The pH active species HCO − 3 (bicarb) then reacts with the water present. Bicarb is amphoteric and therefore can react with water as an acid or base. Identify the appropriate equilibrium reaction for the type of reaction and identify the appropriate equilibrium constant. Recall that K a1 refers to the first dissociation of carbonic acid ( H 2 CO 3 ) and K a2 refers to the second. It might be helpful to write the stepwise dissociation reactions. For the acid write the products and identify the appropriate K value.