Manganese porphyrin derivatives as ionophores for thiocyanate-selective electrodes: the influence of porphyrin substituents and additives on the response properties

The potentiometric response properties of several electrodes based on manganese(III) tetraphenylporphyrin derivatives, including three new ionophores, 5,10,15,20-tetrakis(4-aminophenyl)porphine manganese(III) chloride (Mn(TNH2PP)Cl) (1), 5,10,15,20-tetratolylporphine manganese(III) chloride (Mn(TTP)Cl) (2), 5,10,15,20-tetrakis(pentafluorophenyl)porphine manganese(III) chloride (Mn(TF5PP)Cl) (3), and two previously reported thiocyanate ionophores, octabromo-5,10,15,20-tetraphenylporphine manganese(III) chloride (Mn(Br8TPP)Cl) (4), and 5,10,15,20-tetraphenylporphine manganese(III) chloride (Mn(TPP)Cl) (5) incorporated into plastisized poly(vinyl chloride) (PVC) membranes were investigated. The membranes were directly coated on the surface of graphite electrodes. The effects of membrane matrix, and several membrane additives including carboxylated PVC and lipophilic cationic and anionic additives on the response characteristics were investigated. The sensitivity, working range, detection limit and extent of interference from foreign ions show a significant dependence on the concentration and type of ionophore and additives. The (Mn(TTP)Cl), with highest lipophilicity among the ionophores studied, generally shows better potentiometric selectivity for thiocyanate over a number of common anions (log KSCN,j, (SSM): CH3COO−, −3.66; Br−, −2.86; Cl−, −3.88; CN−, −3.71; F−, −3.44; I−, −2.49; NO3−, −2.90; NO2−, −3.88; C2O42−, −5.44; H2PO4−, −4.38; SO42−, −5.35). The optimized membrane based on this ionophore demonstrates Nernstian response over a wide linear range (10−7 to 10−1 M) of thiocyanate concentration. It has a fast response time, sub-micromolar detection limit (∼5×10−8 M thiocyanate), is easy to prepare and could be used for at least 2 months. The electrode could be used over a relatively wide pH range of 3–8. The electrodes were applied to the potentiometric titration of silver ion and determination of thiocyanate in urine samples.