In the field of process engineering, there are many factors that demand the knowledge of one part of the state vector, if not the full one. Among others, the implementation of nonlinear control methods as well as monitoring of some relevant process variables can be mentioned. The purpose of this paper is to introduce a nonlinear state observer of full order (i.e., the order of the observer matches the order of the nonlinear system observed) in order to estimate all of the state variables of the process. Though the estimated states can be used for many purposes, in this work we aim at using the estimations for output reference tracking. In particular, we deal with the pH neutralization process, which is widely recognized as a difficult problem for the purpose of control. To achieve the pH control goal, a simple algorithm is proposed which lies on a stable reference error dynamics model. Finally, computer simulations are developed for showing the performance of both the nonlinear observer and the control strategy based on pH measurement. For this purpose, a neutralization reaction between a strong acid and a strong base in the presence of a buffer agent is dealt with.