PO43

PO4^3−, commonly referred to as orthophosphate, is the trianion derived from phosphoric acid (H3PO4) by removal of all three protons. It is the principal inorganic phosphate species in neutral to basic environments and occurs widely in geology, biology, and environmental systems. The anion adopts a tetrahedral geometry around phosphorus and carries a formal charge of −3. Through resonance, the four P–O bonds are equivalent in character, giving partial double-bond character to some bonds.

In aqueous solution, phosphate exists in several protonation states: H3PO4, H2PO4−, HPO4^2−, and PO4^3−. The pKa values are about 2.15, 7.20, and 12.35, respectively, so PO4^3− becomes the dominant species at high pH (above roughly pH 12). This speciation underpins its behavior as a buffer and its coordination chemistry as a ligand.

Occurrence and significance extend across minerals, biology, and the environment. Phosphate minerals such as apatite form the primary phosphate reservoirs in rocks. Biologically, phosphate groups are central to DNA, RNA, ATP, and phospholipids, making phosphate vital for energy transfer and information storage. In ecosystems, phosphate is a key plant nutrient and a common pollutant source: excessive phosphate from fertilizers can contribute to eutrophication in surface waters.

Chemically, orthophosphate forms various salts (e.g., sodium, potassium, calcium phosphates) and phosphoric esters in organic synthesis. It also participates in the formation of condensed phosphates (pyrophosphate, tripolyphosphate) and acts as a multidentate ligand in coordination chemistry.

Note: PO4^3− is the conventional notation; PO43 without charge is often used informally, but the correct ionic form in most contexts is PO4^3−.