Liquid membrane electrodes based on some metalloporphyrin derivatives exhibit an anti-Hofmeister selectivity pattern: NO2– > SCN– > I– > F– > Br– > Cl– > ClO4– > NO3–. These derivatives include chloro(5,10,15,20-tetraphenylporphyrinato)indium, chloro[5,10,15,20-tetra(p-methoxyphenyl)porphyrinato]indium and chloro(2-nitro-5,10,15,20-tetraphenylporphyrinato)indium. The selectivity pattern is interpreted on the basis of the coordination chemistry of metalloporphyrins. The membrane electrode with optimum composition shows a linear potentiometric response towards nitrite ion in the range 6.3 × 10–5–0.1 mol l–1 with a slope of 51 mV/pNO2– and a detection limit of 5.0 × 10–6 mol l–1. The electrode was applied to the determination of nitrite in Chinese cabbage.
