oksalaattiioni

Oxalate ion, known in Finnish as oksalaatti-ioni, is the dianion with the chemical formula C2O4^2-. It is the conjugate base of oxalic acid (ethanedioic acid) and forms salts with various cations, such as potassium oxalate (K2C2O4) and calcium oxalate (CaC2O4). The ion consists of two carboxylate groups connected by a single C–C bond; the negative charge is delocalized over the four oxygen atoms through resonance, giving a relatively stable, planar structure.

In solution, oxalate acts as a bidentate ligand, often binding metals through its two oxygen atoms to form chelate rings. This versatility makes oxalate a common ligand in coordination chemistry, contributing to various metal complexes and affecting redox properties. In aqueous solution, oxalate exists primarily as C2O4^2−, with its protonated forms derived from oxalic acid. Oxalic acid has pKa values around 1.27 (first proton) and 4.27 (second proton), which means oxalate predominates at neutral and mildly alkaline pH, while oxalic acid dominates under strongly acidic conditions.

Oxalate occurs naturally in many plants and animals as a metabolic byproduct or as a storage form of calcium. Calcium oxalate crystals are well known in plants for structural purposes and are also a common byproduct implicated in human kidney stones when oxalate accumulates in urine. Organisms metabolize oxalate through pathways that can lead to its breakdown or excretion, reducing potential toxicity.

Applications of the oxalate ion include its use in chelation chemistry, serving as a ligand to form stable metal complexes, and in photometric and electrochemical methods such as oxalate-based actinometers. Safety considerations note that high levels of oxalate salts can contribute to kidney stone formation and may be toxic if ingested in large amounts.