The solubility of salt of weak acid like $\mathrm{Na}_{3} \mathrm{PO}_{4}$ increases at lower $\mathrm{pH}$ because as lower $\mathrm{pH}$ concentration of union $\mathrm{X}^{-}$decreases due to protonation so $\mathrm{X}^{-}$decrease and solubility of $\mathrm{MX}$ increases.

$\mathrm{MX}+\mathrm{M}_{\text {(aq) }}^{+}+\mathrm{X}_{\text {(aq) }}^{-} \quad \ldots . \text { (Eq.-i) }$

$\mathrm{K}_{s p}=\left[\mathrm{M}^{+}\right]\left[\mathrm{X}^{-}\right]\quad \ldots(\mathrm{Eq} .-\mathrm{ii})$

MX is the salt of weak acid ($HX$) ionization of weak acid is as follows.

$\mathrm{HX}_{\text {(aq) }} \square \mathrm{H}_{\text {(aq) }}^{+}+\mathrm{X}_{\text {(aq) }}^{-} \ldots \text { (Eq.-$iii$) }$

Note : In this salt and weak acid the common ion is $\mathrm{X}^{-}$.

(Eq.-$iii$) ionization constant for weak acid

$\mathrm{K}_{a}=\frac{\left[\mathrm{H}_{\text {(aq) }}^{+}\right]\left[\mathrm{X}_{\text {(aq) }}^{-}\right]}{\mathrm{HX}_{\text {(aq) }}} \ldots . . \text { (Eq.-iv) }$

$\therefore\frac{\mathrm{K}_{a}}{\left[\mathrm{H}^{+}\right]}=\frac{\left[\mathrm{X}^{-}\right]}{[\mathrm{HX}]}$

$\frac{\left[\mathrm{H}^{+}\right]}{\mathrm{K}_{a}}+1=\frac{[\mathrm{HX}]}{\left[\mathrm{X}^{-}\right]}+1$

$\therefore\frac{\left[\mathrm{H}^{+}\right]+\mathrm{K}_{a}}{\mathrm{~K}_{a}}=\frac{[\mathrm{HX}]+\left[\mathrm{X}^{-}\right]}{\left[\mathrm{X}^{-}\right]}$

$\frac{\mathrm{K}_{a}}{\left[\mathrm{H}^{+}\right]+\mathrm{K}_{a}}=\frac{\left[\mathrm{X}^{-}\right]}{[\mathrm{HX}]+\left[\mathrm{X}^{-}\right]}=\mathrm{f} \quad \ldots(\text { Eq.-v) }$

So $\left[\mathrm{H}^{+}\right]$increases $\mathrm{pH}$ decreases and with of ' $\mathrm{f}$ ' decrease.