New polymeric membrane (PME), modified carbon paste (MCPE), and coated wire (CWE) selenite ion-selective electrodes based on 5,10,15,20-tetrakis-(4-methoxyphenyl)-21H,23H-porphyrin-Co(II) (CoTMeOPP) are reported. The best composition was the electrode containing 2% CoTMeOPP as the active material and 49%TCP as plasticizer. The electrodes reveal a Nernstian behavior over a concentration range of 5.5 x 10(-5) to 1.1 x 10(-2) M for PME, 5.2 x 10(-5) to 1.2 x 10(-2) M for MCPE and 1.2 x 10(-4) to 4.4 x 10(-3) M for CWE. The potentiometric response is pH dependent, since selenous acid is a diprotic acid. The slope of the selenite PVC electrode was -57.0 mV for the monovalent anion at pH 6.47, and -26.0 mV for the divalent anion at pH 11.00. The detection limits were 3.4 x 10(-5) and 4.7 x 10(-5) M at pH values 6.47 and 11.00, respectively. The electrodes manifest advantages of low resistance, very short response time (15 s), and most importantly good selectivities relative to a wide variety of other anions. In fact, the proposed selenite ion-selective electrodes show a great improvement compared to previously reported electrodes for selenite ion. The electrode was used for the determination of selenite in selenite/selenate mixture, in sodium selenite raw material powder, and in VitaFit Selenium ACE antioxidant tablets with recovery ranges of 90.0-103.3%. (C) 2010 Elsevier B.V. All rights reserved.