A polyvinylchloride (PVC) membrane optode was prepared by using 1,2-di(o-salicylaldiminophenylthio)ethane (H2DSALPTE) as a chelating reagent for selective determination of Ni(II) ions. The optimized membranes incorporating H2DSALPTE as an ionophore, sodium tetraphenylborate (NaTPB) as an anion excluder, and tributyl phosphate (TBP) as a solvent mediator were prepared and applied for determination of Ni(II) ions. The membrane responded to Ni(II) ion by changing the color from yellow to green in an acetate buffer solution at pH 6.0. The best performance was observed when the membrane having a composition of 3.17% ligand, 31.64% PVC, 63.29% TBP, and 1.90% NaTPB. The membrane can be worked well over a wide concentration range from 1.0 × 10？5 to 5.0 × 10？3？M. The membrane exhibited a detection limit of 8.51 × 10？6？M, and the response time of the membrane was within 7–12？min depending on the concentration of Ni(II) ions. The selectivity of the probe towards nickel determination was found to be very good. Experimental results showed that the probe could be used as an effective tool in analyzing the Ni(II) content of water samples. 1. Introduction Over the years, nickel has been regarded as a potentially toxic metal. The main source of nickel in aquatic systems is decomposition or degradation of rocks and soil, biological cycles and especially industrial processes, and water disposal [1]. The maximum recommended concentration of Ni(II) ions in drinking water for livestock is 2.5？mg/mL [2]. Nickel is well known as a toxic metal that can cause cancer of nasal lungs, dermatitis, asthma, and disorders of central nervous system [3]. Thus, due to the urgent need for selective monitoring of Ni(II) in many industrial, environmental, and food samples, there have been a number of recent reports in the literature [4–8]. A number of methods, such as atomic absorption spectrometry (AAS), flame atomic absorption spectrometry-electrothermal atomization (AAS-ETA) [9–12], inductively coupled plasma-atomic emission spectroscopy (ICP-AES), and flame photometry [13], can be used for determination of nickel. However, these methods generally require sample pretreatment and infrastructure backup and are, therefore, not very convenient for routine analysis of large number of environmental samples. Thus, there is critical need for the development of selective, portable, inexpensive diagnostic tool for the determination of nickel. Optical chemical sensors or optodes have become an important research area since their introduction two decades ago. Optical sensors are compact and ideally
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