Chlorine is widely used as a disinfectant in the water treatment industry for inactivation of pathogenic micro-organisms such as Cryptosporidium and Escherichia coli. Chlorine in aqueous solutions is used for a number of purposes, such as disinfection and bleaching, and also as a powerful oxidizing agent in various manufacturing processes. It is known that chlorine is highly toxic, and additionally it can further react with other substances, such as phenols in environment to produce chlorophenols, which are suspected to be carcinogens. Furthermore, chlorine provides strong and persistent odor, even at low concentrations. In water treatment, the concentration of free residual chlorine should be strictly controlled, namely neither too low nor too high. Free residual chlorine with too low of a level cannot kill pathogenic bacteria and viruses in water effectively to keep water clear and clean. However, free residual chlorine with too high of a level may also be harmful, because the excess free chlorine would react with organic materials existing in the water to produce a large number of undesirable byproducts.¹ With increasing public awareness of water quality and tighter public health regulations and practices, such as point-of-use sampling and analysis, a robust, reliable, low-cost, and portable free chlorine sensor would be highly desirable. Several promising materials for free chlorine sensing with linear response have recently been reported in the literature.^2-5 However, the sensing materials are either expensive (e.g. glassy carbon, gold, diamond, graphene, carbon nanotubes) or made from potentially hazardous materials (e.g. benzethonium chloride, aniline oligomers).

In this work a complete analysis of physical and electrochemical properties of pencil graphite and pencil drawn paper based electrodes has been performed to detect free available chlorine (FAC). Cyclic voltammetry and amperometry were used to study the electro-catalytic properties. Excellent catalytic activity has been obtained for free chlorine detection using both pencil graphite and paper based electrodes with a sensitivity of 50 µAmM^-1cm^-2 and 29 µAmM^-1cm^-2 respectively and a linearity range up to 8 mM for both platforms. Also, limit of detect of both electrodes has been found to be 46 and 240 µM, respectively. There is no response obtained for common interfering ions in water such as NO₃^-, SO₄^2-, CO₃^2-, Cl^- and HCO₃^-. Both the sensors were evaluated for their storage stability and response time and thereby can be applied as a low-cost electrochemical sensor for determining free chlorine in water.

Acknowledgment

Authors acknowledge financial support from the Ministry of Science & Technology, Bangladesh funded project "FACSens" under the special allocation to Science & Technology Activity (2015/16) programme and Science Foundation Ireland funded project “SweatSens” under the grant agreement No. 14/TIDA/2455.

Corresponding authors' email addresses: mamun.jamal@chem.kuet.ac.bd and kafil.mahmood@tyndall.ie

Reference

1. Rook, J. J. Water Treat. Exam.1974, 23, 234.

2. S. pan, M. J. Deen and R. Ghosh, Anal. Chem., 2015, 87, 10734-10737.

3. Salazar, P. Martín, M. García-García, F. J. Gonzá lez-Mora, J. L. Gonzá lez-Elipe, A. R. Sens. Actuators, B2015, 213, 116.

4. Olivé -Monllau, R. Orozco, J. Ferná ndez-Sá nchez, C. Baeza, M. Bartrolí, J. Jimenez-Jorquera, C. Cé spedes, F.Talanta2009, 77(5),1739.

5. N. Dossi, R. Toniolo, F. Terzi, F. Impellizzieri, and G. Bontempelli, Electrochim. Acta., 2014, 146, 518–524.

Figure 1. (a) SEM image of bare 2B Pencil graphite electrode, (b) EDX of 2B pencil graphite electrode.