Diprotic
Two acidic protons can be removed stepwise.
The second deprotonation starts from an already negatively charged hydrogen oxalate ion.
Molecular compound · dicarboxylic acid
Oxalic acid
C₂H₂O₄
Oxalic acid is C₂H₂O₄, the simplest dicarboxylic acid. Its two carboxyl groups are directly bonded through their carbon atoms, giving HOOC–COOH.
Hydrate distinction: anhydrous oxalic acid and oxalic acid dihydrate are different solid forms with different formulas and molar masses. This master is for C₂H₂O₄.
Essentials
Two –COOH groups make oxalic acid diprotic.
The compact HOOC–COOH structure shows two carboxylic acid groups and no hydrocarbon spacer between them.
Detail
The full 2D structure contains two adjacent carbonyl/carboxyl systems.
Each carboxyl group has its own carbonyl and hydroxyl oxygen in the neutral acid. Electron distribution changes after deprotonation.
Oxalate
After full deprotonation, C₂O₄²⁻ can coordinate metal ions.
Its oxygen donor sites make oxalate important in coordination and precipitation chemistry.
Understand
Oxalic acid loses two protons in distinct equilibria.
The acid first forms hydrogen oxalate, then oxalate. The two equilibria are not identical because charge and solvation change after the first proton is removed.
Calcium chemistry
Oxalate can form sparingly soluble salts with some metal ions.
That behavior follows from the dianion's charge and coordination geometry, not simply from the presence of oxygen atoms.
Reference identifiers
Names and identifiers that pin down the chemical identity.
| Formula | C₂H₂O₄ / (COOH)₂ |
|---|---|
| CAS Registry Number | 144-62-7 |
| PubChem CID | 971 |
Advanced
Push past the first model without losing the simple picture.
Is oxalic acid dihydrate the same formula?
No. The dihydrate includes two waters of crystallization per oxalic-acid unit.
Are the two acid dissociations equally strong?
No. The second begins from a negatively charged ion and is less favorable.
Why can oxalate bind through two oxygens?
Its geometry and delocalized carboxylate groups can provide multiple donor sites to a metal center.
Constituent elements