$$\mathrm{FeO_4^{2 - }\buildrel { + 2.2V} \over \longrightarrow F{e^{3 + }}\buildrel { + 0.70V} \over \longrightarrow F{e^{2 + }}\buildrel { - 0.45V} \over \longrightarrow F{e^0}}$$

$E_{FeO_4^{2 - }/F{e^{2 + }}}^\theta $ is $x \times {10^{ - 3}}$ V. The value of $x$ is _________