Department of Chemistry, Morgan State University, Baltimore, MD 21251, USA

Abstract

Cyclic voltammetry was used in cetyltrimethylammonium bromide (CTAB) micellar solution to determine the half-wave potentials of selected chlorophenols, CPs. It is observed that all the electrochemical parameters of the studied CPs decrease with an increase in the number of chlorine atom(s) attached to the parent compound. The mathematical relationship between the obtained E_1/2 and the number of chlorine atoms in the parent compound is given. The formal potentials, E^0', of the CPs are approximated from the obtained half-wave potentials.

Keywords: cyclic voltammetry, half-wave, potential, formal, chlorophenols, electrochemical, parameters.

Introduction

Chlorophenols (CPs) are halogenated phenols. They are used not only as herbicides and bactericides but also in wood preservatives and pharmaceuticals. As a result of their numerous uses, they are eventually released to the environment. They are known to be toxic and listed as toxic chemicals [1]. The EPA has subsequently listed them as carcinogenic [2, 3]. These toxic/carcinogenic chemicals require analytical technique in detecting and detoxifying them. One of the major analytical methods used for detecting and characterizing the CPs is electrochemistry [4-12]. The parameter that is often used is the half-wave potential (E_1/2) [4-6]. This parameter has been used in quantitative structure-property relationship (QSPR) [13-15] and in correlation with the physical and chemical properties of other compounds [16-19]. E_1/2 has also been predicted using the quantitative structure-electrochemistry relationship (QSER) [20, 21]. However, to the authors' knowledge, the E_1/2 has not been used in characterizing the CPs. It is the theme therefore of this work to determine the influence of chloride ion attached to the parent compound, phenol, on the E_1/2 of selected CPs for their characterization. Most CPs are not soluble in water, the experiment in this work is therefore conducted in a micellar solution of 1.0 mM cetyltrimethylammonium bromide (CTAB).

Experimental

Chemical

All the CPs used in this work were all purchased from Aldrich Chemical Co. They are 99.0 % pure or are the purest grade from the manufacturer. A 99.0 % pure CTAB was obtained from Acros Organics. ACS reagent grade potassium chloride (KCl) was obtained from Fisher Scientific Co.

Apparatus

All electrochemical experiments were conducted using a three-electrode system comprising of the working electrode (1.0 mm diameter) Glassy Carbon Electrode (GCE) obtained from Cypress Systems, a wound platinum wire as the counter electrode and a commercial calomel electrode as the reference electrode which was obtained from Fisher Scientific in a 1-compactment electrochemical cell. A computer-controlled electrochemical analyzer system supplied by Cypress Systems, Model CS-10190 was used to obtain the requisite voltammograms of the CPs. The polisher, Metaserv®200, a brown polishing cloth, to resurface the electrode surface and 0.05 micron α-aluminum powder, used in conjunction with the polishing cloth were obtained from Buehler Ltd.

Methodology

Results and discussion

As can be seen in this figure, the peak potential and the peak current of the studied CPs are decreasing with an increase in the number of chlorine atoms in each congener. This observation is in accord with what is observed by other workers [22, 23].

In this equation R and T are the universal gas constant and temperature in Kelvin scale, respectively. α, n and F are the electron transfer coefficient, the number of electron(s) involved in the reaction and Faraday's constant, respectively. i is the observed current and id is the diffusion or peak current. The plots gave intercepts which were taken as E_1/2.

As can be seen, the plot is quite linear with the coefficient of determination, r², of 0.9926 and a regression coefficient, r, of 99.63 %. From this plot a useful mathematical relationship that describes the obtained E_1/2 of the CPs is: