Squaric acid

Squaric acid
Names
	Preferred IUPAC name 3,4-Dihydroxycyclobut-3-ene-1,2-dione
	Other names Quadratic acid; Cyclobutenedioic acid
Identifiers
CAS Number	2892-51-5 ;
3D model (JSmol)	Interactive image;
ChemSpider	16919 ;
ECHA InfoCard	100.018.875
EC Number	220-761-4;
PubChem CID	17913;
UNII	SVR9D0VODW ;
CompTox Dashboard (EPA)	DTXSID9049409 ;
	InChI InChI=1S/C4H2O4/c5-1-2(6)4(8)3(1)7/h5-6H Key: PWEBUXCTKOWPCW-UHFFFAOYSA-N ; InChI=1/C4H2O4/c5-1-2(6)4(8)3(1)7/h5-6H Key: PWEBUXCTKOWPCW-UHFFFAOYAC;
	SMILES c1(c(c(=O)c1=O)O)O;
Properties
Chemical formula	C4H2O4
Molar mass	114.056 g·mol−1
Appearance	white crystalline powder
Melting point	> 300 °C (572 °F; 573 K)
Acidity (pKa)	pKa1 = 1.5; pKa2 = 3.4
Hazards
	GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H314
Precautionary statements	P260, P280, P301+P330+P331, P303+P361+P353, P304+P340+P310, P305+P351+P338
Flash point	190 °C (374 °F; 463 K)
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) Infobox references

Squaric acid or quadratic acid (so named because its four carbon atoms approximately form a square) is a diprotic organic acid with the chemical formula C₄O₂(OH)₂.

The conjugate base of squaric acid is the hydrogensquarate anion HC₄O−4; and the conjugate base of the hydrogensquarate anion is the divalent squarate anion C₄O2−4. This is one of the oxocarbon anions, which consist only of carbon and oxygen.

Squaric acid is a reagent for chemical synthesis, used for instance to make photosensitive squaraine dyes and inhibitors of protein tyrosine phosphatases.