In this thesis, the mathematical models for pH process are reviewed in the first. These models consist of two static and dynamic parts. The static part of model is derived based on ion’s dissociation, chemical equilibrium and electro neutrality equation in reactor. The dynamic part is obtained based on conservation of mass for each species in a CSTR reactor with constant volume. This type of modeling is named full order model or modeling based-on reaction invariants. Since implementation of this mode is more sophisticated when the number of state variables increases, the reduced order model which only has one state has been used. One of the significant characteristics of reduced order model is not needing to state observers. Also it can be used for estimation of feed stream titration curve easily. The main object of this thesis is to propose a controller based on reduced order model and compare with the other methods as below: a) Designing of adaptive and non-adaptive feedback linearizing controller based on reduced order model and compare it with linearizing controller based on full order model. Since nonlinear Leunberger like observers and extended Leunberger observers are implemented for estimating of states in full order model. Simulation and experimental results indicate the better performance of extended Leunberger observer with less computation. Also a method is proposed for prevention of singularity of observer gain in Leunberger like observer when strong acid and base are presented together. Finally is shown that the performance of feedback linearization controller based on reduced order model is better than controller based on full order model considerably. b) Review on pH control based on SAE method which proposed in literature and compare it with linearizing controller. First SAE method is introduced. Then a comparison is preformed between two SAE controllers through simulation. Finally the closed-loop performances of these controllers are evaluated through experimental studies. Results show that the SAE method is not suitable controller for common pH process in spite of good reports of this method in literature. c) Review on backstepping(BS) controller method which proposed for pH control in literature. It is shown that the BS controller proposed in the literature is identical with exact input-output linearization. To avoid difficulties related to state estimator design, these controllers have been designed based on the reduced order model of pH. Through simulation, the performance of linearizing controller, designed based on reduced order model, has been compared with performance of adaptive backstepping controller. Results indicate that the non-adaptive version of linearizing controller has a better performance in spite of its simpler design......