Yes, I know. A student in chemistry thesis here in my department was cleaning a waste of [Fe(CN)6]4- using HCl!!!!!!! He was a jerk, in fact he was moved to a compilative thesis instead of an experimental one, eh eh eh....
In the end, this is my idea:
a solution of (substituted) benzyl chloride and K4[Fe(CN)6] is first treated with H3PO4 => Fe3(PO4)2 precipitated and in soln there is Ar-CH2Cl and HCN. Then NaOH soln is added, CN- is formed and it will react with Ar-CH2Cl forming Ar-CH2CN.
It'd be preferred a closed flask, so HCN dispersion from reaction flask is avoided. I think that a PTC (like a quaternary ammonium salt) is useful, isnt it?