1. n. [Drilling Fluids]

An anion with formula CO₃^–2. Carbonate chemistry involves a pH-dependent equilibrium between H₂O, H⁺, OH^–, CO₂, HCO₃^– and CO₃^–2. At low pH, carbon dioxide [CO₂] dominates. As pH rises from acidic toward neutral, HCO₃^– ions dominate. As pH rises above neutral, CO₃^–2 ions dominate. If no component is lost from the system (such as CO₂ gas evolving), changing pH up and down continually reverses the relative proportion of the carbonate species. Carbonates play several important roles in water mud chemistry.

• One role is the corrosion of metals by acidic CO₂.
• A second is the formation of calcium carbonate [CaCO₃] scale on surfaces by carbonate and calcium ion reactions.
• Another role is in the chemistry of deflocculated mud, where bicarbonate ions prevent attachment of deflocculants such as lignosulfonate, onto clay edge charges.

See: alkalinity,  buffered mud,  calcium hydroxide,  calcium sulfate,  carbonate test,  Garrett Gas Train,  hydrolysis,  lignite,  sodium bicarbonate,  sodium carbonate