Finally, at a pH of around 3, the most prevalent form of phosphoric acid is H 2 PO - 4 (the pKa of the first deprotonation event of this species amounts to 2.14). This species readily associates with the formed oxocarbenium in a barrierless process that is exergonic by 22.3 kcal/mol. During this step, the α and β isomers are resolved thermodynamically, and the β ribofuranose-phosphoric acid is most stable by 4.1 kcal/mol.