$2 \mathrm{NaCN}+\mathrm{FeSO}_{4} \longrightarrow \mathrm{Fe}(\mathrm{CN})_{2}+\mathrm{Na}_{2} \mathrm{SO}_{4}$

$F e(C N)_{2}+4 N a C N \longrightarrow N a_{4}\left[F e(C N)_{6}\right]$

$3 N a_{4}\left[F e(C N)_{6}\right]+4 F e^{3+} \longrightarrow F e_{4}\left[F e(C N)_{6}\right]_{3}+12 N a^{+}$

It is ferri-ferrocyanide