(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 mudchemistry. 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 aslignosulfonate, onto clay edge charges. See: alkalinity, buffered mud, calcium hydroxide, calcium sulfate, carbonate test, Garrett Gas Train, hydrolysis, lignite, sodium bicarbonate, sodium carbonate