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 C2O4(2−). Rubidium oxalate forms a monohydrate Rb2C2O4*H2O.

Preparation
Rubidium carbonate and oxalic acid react to form rubidium oxalate:


 * Rb2CO3 + H2C2O4 → Rb2C2O4 + H2O + CO2↑

Rubidium oxalate can also be obtained via the thermal decomposition of rubidium formate:


 * 2 HCOORb → Rb2C2O4 + H2↑

Properties
From an aqueous solution, rubidium oxalate crystallizes as a monohydrate Rb2C2O4*H2O in the monoclinic crystal system. and is isomorphic to potassium oxalate monohydrate K2C2O4*H2O. 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). Freshly prepared anhydrous rubidium oxalate initially contains mainly the monoclinic form, but this slowly transforms irreversibly into the orthorhombic form. In 2004, two more high-temperature forms of rubidium oxalate were discovered.

Crystal data of the different forms of rubidium oxalate

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

The decomposition of rubidium oxalate with the release of carbon monoxide and subsequently carbon dioxide and oxygen takes place at 507–527 C.


 * 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 and forms monoclinic crystals, 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.

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.

Rubidium oxalate reacts with hydrogen fluoride to form a hydrofluoridate salt (RbHC2O4*HF):


 * Rb2C2O4 + 2 HF → RbHC2O4*HF + RbF