Processes for producing acetic acid are presented herein. One or more embodiments include processes for controlling downstream water concentration in acetic acid production process including contacting methanol and carbon monoxide in the presence of a reaction medium under carbonylation conditions sufficient to form a carbonylation product including acetic acid, wherein the reaction medium includes a carbonylation catalyst, water in an upstream water concentration of from 1 wt. % to 14 wt. % water, and a tertiary phosphine oxide; recovering acetic acid from the carbonylation product; and controlling a downstream water concentration by determining a target water concentration and introducing the tertiary phosphine oxide to the reaction medium at a rate, basicity, concentration or combination thereof sufficient to provide a downstream water concentration within 1 wt. % of the target water concentration.
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