Chrome alum
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| Names | |
|---|---|
| IUPAC name
Chromium potassium sulfate
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| Other names
Chromium alum
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| Properties | |
| KCr(SO4)2 (anhydrous) KCr(SO4)2 · 12 H2O (dodecahydrate) | |
| Appearance | Purple to black crystals (dodecahydrate) |
| Odor | Odorless |
| Density | 2.657 g/cm3 (20 °C, anhydrous)[1] 1.826 g/cm3 (25 °C, dodecahydrate)[2] |
| Melting point | 75–90 °C (167–194 °F; 348–363 K) (with decomposition) |
| Anhydrous Insoluble Dodecahydrate 208.5 g/L (18 °C, 19 h)[3] 225.8 g/L (18 °C, 14 d)[3] 282.2 g/L (18 °C, 97 d)[3] | |
| Solubility | Reacts with bases >100 g/L in glycerol Insoluble in 70% ethanol Soluble in mineral acids |
| Vapor pressure | ~0 mmHg |
| Hazards | |
| Safety data sheet | [1] (hydrate) |
| Flash point | Non-flammable |
| Related compounds | |
| Related compounds
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Chromium(III) sulfate |
| Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). | |
| Infobox references | |
Chrome alum, or potassium chromium(III) sulfate is an inorganic compound with the formula KCr(SO4)2 and most commonly found as the dodecahydrate. It is the double salt of chromium(III) sulfate and potassium sulfate.
Contents
Properties
Chemical
Solutions of chrome alum show the typical reactions of aqueous chromium(III) salts. The anhydrous salt is insoluble in water, while the dodecahydrate is soluble. Due to formation of various complexes in solution, also known as the "green form" of chrome alum, the solubility slowly rises over time (approaching equilibrium after hundreds of days). With increasing temperature, not only is the equilibrium reached faster, the fraction of the green form in equilibrium also rises. Consequently, the fraction of the purple form drops from 52.3% (19 °C, 10% solution) to 17.5% (42.5 °C, 10% solution).[3] The green form consists of a multitude of different sulfato and μ-OH-complexes (polynuclear).[4][5][6]
Chrome alum forms isomorphous crystals with potassium alum. Solutions of both salts will cocrystallize, however the crystals will contain less chromium than the solution. The following data are for crystals grown from solutions prepared at <40 °C and crystallized at 12-14 °C by evaporation.[7]
| Molar fraction Cr/Al in solution | Molar fraction Cr/Al in crystals |
|---|---|
| 0.0 | 0.0 |
| 10.0 | 4.6 |
| 20.0 | 11.3 |
| 30.0 | 22.1 |
| 40.0 | 29.4 |
| 50.5 (sic!) | 41.9 |
| 60.0 | 52.1 |
| 70.0 | 61.7 |
| 80.0 | 71.0 |
| 90.0 | 89.6 |
| 100.0 | 100.0 |
Other literature data is sparse and partially conflicting.[8][9][10]
Physical
The dodecahydrate forms large octahedral crystals which look almost black. They easily dehydrate and disintegrate even at room temperature.
Availability
Chemical suppliers or places selling crystal growing supplies.
Preparation
Reduction of potassium dichromate with aq. sulfuric acid and ethanol yields chrome alum. Care must be taken to keep the solution cold.
- K2Cr2O7 + 4 H2SO4 + 3 CH3CH2OH + 17 H2O → 2 KCr(SO4)2·12 H2O + 3 CH3CHO
Sulfur dioxide can also be used.
Projects
- Grow crystals
- Hardener for gelatine emulsions in photographic film
- Leather tanning
Handling
Safety
Soluble chromium(III) salts are considered to be of low toxicity. Allergenic potential is discussed.[11]
Storage
Storage at room temperature in closed plastic bottles is recommended.
Disposal
Precipitate chromium(III) as the hydroxide before disposal.
References
- ↑ F. Kraus, A. Fricke, H. Querengässer, Z. anorg. Ch. 1929, 181, 38-54. Cited in: Gmelins Handbuch der Anorganischen Chemie, Chrom Teil B, 8th edition 1962, p. 604
- ↑ H. P. Klug, J. Am. Soc 1940, 62, 2992-2993
- ↑ 3.0 3.1 3.2 3.3 C. Montemartini, E. Vernazza, Ind. chim. 1933, 8, 445-446. Cited in: Gmelins Handbuch der Anorganischen Chemie, Chrom Teil B, 8th edition 1962, p. 606, 637
- ↑ Gmelins Handbuch der Anorganischen Chemie, Chrom Teil B, 8th edition 1962, p. 636
- ↑ H. T. Hall, H. Eyring, J. Am. Chem. Soc. 1950, 72, 2, 782–790, https://doi.org/10.1021/ja01158a036
- ↑ H. Sueda, Bull. Chem. Soc. Jpn. 1937, 12, 12, 524–536, https://doi.org/10.1246/bcsj.12.524
- ↑ C. H. Rehberg, Neues Jahrbuch für Mineralogie, Geologie und Paläontologie, Abteilung A, 1950, 80, 1
- ↑ E. Kirkova, R. Nikolaeva, Krist. Techn. 1973, 8, 463-470, https://doi.org/10.1002/crat.19730080409
- ↑ P. J. Brendt, Phys. Rev. B 1970, 2, 4366, https://doi.org/10.1103/PhysRevB.2.4366
- ↑ R. Krishnaswamy, M. Rajalakshmi, Indian J. Phys. 1987, 61A, 469-472, https://archive.org/details/in.ernet.dli.2015.459814/page/n554/mode/1up
- ↑ Gmelins Handbuch der Anorganischen Chemie, Chrom Teil A - 1, 8th edition 1962, p. 276
