Potassium dichromate

Potassium dichromate
Names:	potassium dichromate(VI) potassium bichromate dipotassium dichromate bichromate of potash
Formula:	K2Cr2O7
Molar mass:	294.181 g/mol
Density:	2.684 g/cm3
Crystal system:	triclinic (α) a=7.3807 Å, b=7.4593 Å, c=13.391 Åα=96.205°, β=98.033°, γ=90.914° monoclinic (β) a=13.0339 Å, b=7.375 Å, c=7.4672 Åα=90°, β=91.923°, γ=90°
Shapes:	https://en.crystalls.info/w/uploads/media/9/9a/Truncated_deformed_hexagonal_prism.json Truncated deformed hexagonal prism https://en.crystalls.info/w/uploads/media/c/ca/Truncated_rhombic_prism.json Truncated rhombic prism https://en.crystalls.info/w/uploads/media/f/fd/Beveled_truncated_rhombic_prism.json Beveled truncated rhombic prism https://en.crystalls.info/w/uploads/media/5/50/Beveled_tetragonal_prism.json Beveled tetragonal prism https://en.crystalls.info/w/uploads/media/a/a4/Truncated_tetragonal_bipyramid.json Truncated tetragonal bipyramid https://en.crystalls.info/w/uploads/media/6/68/Irregular_pinacoidal_prism.json Irregular pinacoidal prism
Color:	orange red
Melting point:	398 °C671.15 K <br />748.4 °F <br />1,208.07 °R <br />
Boiling point:	500 °C773.15 K <br />932 °F <br />1,391.67 °R <br />
Decomposition point:	241.6 °C514.75 K <br />466.88 °F <br />926.55 °R <br /> (α) 610 °C883.15 K <br />1,130 °F <br />1,589.67 °R <br /> (β)
Refractive index:	1.738
Stability:	stable
Hardness:	strong
Toxicity:	poisonous LD50=25mg/kg

Description

Inorganic compound, salt of alkaline metal potassium and inorganic dichromic acid. Doesn't form water hydrates.

Minerals

Occurs in nature as lopezite mineral.

Precursors

Reaction between chromium(III) oxide, hydroxide or potassium carbonate and potassium nitrate

Chemical equation:

Cr₂O₃ + 2KOH + 3KNO₃ = K₂Cr₂O₇ + 3KNO₂ + H₂O

Cr₂O₃ + K₂CO₃ + 3KNO₃ = K₂Cr₂O₇ + 3KNO₂ + CO₂↑

For preparation of 100.00g of potassium dichromate а 51.67g of chromium(III) oxide, 46.98g of potassium carbonate or 38.14g of potassium hydroxide and 103.10g of potassium nitrate is required.

Add to the fireproof ceramic or graphite crucible a reagent mixture and heat it until it become melted with occasional stirring. After the reaction will stop, cool the crucible and separate compound.Add resulting fused mass to the flask with boiling water until solution become saturated. Then filter out the solution and cool slowly. Excess of potassium dichromate forms a crystalline precipitate, when most of impurities remain in the solution.
Separate this precipitate from the stock solution, add another portion of mixture, and repeat this process several times, using the same solution again and again. Finally, the precipitated potassium dichromate is used for crystal growing, and solution with accumulated impurities is thrown away.

Influence of impurities

Addition of sulfuric acid (until solution becomes red) positively influences on crystal shape - mostly smooth monocrystallines are forming

Notes

Concentrated aqueous solutions may damage the skin.

Storage conditions

Keep in its original form or under several layers of varnish at average humidity and room temperature. Do not keep crystals near heaters.

Solubility

TableGraph

Temperature	g/100,00 g water
0°C273.15 K <br />32 °F <br />491.67 °R <br />	4.7
10°C283.15 K <br />50 °F <br />509.67 °R <br />	7.8
20°C293.15 K <br />68 °F <br />527.67 °R <br />	12.48
25°C298.15 K <br />77 °F <br />536.67 °R <br />	15
30°C303.15 K <br />86 °F <br />545.67 °R <br />	18.2
40°C313.15 K <br />104 °F <br />563.67 °R <br />	25.9
60°C333.15 K <br />140 °F <br />599.67 °R <br />	45.6
80°C353.15 K <br />176 °F <br />635.67 °R <br />	73.01
100°C373.15 K <br />212 °F <br />671.67 °R <br />	100

Insoluble in ethanol, acetone and ammonia.

Gallery

• 

  Potassium dichromateGleb Vertyankin, VKontakteGleb VertyankinVKontakte

• 

  Growth from solution with addition of sulfiric acidPotassium dichromate, Vladimir Semikin, VKontakteVladimir SemikinVKontakte

• 

  Potassium dichromateSergey Sachkov, VKontakteSergey SachkovVKontakte

• 

  Potassium dichromateSergey Sachkov, VKontakteSergey SachkovVKontakte

• 

  Potassium dichromateSergey Sachkov, VKontakteSergey SachkovVKontakte

• 

  Potassium dichromateSergey Sachkov, VKontakteSergey SachkovVKontakte

• 

  Growth during 1.5 months from solution with addition of sulfiric acidPotassium dichromate, Rostislav Dolgikh, VKontakteRostislav DolgikhVKontakte

• 

  Growth during 1.5 months from solution with addition of sulfiric acidPotassium dichromate, Rostislav Dolgikh, VKontakteRostislav DolgikhVKontakte

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Sources

• R.A.Kiper Properties of compounds. Handbook
• T.Weakley, Phase transitions in K2Cr2O7 and structural redeterminations of phase II
• S.Krivovichev, β-K2Cr2O7

Navigation

Potassium compounds
Salts	Potassium bromate (KBrO3) • Potassium bromide (KBr) • Potassium chlorate (KClO3) • Potassium chloride (KCl) • Potassium chromate (K2CrO4) • Potassium dihydrophosphate (KH2PO4) • Potassium dichromate (K2Cr2O7) • Potassium iodide (KI) • Potassium hydrofluoride (KHF2) • Potassium hydrophosphate (K2HPO4) • Potassium hydrophthalate (KC8H6O4) • Potassium metavanadate (KVO3) • Potassium nitrate (KNO3) • Potassium sulfate (K2SO4) • Potassium thiocyanate (KNCS)
Double salts	Aluminum-potassium sulfate (KAl(SO4)2) • Antimony(III)-potassium tartrate (K2Sb2(C4H2O6)2) • Chromium(III)-potassium sulfate (KCr(SO4)2) • Cobalt(II)-potassium sulfate (K2Co(SO4)2) • Copper(II)-potassium sulfate (K2Cu(SO4)2) • Indium(III)-potassium sulfate (KIn(SO4)2) • Iron(II)-potassium sulfate (K2Fe(SO4)2) • Iron(III)-potassium sulfate (KFe(SO4)2) • Lithium-potassium sulfate (KLiSO4) • Magnesium-potassium sulfate (K2Mg(SO4)2) • Magnesium-potassium sulfate-chromate (K2CrO4 · MgSO4) • Manganese(II)-potassium sulfate (K2Mn(SO4)2) • Nickel-potassium sulfate (K2Ni(SO4)2) • Potassium-sodium tartrate (KNaC4H4O6) • Vanadium(III)-ammonium sulfate (NH4V(SO4)2) • Zinc-potassium sulfate (K2Zn(SO4)2)
Complexes	Potassium dioxalatocuprate(II) (K2[Cu(C2O4)2]) • Potassium hexanitrocobaltate(III) (K3[Co(NO2)6]) • Potassium hexachlororhodiate(III) (K3[RhCl6]) • Potassium hexacyanocobaltate(III) (K3[Co(CN)6]) • Potassium hexacyanoferrate(II) (K4[Fe(CN)6]) • Tripotassium-sodium hexacyanoferrate(II) (K3Na[Fe(CN)6]) • Potassium hexacyanoferrate(III) (K3[Fe(CN)6]) • Potassium hexacyanochromate(III) (K3[Co(CN)6]) • Potassium tetraiodobismuthate (К[BiI4]) • Potassium tetraiodomercurate(II) (K2[HgI4]) • Potassium trioxalatoaluminate (K3[Al(C2O4)3]) • Potassium trioxalatocobaltate(III) (K3[Co(C2O4)3]) Potassium trioxalatoferrate(III) (K3[Fe(C2O4)3]) • Potassium-sodium trioxalatoferrate(III) (K5Na[Fe(C2O4)3]2) • Dipotassium-sodium trioxalatoferrate(III) (K2Na[Fe(C2O4)3]) • Potassium trioxalatochromate(III) (K3[Cr(C2O4)3])