Rubidium oxalate

Rubidium oxalate is a chemical compound with the chemical formula Rb₂C₂O₄. It is a rubidium salt of oxalic acid. It consists of rubidium cations Rb⁺ and oxalate anions C₂O2−4. Rubidium oxalate forms a monohydrate Rb₂C₂O₄·H₂O.

Rubidium oxalate

Identifiers
CAS Number	10010-65-8 anhydrous Y 7243-75-6 monohydrate Y
3D model (JSmol)	Interactive image
ChemSpider	10760835
PubChem CID	13955568
InChI InChI=1S/C2H2O4.2Rb/c3-1(4)2(5)6;;/h(H,3,4)(H,5,6);;/q;2*+1/p-2 Key: DUXDETQJUQZYEX-UHFFFAOYSA-L
SMILES [Rb+].[O-]C(=O)C(=O)[O-].[Rb+]
Properties
Chemical formula	Rb2C2O4
Molar mass	258.954 g·mol−1
Appearance	colourless crystals
Density	2.76 g/cm3 (monohydrate)
Solubility in water	soluble
Related compounds
Other anions	Rubidium carbonate Rubidium acetate
Other cations	Lithium oxalate Sodium oxalate Potassium oxalate Caesium oxalate
Related compounds	Rubidium hydroxide Oxalic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Preparation

Rubidium carbonate and oxalic acid react to form rubidium oxalate:^[1]

Rb₂CO₃ + H₂C₂O₄ → Rb₂C₂O₄ + H₂O + CO₂↑

Rubidium oxalate can also be obtained via the thermal decomposition of rubidium formate:^[2]

2 HCOORb → Rb₂C₂O₄ + H₂↑

Properties

From an aqueous solution, rubidium oxalate crystallizes as a monohydrate Rb₂C₂O₄·H₂O in the monoclinic crystal system.^[3] and is isomorphic to potassium oxalate monohydrate K₂C₂O₄·H₂O.^[4] Two forms of the anhydrous form (Rb₂C₂O₄) exist at room temperature: one form is monoclinic and isotypic to caesium oxalate (Cs₂C₂O₄), the other is orthorhombic and isotypic to potassium oxalate (K₂C₂O₄).^[5] Freshly prepared anhydrous rubidium oxalate initially contains mainly the monoclinic form, but this slowly transforms irreversibly into the orthorhombic form.^[6] In 2004, two more high-temperature forms of rubidium oxalate were discovered.^[7]

Crystal data of the different forms of rubidium oxalate

Form	Crystal system	Space group	a in Å	b in Å	c in Å	β	Z
Alpha[5]	monoclinic	P21/c	6.328	10.455	8.217	98.016°	4
Beta[5]	orthorhombic	Pbam	11.288	6.295	3.622	—	2
Monohydrate[8]	monoclinic	C2/c	9.617	6.353	11.010	109.46°	4

The standard enthalpy of formation of the crystalline rubidium oxalate is 1325.0 ± 8.1 kJ/mol.^[9]

The decomposition of rubidium oxalate with the release of carbon monoxide and subsequently carbon dioxide and oxygen takes place at 507–527 °C (945–981 °F; 780–800 K).^[6][2]

Rb₂C₂O₄ → Rb₂CO₃ + CO↑

Rb₂CO₃ → Rb₂O + CO₂↑

2 Rb₂O → 4 Rb + O₂↑

In addition to the neutral rubidium oxalate Rb₂C₂O₄, there is also an acidic salt, rubidium hydrogen oxalate with the formula RbHC₂O₄, which is isomorphic to potassium hydrogen oxalate KHC₂O₄^[10] and forms monoclinic crystals,^[11] and an acidic dioxalate with the formula RbHC₂O₄·H₂C₂O₄, which exists as a dihydrate, has a density of 2.125 g/cm³ at 18 °C and a solubility of 21 g/L at 21 °C.^[12]

Upon evaporation of a solution in hydrogen peroxide, rubidium oxalate forms a monoperhydrate of the formula Rb₂C₂O₄·H₂O₂, which forms monoclinic crystals that are relatively stable in air.^[13]

Rubidium oxalate reacts with hydrogen fluoride to form a hydrofluoridate salt (RbHC₂O₄·HF):^[14]

Rb₂C₂O₄ + 2 HF → RbHC₂O₄·HF + RbF