Metal phosphate buffers A calibration solution contains $\pu{0.02M}$ $\ce{Cr^3+}$ and $\pu{0.02M}$ $\ce{Cu^2+}$. In order to control the solution $\mathrm{pH} = 12$, a phosphate buffer is used with the weak acid being $\ce{HPO4^2-}$ and the weak base being $\ce{PO4^3-}$. The maximum permissible change in $\mathrm{pH}$ is 0.2 units, how many moles of strong acid can be buffered per mole of weak base present?
  $\ce{H3PO4}$: $K_\mathrm{a3}$ = $4.2 \times 10^{-13}$


I have tried to formulate an equation for the reaction but I'm not sure which metal ion is/if both are taking part and therefore finding it hard to produce a $K_\mathrm{a}$ expression.
I think phosphate buffer is the solution here. Reason:

$K_\mathrm{sp}$ of $\ce{CrPO4}$ at $\pu{25 ^\circ C}$ is $2.4 \times 10^{-23}$ while that of $\ce{Cu3(PO4)2}$ at $\pu{25 ^\circ C}$ is $1.4 \times 10^{-37}$, thus both ions will precipitate since concentration of each ion is $\pu{0.02 M}$.