Electroanalytical Determination of Morpholine as a Corrosion Inhibitor at a Cathodically Pretreated Boron-Doped Diamond Electrode

An electroanalytical method for the determination of morpholine, a corrosion inhibitor, was developed at a cathodically pretreated boron-doped diamond electrode (BDDE). The voltammetric response of morpholine at the BDDE in 0.1 mol L-1 KCl (pH 10) shows an irreversible oxidation process at approximately 1.3 V vs. Ag/AgCl in 3.0 mol L-1 KCl. Using cyclic voltammetry, the number of electrons involved in the morpholine electroxidation mechanism was found to be 1. The application of chronoamperometry showed that the apparent diffusion coefficient (D-0) was 2.99 x 10(-6) cm(2) s(-1). Using square wave voltammetry under the optimized conditions (frequency of 30.0 Hz, pulse amplitude of 100 mV and step potential of 20 mV at pH 10.0), the developed method provided limits of detection and quantification of 2.1 and 6.9 mg L-1, respectively, with a linear range from 5.0 to 100.0 mg L-1 (r = 0.991). Intraday (n = 10) and interday (two consecutive day) precision values assessed as the relative standard deviation for solutions containing 30.0, 60.0, and 90.0 mg L-1 of morpholine were from 0.41 to 5.86% and 0.92 to 3.19%, respectively. The feasibility of the method for the interference-free determination of morpholine was verified by the analysis of synthetic boiler water samples containing CaCO3, Na2SO3, Na3PO4, FeCl3, and humic acid as organic matter. In addition, hydrazine was added as a possible interfering compound because of its widespread use in corrosion inhibition. Recovery values from 90.9 to 109.4% were obtained in the synthetic boiler water, thereby attesting to the accuracy of the method.