Rubidium oxalate
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| Properties | |||
| Rb2C2O4 | |||
| Molar mass | 258.954 g·mol−1 | ||
| Appearance | colourless crystals | ||
| Density | 2.76 g/cm3 (monohydrate) | ||
| soluble | |||
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Std enthalpy of formation (ΔfH⦵298) |
1325.0 ± 8.1 kJ/mol[1] | ||
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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.
From an aqueous solution, rubidium oxalate crystallizes as a monohydrate Rb2C2O4·H2O in the monoclinic crystal system[2] and is isomorphic to potassium oxalate monohydrate K2C2O4·H2O.[3] 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).[4] Freshly prepared anhydrous rubidium oxalate initially contains mainly the monoclinic form, but this slowly transforms irreversibly into the orthorhombic form.[5] In 2004, two more high-temperature forms of rubidium oxalate were discovered.[6]
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[4] | monoclinic | P21/c | 6.328 | 10.455 | 8.217 | 98.016° | 4 |
| Beta[4] | orthorhombic | Pbam | 11.288 | 6.295 | 3.622 | — | 2 |
| Monohydrate[7] | monoclinic | C2/c | 9.617 | 6.353 | 11.010 | 109.46° | 4 |
Preparation
Rubidium carbonate and oxalic acid react to form rubidium oxalate:[8]
- Rb2CO3 + H2C2O4 → Rb2C2O4 + H2O + CO2↑
Rubidium oxalate can also be obtained via the thermal decomposition of rubidium formate:[9]
- 2 HCOORb → Rb2C2O4 + H2↑