# What is an alternate way of preparing a sodium hydrogen phosphate/sodium dihydrogen phosphate buffer through titrating an acid with a strong base?

So my friend and I are stumped. This is in the context of achieving the sodium hydrogen phosphate/sodium dihydrogen phosphate buffer by titrating the acid with a strong base.

My friend says the answer would be H3PO4 + Na3PO4. In my case, I'd think that sodium hydrogen phosphate would be the acid (but I genuinely am not sure) that we'd titrate in that scenario and all we'd need then is a strong base (although I honestly don't know how to figure out what strong base in particular you must use.)

What's correct? Or what would be the correct way?

• Start with writing the 3 equilibrium chemical reactions between 4 forms of phosphate. You will see what is an acid and when. Aug 14 '20 at 14:35
• Any process treating any mixture of $\ce{Na_{n}H_{3-n}PO4}$ ( n=0..3) by any of $\ce{Na_{n}H_{3-n}PO4}$ or $\ce{NaOH}$ to produce solution of $\ce{NaH2PO4 + Na2HPO4}$ with the desired $\mathrm{pH}$ or the molar ratio/concentration is a correct way. But most handy may be titration of $\ce{H3PO4}$ or $\ce{NaH2PO4}$ by $\ce{NaOH}$. Aug 14 '20 at 14:55
• Both methods are correct. You may mix equal amounts of $\ce{H3PO4}$ and $\ce{Na3PO4}$. Or you may mix n moles of $\ce{NaH2PO4}$ and n / $2$ moles of NaOH. Aug 14 '20 at 14:58
• What is your target ph? Aug 14 '20 at 16:32
• Also you have to consider ionic strength of your buffer in addition to pH. Aug 14 '20 at 17:17

You want a mixture of $$\ce{HPO4^2-}$$ and $$\ce{H2PO4^-}$$. You can start with any species (in a solution with a certain pH) and add another species whose pH is "on the other side" of the desired pH. If the pH of your starting solution is lower than the desired pH, you can also titrate with NaOH.