Advanced control of pH is largely a chemical modeling problem, because the modeling of the process can have a profound effect on the attainable control quality. The literature on pH control shows much confusion and misunderstanding concerning process modeling, resulting in misleading recommendations and inefficient control solutions. The present paper discusses pH modeling for control mainly from a chemical point of view, including such rarely treated situations as precipitation and formation of complexes in the system. Approximate models based on the chemistry of the system show a favorable structure, which allows adaptation to real unknown systems with a minimum of parameters.