# Could it be possible to make a buffer of pH 12.3 from solutions of disodium hydrogen phosphate and trisodium phosphate?

Mar 21, 2017

Yes, it could.

#### Explanation:

One type of buffer consists of a weak acid and its conjugate base.

In this question, the hydrogen phosphate ion is the weak acid and the phosphate ion is its conjugate base.

The chemical equation is

underbrace("HPO"_4^"2-")_color(red)("weak acid") + "H"_2"O" ⇌ "H"_3"O"^"+" + underbrace("PO"_4^"3-")_color(red)("conjugate base"); K_text(a) = 4.8 × 10^"-13"

Per the Henderson-Hasselbalch equation, the pH of the buffer is given by

color(blue)(bar(ul(|color(white)(a/a)"pH" = "p"K_text(a) + log((["PO"_4^"3-"])/(["HPO"_4^"2-"]))color(white)(a/a)|)))" "

"p"K_text(a) = "-log"(K_text(a)) = "-log"(4.8 × 10^"-13") = 12.32

If ["PO"_4^"3-"] = ["HPO"_4^"2-"], "then pH" = pK_text(a) = 12.32

Thus, you should be able to use solutions of ${\text{Na"_2"HPO}}_{4}$ and ${\text{Na"_3"PO}}_{4}$ to prepare buffers with $\text{pH}$ values between 11.3 and 13.3.