Difference between revisions of "Chrome alum"

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| OtherNames = Chromium alum
 
| OtherNames = Chromium alum
 
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| Appearance = Purple to black crystals (dodecahydrate)
 
| Appearance = Purple to black crystals (dodecahydrate)
 
| BoilingPt =  
 
| BoilingPt =  
−
| BoilingPtC =
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| BoilingPtC = 400
 
| BoilingPt_ref =  
 
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| BoilingPt_notes =
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| BoilingPt_notes = (decomposes)
 
| Density = 2.657 g/cm<sup>3</sup> (20 °C, anhydrous)<ref name="density_gmelin">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</ref><br>1.826 g/cm<sup>3</sup> (25 °C, dodecahydrate)<ref>H. P. Klug, J. Am. Soc 1940, 62, 2992-2993</ref>
 
| Density = 2.657 g/cm<sup>3</sup> (20 °C, anhydrous)<ref name="density_gmelin">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</ref><br>1.826 g/cm<sup>3</sup> (25 °C, dodecahydrate)<ref>H. P. Klug, J. Am. Soc 1940, 62, 2992-2993</ref>
 
| Formula = KCr(SO<sub>4</sub>)<sub>2</sub> (anhydrous)<br>KCr(SO<sub>4</sub>)<sub>2</sub> · 12 H<sub>2</sub>O (dodecahydrate)
 
| Formula = KCr(SO<sub>4</sub>)<sub>2</sub> (anhydrous)<br>KCr(SO<sub>4</sub>)<sub>2</sub> · 12 H<sub>2</sub>O (dodecahydrate)
 
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| LogP =  
 
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| MolarMass = 283.22 g/mol (anhydrous)<br>499.4 g/mol (dodecahydrate)
 
| MeltingPt =  
 
| MeltingPt =  
 
| MeltingPtC = 75-90
 
| MeltingPtC = 75-90
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| Section3 = {{Chembox Structure
 
| Section3 = {{Chembox Structure
 
| Coordination =  
 
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| CrystalStruct = Cubic
 
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}}
 
}}
  
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Chrome alum, or potassium chromium(III) sulfate is an inorganic compound with the formula KCr(SO<sub>4</sub>)<sub>2</sub> and most commonly found as the dodecahydrate. It is the double salt of [[Chromium(III) sulfate]] and [[Potassium sulfate]].
+
'''Chrome alum''', or '''potassium chromium(III) sulfate''' is an inorganic compound with the formula '''KCr(SO<sub>4</sub>)<sub>2</sub>''' and most commonly found as the dodecahydrate. It is the double salt of [[chromium(III) sulfate]] and [[potassium sulfate]].
  
 
==Properties==
 
==Properties==
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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).<ref name="solubility_gmelin"/> The green form consists of a multitude of different sulfato and μ-OH-complexes (polynuclear).<ref>Gmelins Handbuch der Anorganischen Chemie, Chrom Teil B, 8th edition 1962, p. 636</ref><ref>H. T. Hall, H. Eyring, J. Am. Chem. Soc. 1950, 72, 2, 782–790, https://doi.org/10.1021/ja01158a036</ref><ref>H. Sueda, Bull. Chem. Soc. Jpn. 1937, 12, 12, 524–536, https://doi.org/10.1246/bcsj.12.524</ref>
 
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).<ref name="solubility_gmelin"/> The green form consists of a multitude of different sulfato and μ-OH-complexes (polynuclear).<ref>Gmelins Handbuch der Anorganischen Chemie, Chrom Teil B, 8th edition 1962, p. 636</ref><ref>H. T. Hall, H. Eyring, J. Am. Chem. Soc. 1950, 72, 2, 782–790, https://doi.org/10.1021/ja01158a036</ref><ref>H. Sueda, Bull. Chem. Soc. Jpn. 1937, 12, 12, 524–536, https://doi.org/10.1246/bcsj.12.524</ref>
  
−
Chrome alum forms isomorphous crystals with potassium alum. Solutions of both salts will cocrystallize, however the crystals will contain less chromium than the solution.
+
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.<ref>C. H. Rehberg, Neues Jahrbuch für Mineralogie, Geologie und Paläontologie, Abteilung A, 1950, 80, 1</ref>
 +
 
 +
{| class="wikitable sortable"
 +
!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.<ref>E. Kirkova, R. Nikolaeva, Krist. Techn. 1973, 8, 463-470, https://doi.org/10.1002/crat.19730080409</ref><ref>P. J. Brendt, Phys. Rev. B 1970, 2, 4366, https://doi.org/10.1103/PhysRevB.2.4366</ref><ref>R. Krishnaswamy, M. Rajalakshmi, Indian J. Phys. 1987, 61A, 469-472, https://archive.org/details/in.ernet.dli.2015.459814/page/n554/mode/1up</ref>
  
 
===Physical===
 
===Physical===
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==Preparation==
 
==Preparation==
−
Reduction of potassium dichromate with aq. sulfuric acid and ethanol yields chrome alum. Care must be taken to keep the solution cold.
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Reduction of [[potassium dichromate]] with aq. sulfuric acid and ethanol yields chrome alum. Care must be taken to keep the solution cold.
 +
 
 +
: K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> + 4 H<sub>2</sub>SO<sub>4</sub> + 3 CH<sub>3</sub>CH<sub>2</sub>OH + 17 H<sub>2</sub>O →  2 KCr(SO<sub>4</sub>)<sub>2</sub>·12 H<sub>2</sub>O + 3 CH<sub>3</sub>CHO
  
−
: K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> + 4 H<sub>2</sub>SO<sub>4</sub> + 3 CH<sub>3</sub>CH<sub>2</sub>OH + 17 H<sub>2</sub>O -> 2 KCr(SO<sub>4</sub>)<sub>2</sub> · 12 H<sub>2</sub>O + 3 CH<sub>3</sub>CHO
+
[[Sulfur dioxide]] can also be used.
  
 
==Projects==
 
==Projects==
−
* Grow crystals
+
*Grow crystals
 +
*Hardener for gelatine emulsions in photographic film
 +
*Leather tanning
  
 
==Handling==
 
==Handling==
 
===Safety===
 
===Safety===
−
...
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Soluble chromium(III) salts are considered to be of low toxicity. Allergenic potential is discussed.<ref>Gmelins Handbuch der Anorganischen Chemie, Chrom Teil A - 1, 8th edition 1962, p. 276</ref>
  
 
===Storage===
 
===Storage===
−
...
+
Storage at room temperature in closed plastic bottles is recommended.
  
 
===Disposal===
 
===Disposal===
−
...
+
Precipitate chromium(III) as the hydroxide before disposal.
  
 
==References==
 
==References==
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[[Category:Inorganic compounds]]
 
[[Category:Inorganic compounds]]
 
[[Category:Chromium compounds]]
 
[[Category:Chromium compounds]]
 +
[[Category:Potassium compounds]]
 
[[Category:Sulfates]]
 
[[Category:Sulfates]]
 +
[[Category:Chemicals for crystal growing]]
 +
[[Category:Easily prepared chemicals]]
 +
[[Category:Solids]]

Latest revision as of 11:41, 24 August 2025

Potassium chromium(III) sulfate
Chrome alum.jpg
Chrome alum large.jpg
Names
IUPAC name
Chromium potassium sulfate
Other names
Chromium alum
Properties
KCr(SO4)2 (anhydrous)
KCr(SO4)2 · 12 H2O (dodecahydrate)
Molar mass 283.22 g/mol (anhydrous)
499.4 g/mol (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)
Boiling point 400 °C (752 °F; 673 K) (decomposes)
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
 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.

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

  1. ↑ 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
  2. ↑ H. P. Klug, J. Am. Soc 1940, 62, 2992-2993
  3. ↑ 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
  4. ↑ Gmelins Handbuch der Anorganischen Chemie, Chrom Teil B, 8th edition 1962, p. 636
  5. ↑ H. T. Hall, H. Eyring, J. Am. Chem. Soc. 1950, 72, 2, 782–790, https://doi.org/10.1021/ja01158a036
  6. ↑ H. Sueda, Bull. Chem. Soc. Jpn. 1937, 12, 12, 524–536, https://doi.org/10.1246/bcsj.12.524
  7. ↑ C. H. Rehberg, Neues Jahrbuch für Mineralogie, Geologie und Paläontologie, Abteilung A, 1950, 80, 1
  8. ↑ E. Kirkova, R. Nikolaeva, Krist. Techn. 1973, 8, 463-470, https://doi.org/10.1002/crat.19730080409
  9. ↑ P. J. Brendt, Phys. Rev. B 1970, 2, 4366, https://doi.org/10.1103/PhysRevB.2.4366
  10. ↑ R. Krishnaswamy, M. Rajalakshmi, Indian J. Phys. 1987, 61A, 469-472, https://archive.org/details/in.ernet.dli.2015.459814/page/n554/mode/1up
  11. ↑ Gmelins Handbuch der Anorganischen Chemie, Chrom Teil A - 1, 8th edition 1962, p. 276

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