Potassium dichromate

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  Potassium dichromate   
Names: potassium dichromate(VI)
potassium bichromate
dipotassium dichromate

Formula: K2Cr2O7
SMILES: [O-][Cr](=O)(=O)O[Cr](=O)(=O)[O-].[K+].[K+]
Molar mass: 294.181 g/mol
Density: 2.684 g/cm3
Crystal system: triclinic (α)
a=7.3807 Å
7.3807 Å
0.7381 nm
, b=7.4593 Å
7.4593 Å
0.7459 nm
, c=13.391 Å
13.3910 Å
1.3391 nm
α=96.205°
96°12′18.0″  
, β=98.033°
98°1′58.8″  
, γ=90.914°
90°54′50.4″  

monoclinic (β)
a=13.0339 Å
13.0339 Å
1.3034 nm
, b=7.375 Å
7.3750 Å
0.7375 nm
, c=7.4672 Å
7.4672 Å
0.7467 nm
α=90°
90°  
, β=91.923°
91°55′22.8″  
, γ=90°
90°  

Shapes:
https://media.crystalls.info/w/uploads/media/Truncated_deformed_hexagonal_prism.json
https://media.crystalls.info/w/uploads/media/Truncated_rhombic_prism.json
https://media.crystalls.info/w/uploads/media/Beveled_truncated_rhombic_prism.json
https://media.crystalls.info/w/uploads/media/Beveled_tetragonal_prism.json
https://media.crystalls.info/w/uploads/media/Truncated_tetragonal_bipyramid.json
https://media.crystalls.info/w/uploads/media/Irregular_pinacoidal_prism.json

Color: orange
red
Melting point: 398 °C
671.15 K
748.4 °F
1,208.07 °R

Boiling point: 500 °C
773.15 K
932 °F
1,391.67 °R

Decomposition point: 241.6 °C
514.75 K
466.88 °F
926.55 °R
(α)
610 °C
883.15 K
1,130 °F
1,589.67 °R
(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 compound precipitates as a crystalline precipitate, most of impurities remain in the solution.
Separate this precipitate from the stock solution, and repeat this process for several times, use the same solution at each next step. Finally, the precipitated compound is used for crystal growing.

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

Store in its original form or under several layers of varnish at average humidity and room temperature. Do not store in matchboxes or cotton wool and do not heat.

Solubility

Temperatureгр/100,00 гр water
0°C
273.15 K
32 °F
491.67 °R
4.7
4.7 g/100g
4.489 %
10°C
283.15 K
50 °F
509.67 °R
7.8
7.8 g/100g
7.236 %
15°C
288.15 K
59 °F
518.67 °R
~10.1
10.1 g/100g
9.173 %
20°C
293.15 K
68 °F
527.67 °R
12.48
12.48 g/100g
11.095 %
25°C
298.15 K
77 °F
536.67 °R
15
15 g/100g
13.043 %
30°C
303.15 K
86 °F
545.67 °R
18.2
18.2 g/100g
15.398 %
40°C
313.15 K
104 °F
563.67 °R
25.9
25.9 g/100g
20.572 %
50°C
323.15 K
122 °F
581.67 °R
~35.8
35.8 g/100g
26.362 %
60°C
333.15 K
140 °F
599.67 °R
45.6
45.6 g/100g
31.319 %
70°C
343.15 K
158 °F
617.67 °R
~59.3
59.3 g/100g
37.225 %
80°C
353.15 K
176 °F
635.67 °R
73.01
73.01 g/100g
42.2 %
90°C
363.15 K
194 °F
653.67 °R
~86.5
86.5 g/100g
46.381 %
100°C
373.15 K
212 °F
671.67 °R
100
100 g/100g
50 %
Insoluble in ethanol, acetone and ammonia.

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