Question

Question #bb8ec

Answer 1

You can make phosphate buffers for pH 2, 7, and 12.

Explanation:

Phosphoric acid has three dissociation constants.

(1) `"H"_3"PO"_4 + "H"_2"O" ⇌ "H"_3"O"^+ + "H"_2"PO"_4^"-"; "p"K_text(a1) = 2.15`

(2) `"H"_2"PO"_4^"-" + "H"_2"O" ⇌ "H"_3"O"^"+" + "HPO"_4^"2-"; "p"K_text(a2) = 7.20`

(3) `"H"_3"PO"_4^"2-" + "H"_2"O" ⇌ "H"_3"O"^"+" + "PO"_4^"3-"; "p"K_text(a1) = 12.35`

Thus, you could make a buffer of pH 2 by preparing a solution of phosphoric acid and sodium dihydrogen phosphate (Equation 1).

You could make a buffer of pH 7 by preparing a solution of sodium dihydrogen phosphate and sodium hydrogen phosphate (Equation 2).

You could make a buffer of pH 12 by preparing a solution of sodium hydrogen phosphate and sodium phosphate (Equation 3).

The ranges for each buffer would be `"p"K_text(a) ± "1 pH unit"`.