Poly(vinyl chloride) membrane electrode for a stimulant, phentermine, using tris(2-ethylhexyl) phosphate as a solvent mediator

Tris(2-ethylhexyl) phosphate was an effective solvent mediator for constructing a poly(vinyl chloride) (PVC) membrane electrode responsive to a stimulant, phentermine, in combination with an ion-exchanger, sodium tetrakis[3,5-bis(2-methoxyhexafluoro-2-propyl)phenyl]borate (NaHFPB). This electrode discriminated between phentermine and analogous compounds and showed remarkably little interference by lipophilic quaternary ammonium ions, as well as inorganic cations, to almost the same degree of an electrode using the recently developed phentermine ionophore, N,N-dioctadecyl-N′,N′-dipropyl-3,6-dioxaoctanediamide. The present electrode exhibited a near-Nernstian response to phentermine in the concentration range of 2×10−6 to 1×10−2 M with a slope of 58.8 mV per concentration decade in 0.1 M MgCl2. The limit of detection was 1×10−6 M. This electrode was applied to determine phentermine in ion-exchange resin complexes containing this stimulant.