A laboratory is studying the precipitation of ferric phosphate from aqueous solution

CHEMISTRY · Multi-part · AP · Chemistry · Acids and Bases · Acids and Bases

Problem

A laboratory is studying the precipitation of ferric phosphate from aqueous solution. Solid FePO

_4

(s) is known to form when Fe

^{3+}

and PO

_4^{3-}

ions are present in sufficient concentrations. The laboratory wishes to prepare a buffer so that FePO

_4

(s) will precipitate only if the pH exceeds a particular threshold, ensuring that trace amounts of Fe

^{3+}

remain soluble at lower pH values. Relevant data (25 °C):

Reaction	Equilibrium constant
FePO $_4$ (s) ⇌ Fe $^{3+}$ + PO $_4^{3-}$	$K_{sp}=1.0\times10^{-22}$
H $_3$ PO $_4$ ⇌ H $^+$ + H $_2$ PO $_4^-$	$K_{a1}=7.1\times10^{-3}$
H $_2$ PO $_4^-$ ⇌ H $^+$ + HPO $_4^{2-}$	$K_{a2}=6.3\times10^{-8}$
HPO $_4^{2-}$ ⇌ H $^+$ + PO $_4^{3-}$	$K_{a3}=4.2\times10^{-13}$

(a) Derive an expression that relates the concentration of PO

_4^{3-}

to the pH of the solution, assuming that the total phosphate in solution is dominated by the H

_2

_4^-

/ HPO

_4^{2-}

/ PO

_4^{3-}

species. (b) Using the expression from part (a), determine the highest pH at which a solution that is 0.010 M in total dissolved phosphate can hold 1.0 × 10

^{-5}

M Fe

^{3+}

without precipitating FePO

_4

(s). Show all reasoning and approximations clearly. (c) Explain qualitatively how the buffer composition would need to be adjusted if the allowable Fe

^{3+}

concentration were lowered to 1.0 × 10

^{-6}

M, and relate your explanation to the species distribution derived in part (a).

Related theory

Browse the problem library