A cation selective electrode based on copper(II) and nickel(II) hexacyanoferrates: dual response mechanisms, selective uptake or adsorption of analyte cations

Abstract

Abstract The potentiometric responses of copper and nickel hexacyanoferrate membrane electrodes were examined for alkali, alkaline earth, heavy metal and ammonium ions. When K2Cu3[Fe(II)(CN)6]2 membranes were electrodeposited on a Cu plate in aqueous K4[Fe(II)(CN)6] solution at applied potentials from +0.20 to +0.40 V vs SCE, the membranes exhibited a near-Nernstian response (55 mV/decade) to K+ ions above 1×10−4 M. When this K2Cu3[Fe(II)(CN)6]2 membrane was immersed and conditioned in 0.1 M (CH3)4N·NO3 (tetramethylammonium nitrate, TMA·NO3) solution prior to potentiometric measurements, the preferential dissolutions of K+ ions into the adjacent solution was observed accompanying negative membrane potential shifts (more than 200 mV). The potentiometric response of copper hexacyanoferrate membranes electrodeposited above 0.40 V vs SCE became however weaker with decreasing K+ content in CuHCF membranes and both the amount of preferential K+ dissolution and the extent of the membrane potential shift became smaller during the membrane conditioning. From these results, it was concluded that negatively charged K+ vacancies at the membrane surface contacting the adjacent electrolyte solution (0.1 M (CH3)4N·NO3) were formed by the preferential K+ dissolution, into which the analyte K+ ions was back-titrated during the potentiometric response process. The potentiometric selectivities of the K2Cu3[Fe(II)(CN)6]2 and KNi[Fe(III)(CN)6] membrane electrodes for alkali metal ions were Cs+>Rb+>K+>Na+>Li+ and no significant responses were observed for any alkaline earth metal ions in the concentration range below 1.0×10−2 M. These selectivities may reflect their dehydration energies needed for their accommodation from the solution side into the negatively charged vacancies formed by the preferential K+ dissolution at the membrane surface. The potentiometric response to a series of ammonium ions was in the following order; NH4+>(CH3)NH3+>(CH3)2NH2+≫(CH3)3NH+, (CH3)4N+, CH3CH2NH3+, (CH3CH2)2NH2+, (CH3CH2)4N+, (CH3)CNH3+, CH3C3H6NH3+, CH3C3H6NH2+CH3, (CH3)2CHCH2NH2+CH3, reflecting the size of the analyte ions. On the contrary, the response to divalent heavy metal cations (selectivity; Cu2+>Pb2+>Zn2+>Mn2+>Ni2+>Cd2+) however seemed to be due to specific adsorption onto the solid surface rather than the uptake of relevant ions into the vacancies formed by the preferential K+ dissolution. The latter mechanism appeared to be similar to the one previously reported for conventional precipitated-based ISEs, such as CuS, CdS and AgX (X=Cl, Br and I).