Rubidium oxalate
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| Rb2C2O4 | |||
| Molar mass | 258.954 g·mol−1 | ||
| Appearance | colourless crystals | ||
| Density | 2.76 g/cm3 (monohydrate) | ||
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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.
Preparation
[edit]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
[edit]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
References
[edit]- ^ Giglio, E.; Loreti, S.; Pavel, N. V. (May 1988). "EXAFS: a new approach to the structure of micellar aggregates". The Journal of Physical Chemistry. 92 (10): 2858–2862. doi:10.1021/j100321a032. ISSN 0022-3654.
- ^ a b Meisel, T.; Halmos, Z.; Seybold, K.; Pungor, E. (February 1975). "The thermal decomposition of alkali metal formates". Journal of Thermal Analysis. 7 (1): 73–80. doi:10.1007/BF01911627. ISSN 0022-5215. S2CID 93705025.
- ^ Ans, Jean d'; Lax, Ellen (1998). Taschenbuch für Chemiker und Physiker (in German). Springer. ISBN 978-3-540-60035-0.
- ^ Pedersen, B. (1966-03-01). "The equilibrium hydrogen–hydrogen distances in the water molecules in potassium and rubidium oxalate monohydrates". Acta Crystallographica. 20 (3): 412–417. doi:10.1107/S0365110X66000951. ISSN 0365-110X.
- ^ a b c Dinnebier, Robert E.; Vensky, Sascha; Panthöfer, Martin; Jansen, Martin (2003-03-10). "Crystal and molecular structures of alkali oxalates: first proof of a staggered oxalate anion in the solid state". Inorganic Chemistry. 42 (5): 1499–1507. doi:10.1021/ic0205536. ISSN 0020-1669. PMID 12611516.
- ^ a b Vensky, Sascha (2004). Konformationsaufklärung anorganischer Oxoanionen des Kohlenstoffs und Festkörpersynthesen durch Elektrokristallisation von Ag3O4 und Na3BiO4 (doctoralThesis thesis) (in German).
- ^ Robert E. Dinnebier, Sascha Vensky, Martin Jansen, Jonathan C. Hanson (2005-02-04), "Crystal Structures and Topological Aspects of the High-Temperature Phases and Decomposition Products of the Alkali-Metal Oxalates M2[C2O4] (M=K, Rb, Cs)", Chemistry - A European Journal, vol. 11, no. 4, pp. 1119–1129, doi:10.1002/chem.200400616, PMID 15624128
{{citation}}: CS1 maint: multiple names: authors list (link) - ^ Takuya Echigo, Mitsuyoshi Kimata (November 2006), "The common role of water molecule and lone electron pair as a bond-valence mediator in oxalate complexes : the crystal structures of Rb2(C2O4) · H2O and Tl2(C2O4)", Zeitschrift für Kristallographie, vol. 221, no. 12, pp. 762–769, Bibcode:2006ZK....221..762E, doi:10.1524/zkri.2006.221.12.762, S2CID 98482669
- ^ Masuda, Y.; Miyamoto, H.; Kaneko, Y.; Hirosawa, K. (February 1985). "The standard molar enthalpies of formation of crystalline rubidium and cesium oxalates". The Journal of Chemical Thermodynamics. 17 (2): 159–164. doi:10.1016/0021-9614(85)90068-0.
- ^ Piccard, Julius (1862). "Beitrag zur Kenntniss der Rubidiumverbindungen". Journal für Praktische Chemie (in German). 86 (1): 449–460. doi:10.1002/prac.18620860163. ISSN 0021-8383.
- ^ Watts, Henry (1866). A Dictionary of Chemistry and the Allied Branches of Other Sciences. Longmans, Green, Longman, Roberts & Green.
- ^ Abegg, Richard Wilhelm Heinrich; Auerbach, Friedrich; Koppel, Ivan (1905). Handbuch der anorganischen Chemie. University of California. Leipzig, S. Hirzel.
- ^ Pedersen, Berit F.; Seip, Hans M.; Santesson, Johan; Holmberg, Pär; Eriksson, G.; Blinc, R.; Paušak, S.; Ehrenberg, L.; Dumanović, J. (1967). "The Crystal Structure of Potassium and Rubidium Oxalate Monoperhydrates, K2C2O4.H2O2 and Rb2C2O4.H2O2". Acta Chemica Scandinavica. 21: 779–790. doi:10.3891/acta.chem.scand.21-0779. ISSN 0904-213X.
- ^ Weinland, R. F.; Stille, W. (1903). "Ueber die Anlagerung von Krystallfluorwasserstoff an Oxalate und an Ammoniumtartrat". Justus Liebig's Annalen der Chemie (in German). 328 (2): 149–153. doi:10.1002/jlac.19033280205.