Talanta 65 (2005) 991–997 The first use of a Rh(III) complex as a novel ionophore for thiocyanate-selective polymeric membrane electrodes Mojtaba Shamsipur a,∗ , Sohrab Ershad a , Naser Samadi a , Ali Reza Rezvani b , Hasan Haddadzadeh b a Department of Chemistry, Razi University, Kermanshah, Iran b Department of Chemistry, Sistan-Balouchestan University, Zahedan, Iran Received 5 May 2004; received in revised form 1 August 2004; accepted 25 August 2004 Available online 29 September 2004 Abstract The feasibility of a newly synthesized Rh(III) complex, Rh[(trpy)(bpy)Cl](PF 6 ) 2 , as a novel ionophore for the preparation of anion-selective polymeric membrane electrodes was tested. The ionophore exhibited anti-Hofmeister behavior with enhanced potentiometric selectivity toward thiocyanate ion compared to other anions. The influence of some experimental parameters such as membrane composition, nature and amount of plasticizer and additive and concentration of internal solution on the potential response of the SCN - sensor were investigated. The electrode exhibits a Nernstian response for SCN - over a wide concentration range (1.0 × 10 -5 to 1.0 × 10 -1 M) with a slope -58.7 ± 0.5 mV per decade and a detection limit of 4.0 × 10 -6 M (0.23 ppm). It could be used in a pH range of 3.0–8.0 and has a fast response time of about 15 s. The proposed sensor was used for the determination of thiocyanate ions in real samples such as urine and saliva of smokers and nonsmokers and, as an indicator electrode, in potentiometric titrations of SCN - ion. © 2004 Elsevier B.V. All rights reserved. Keywords: SCN - -selective electrode; PVC-membrane; Rh[(trpy)(bpy)Cl](PF 6 ) 2 ; Potentiometry 1. Introduction Because of their widespread analytical applications in physiology, process control and environmental analysis, the ionophore-based ion-selective electrodes (ISEs) have been established as one of the most important groups of chemical sensors [1–3]. Thus, in the past two decades, there has been a growing interest in search for ionophores that can chemically recognize specific cations and anions and offer either new or improved selectivities for different ions [4–7]. Classical membrane electrodes for anions are based on quaternary ammonium and phosphoium salts and usually ex- hibit the Hofmeister pattern with the largest selectivity to lipophilic anions [8]. Recently, much attention has been fo- cused on new ion carriers possessing an anti-Hoffmeister ∗ Corresponding author. Tel.: +98 831 4223307; fax: +98 831 4228439. E-mail address: mshamsipur@yahoo.com (M. Shamsipur). selectivity sequence in preparation of anion-selective elec- trodes. Generally, the antiHofmeister anion selectivity is achieved in the case of membrane electrodes doped with an organometallic complex, via specific anion coordination with the central metal ion of the complex; of course, for such a coordination process, the structure of the complex-forming carrier plays an important role in the final selectivity charac- teristics of the resulting electrode [9]. Thiocyanate is present as a normal constituent in mam- malian tissues and body fluids [10]. The tyrostatic effect of the thiocyanate, brought about by inhibiting iodine transport in the thyroid, is well known [11]. Moreover, thiocyanate is present in human body as a metabolic degradation product of sulfur containing compounds in tobacco. It is a detoxication product of cyanide, and its content is higher in the body fluids of smokers [12,13]. Thus, due to the critical importance of selective thiocyanate determination, especially in food, bio- logical and water samples [14], there are several reports on 0039-9140/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2004.08.032