{NaH2PO4}$ solutions. It can be used for describing all mixtures contained in the reference of Cold Spring Harbor.


 1