The classical control theory is based on the design of linear controllers for systems described by linear models. However, there exist some situations where it is not recommended, or even possible, to use a fixed linear controller to control a process. One of those situations arises when the magnitude of the process gain experiences a dramatic variation within the operating range of interest. A classic example of a chemical process where this situation occurs is the pH control around the neutralization point in a continuous stirred tank. In this work, the disturbance attenuation problem for a strong acid - strong base pH control system is addressed. To solve this problem, a nonlinear H_∞ control law is derived based on a nonlinear model previously developed. The attainment of that control law is done with the help of recent mathematical results from the authors concerning the solution of Hamilton-Jacobi inequalities. The nonlinear controller is implemented on an reactor and its performance is compared with a PID control law tuned according to the classical minimum error integral criteria. The obtained results show that the nonlinear H_∞ control theory can be a good alternative to solve this difficult SISO control problem.