phosphoric acid
Summary
Phosphoric acid (H₃PO₄) is a weak triprotic acid whose conjugate bases, the phosphates, form esters and anhydrides in ATP and the DNA backbone. Testable point: its three pKa values (about 2, 7 and 12) mean H₂PO₄⁻ and HPO₄²⁻ are the main forms near physiological pH.
Detail
Meaning: Phosphoric acid, H₃PO₄, has a central phosphorus bonded to one P=O and three OH groups. It loses its protons one at a time, and each pKa is higher than the last because removing H⁺ from a more negative ion is harder. Approximate values are 2.2, 7.2 and 12.4. Alcohols react with it to give phosphate esters. Two phosphates joined give a phosphoanhydride (as in ATP), and a phosphodiester links nucleotides in DNA and RNA. Hot, concentrated H₃PO₄ also works as an acid catalyst to dehydrate alcohols to alkenes.
Equation: pH = pKa + log([A⁻]/[HA]), where A⁻ is the deprotonated form and HA the protonated form (concentrations in mol/L, so the ratio has no unit; pH and pKa have no unit).
On the MCAT: You may be given a pH and asked which phosphate species dominates. When pH equals a pKa, the two neighboring forms are present in equal amounts. Phosphate buffers work best near pH 7.2. You may also be asked to identify an ester versus an anhydride in a structure, or to explain why DNA is negatively charged at physiological pH.
Common confusion: Students treat phosphoric acid as a strong acid like H₂SO₄. It is a weak acid, and its second and third protons are much less acidic than the first. Also, a phosphate ester (P–O–C) is not a phosphoanhydride (P–O–P).
Sources
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
- Organic Chemistry (McMurry)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.