A theoretical model of an electrochemical reduction of nitrate ion is presented. Analytical expressions pertaining to the concentration of nitrate, nitrite, ammonia, and nitrogen are obtained in terms of reaction rate constant. Simple and closed expression of current is also derived. In this paper, a powerful analytical method, called Homotopy Analysis Method (HAM) is used to obtain approximate analytical solutions for a nonlinear ordinary differential equation. The main objective is to propose an analytical method of solution, which do not require small parameters and avoid linearization and physically unrealistic assumptions. The obtained analytical solution in comparison with the numerical ones is found to be in satisfactory agreement. (C) 2011 Elsevier B.V. All rights reserved.