The detection and measurement of hydroxyl ion (OH^-) in a solution is important in many applications, namely treatments of natural water and wastewater. The quantification of OH^- is normally carried out by titration method, but this method is time consuming and needs several reagents for precise analysis. Additionally, titration method cannot detect low concentration of OH^- ions in solution. Alternatively, the hydroxyl ion concentration in any solution could be estimated using pH sensors. Commercial pH probes sense the proton concentration in a solution and the concentration of OH^- is determined by use of the following equation.

pH= 14+log [OH^-] (1)

However, these pH sensors, which are made of glass electrodes, show acidic and alkaline errors at low and high pH values, respectively [1]. Moreover, these pH meters exhibit a sluggish response, especially in alkaline media. Some reported an optical pH sensing for high pH values, but they were not portable [2].

The focus of this investigation is to develop an electrochemical sensor to detect and measure OH ion in a solution. The presence of hydroxyl ion in a solution will be detected using the electrochemical oxidation of nickel hydroxide to oxyhydroxide (eq. 2). Considering this, a simple, low-cost, easy-to-miniaturize nickel- based electrochemical sensor was designed to examine the possibility of on line electrochemical sensing of OH^-. The sensor signal is based on the current produced by the following reaction that happens between the nickel electrode and the hydroxyl ions present in solution;

Ni(OH)_2(s)+OH⁻⇄NiOOH_(s)+H₂O_(l)+e⁻  (2) 

According to this mechanism, the nickel electrode can selectively measure OH^- concentration.

The analysis of the oxidation reaction was evaluated by cyclic voltammetry. Furthermore, the oxidation current at different concentrations of OH^- obtained by chronoamperometry revealed a linear correlation between oxidation current and concentration in alkali range within a short response time (Figure 1). This finding suggests that the sensor can be used for OH^- electrochemical quantification. However, the presence of other species in solution might affect this correlation. For instance, urea, which is widely used in fertilizers and can be found in wastewater, might affect sensor signal. It was observed when urea exists in media, urea oxidation happens and produces current which interferes with hydroxyl ion sensing.

The purpose of this work is finding of mathematical correlation between oxidation current and OH^- concentration in the presence of urea.

References

1. Safavi, A. and H. Abdollahi (1998). "Optical sensor for high pH values." Analytica chimica acta 367(1): 167-173.
2. Safavi, A. and M. Bagheri (2003). "Novel optical pH sensor for high and low pH values." Sensors and Actuators B: Chemical 90(1): 143-150.