Introduction Fluoride (F-) is an important and reactive anion, present in water, mineral, many plants, air and food. Obviously, this element is an important tool in the prevention of tooth decay and caries [1], but excess fluoride can cause dental and skeletal flourosis, kidney failure and nephrolithiasis [2]. This diversity of characteristic, both beneficial and destructive aspects, makes the sensing and measurement of fluoride one of considerable current interest for environmental, biomedical, industrial-process monitoring. Numerous methods have been exploited to measurement of fluoride in different sample matrices [3-5] of which the most important and interesting is using of fluoride ion selective. In this research, we have investigated the potentiometric behavior of electrodes based on polypyrrole doped with four different kinds of dopants, i.e., polypyrrole/perchlorate (Ppy-ClO4) and polypyrrole/chloride (Ppy-Cl), polypyrrole/fluoride (PPy-F), and polypyrrole/nafion (Ppy-Nafion), towards fluoride ion. Method The electrochemical deposition of Ppy-ClO4 was carried out by cycling the potential between 0.0 V to 0.85 V at 50 mV/s for 10 cycles. The electropolymerization was conducted in a three-electrode cell with pencil lead as the working electrode, saturated Ag/AgCl as the reference electrode and platinum wire as the counter electrode. The electrolyte consisted of 0.1 M pyrrole as monomer and 0.1 mg/mL NaClO4 in water. The Ppy-Cl, Ppy-F, Ppy-Nafion film was prepared under the same conditions using aqueous solution of 0.1 NaCl, NaF, and Nafion, respectively, instead of NaClO4. These electrodes then dipped into a solution of 10-2 M NaF about 24 hours for conditioning and used as fluoride potentiometric sensor. The open-circuit potentials (Ecell) of Electrodes were recorded against a saturated Ag/AgCl reference electrode after reaching the steady state in the tested solution. Calibration curves were constructed by plotting the Ecell (mV), against the –Log [F-] in the solution. Results and discussion The potentiometric responses of four differently prepared electrodes, i.e., Ppy-ClO4, Ppy-Cl, Ppy-F, and Ppy-Nafion, have been studied. The results showed that the Ppy-F and Ppy-Nafion electrodes do not exhibit a stable potentiometric response. On the other hand, two sensors based on Ppy-ClO4 and Ppy-Cl exhibited highly reproducible response with a near Nersntian behavior. The Ppy-ClO4 electrode response was linear with a slope of 63.5± 0.07 mV within the concentration range of 5× 10-5 M to 10-1 M of F-. Ppy-Cl electrode response was linear with a slope of 61.8 ± 0.01 mV within the concentration range of 5× 10-5 M to 10-1 M of F-. The response time for both electrodes was less than 15 seconds. The as prepared electrodes were applied in potentiometric titration of fluoride ions with La3+ and also in determination of fluoride in real samples by standard addition method. Conclusion These sensors are comparable with expensive commercial fluoride ion selective sensors due to their fast response and simple, low cost fabrication. Keywords: Flouride, Polypyrrole, Potentiometry, Sensor References (1) Ayoob, S.; Gupta, A. K. Crit. Rev. Env. Sci. Technol. 2006, 36, 433487 (2) 487Dong, B.; Su, Y., Liu, Y.; Yuan, J.; Xu, J.; Zheng, L. J. Colloid Interface Sci. 2011, 356,190. (3) Clark, M. D.; Subramanian, S.; Krishnamoorti, R. J. Colloid Interface Sci. 2011, 354,144. (4) Gehlhausen, J.-M.; Carnahan, J.-W. Anal. Chem.1989, 61, 674 (5) Leon-Gonzalez, M.-E.; Santos-Delgado, M.-J.; Polo-Diez, L.-M.Anal. Chim. Acta1989,219, 329

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