The synthetic semi permeable membrane used in Berkeley-Hartley method is

Question

$\begin {array} {1 1} (1)\;Copper\: Ferrocyanide & \quad (2)\;Copper\: Ferricyanide \\ (3)\;Potassium\: Ferricyanide & \quad (4)\;Potassium\: Ferrocyanide \end {array}$