Cyanide-selective electrode based on Zn(II) tetraphenylporphyrin as ionophore

Receptors that exhibit high selectivity are essential for potentiometric cyanide sensors. Therefore, CN ^- binding to metallotetraphenylporphyrins with different metal centers (i.e., Co(II), Co(III), Zn(II), Ni(II), Cu(II), and Fe(III)) was investigated. All these metalloporphyrins were found to function as neutral ionophores. Co(III) and Fe(III) tetraphenylporphyrins with their positive charges seemed likely to bind up to two axial CN ^- ligands, but only the Co(III) porphyrin was found to strongly bind a second CN ^- ligand. The electrode membranes doped with Zn(II) tetraphenylporphyrin provided the highest selectivity over chloride (logKCN ^-,Cl ^- _pot = -3.71, as opposed to -0.36 for an ionophore-free ISE) and were optimized by adjusting the site-to-ionophore ratio to achieve the highest CN ^- selectivity, with special consideration of interfering ions present in gold mining applications. The Zn(II) tetraphenylporphyrin-based CN ^--selective electrodes exhibited the best discrimination of OH ^-; no pH effect was observed even at pH 11 (logKCN ^-,OH ^-pot = -3.42). The response slopes and unbiased selectivities of the ionophore-free and the ionophore-based electrodes with 25 mol % and 71 mol % cationic sites relative to ionophore showed that the Zn(II) tetraphenylporphyrin forms a 1:1 complex with the target ion CN ^- and 2:1 and 1:1 complexes with the interfering ions OH ^- and S ^2-, respectively. The CN ^- binding constant was 2.3 × 10 ⁶ (mol/kg) ^-1, which is slightly bigger but of the same order of magnitude as for binding of Zn(II) tetraphenylporphyrin to CN ^- in dichloroethane.