Preparation of chromium trioxide (chromium(VI) oxide; chromic anhydride)
The addition of sulfuric acid to a solution of either a chromate or a dichromate liberates chromic acid which is very soluble and can exist in solution in the different forms, H2CrO4, H2Cr2O7 and CrO3, in equilibrium with each other. With the addition of a large excess of concentrated sulfuric acid, water is withdrawn from the hydrated forms and the chromium trioxide separates in the shape of red needles. Chromium trioxide could also be obtained from potassium dichromate, however, due to poor solubility the synthesis is more complicated.
100 grams of sodium dichromate are dissolved in 250 ml of water and the obtained solution is filtered. 200 ml of concentrated sulfuric acid are dropwise added with constant stirring until a slight permanent precipitate of chromium trioxide is formed. The mixture is cooled for half an hour or longer, then slowly, while stirring, 200 ml of concentrated sulfuric acid are added dropwise. The reaction mixture is left to stand over night in order that the crystal meal may become somewhat coarser. The larger crystals (and purer) of chromium trioxide could be obtained by heating the solution with crystal meal with stirring to 100° C and allowing to cool slowly. If this process is repeated once or twice, a more satisfactory product will be obtained. The obtained crystals of chromium trioxide are filtered with a Buchner funnel with a sintered glass disc instead of the usual paper filter (chromium trioxide is a strong oxidizing agent and will oxidize organic material). After draining the crystals completely and pressing the surface and treating with 15 ml of concentrated (65-70%) nitric acid. The nitric acid is removed by suction and the operation is repeated twice with 10 ml of nitric acid each time. Finally, the red crystals of chromium trioxide are dried by suction as much as possible, transferred in a dry evaporating dish and by gentle heating the remaining nitric acid is removed. Dried product is stored in a stoppered bottle. Yield is 60%.
If necessary chromium trioxide is purified by recrystallization. 300 grams of chromium trioxide are dissolved in 180 ml of water. The obtained solution is carefully decanted from any undissolved residue. Very slowly concentrated sulfuric acids is added to the decanted solution. The required amount of sulfuric acid is 100 ml for every 100 grams of the crude chromic anhydride. When the suspension has cooled to room temperature, The crystals are collected, washed in the same manner as in the original preparation using 60 ml of concentrated nitric acid for the first washing and 40 ml each for the second and third washings. The crystals are drained very thoroughly with suction in order to remove as much as possible of nitric acid, then transferred in a porcelain dish and gently heated on a hot plate. When after several hours nitric acid vapors ceases to come off, the pure chromium trioxide is transferred in a stoppered bottle.
Synthetic inorganic chemistry, by A. A. Blanchard, 324-325, 1936
Preparation of chromium trioxide is presented in following video by Doug’s Lab.
Electrochemical synthesis of chromium trioxide
The following synthesis description for the preparation of chromium trioxide was submitted by Jared Ledgard from Kings Chem Guide, 3th edition
Into an apparatus as illustrated in the following, pour into the anode compartment a salt solution prepared by adding and dissolving 100 grams (3.5 oz.) of potassium dichromate into 700 milliliters (23.7 fluid oz.) of warm water.
Thereafter, pour into the cathode compartment, a salt solution prepared by adding and dissolving 400 grams (14.1 oz.) of Epsom salt into 1700 milliliters (57.5 fluid oz.) of water. Thereafter, put in place the electrodes as illustrated, and then attach the power clamps; thereafter, turn on the power supply and electrolysize the cell at 6 to 12 volt at 6 amp DC for 24 hours. After about 24 hours, discontinue the power supply, open the cell, remove the electrodes, and then pull-out the clay pot and pour its contents into a clean beaker. Thereafter, filter this anode liquid to remove insoluble impurities, and then pour the filtered anode liquid onto a shallow pan or tray and allow it to air-evaporate forming beautiful dark red crystals of pure chromium trioxide. The cathode liquid will contain allot of white precipitate of magnesium hydroxide, which can be collected by filtration if desired; after removing the insoluble magnesium hydroxide, the filtered liquid will contain dissolved potassium sulfate, which can be collected by recrystallization if desired or simply discarded.
Kings Chem Guide Third Edition, by Jared Ledgard, 293-294, 2014.
chromic anhydride, chromium trioxide
Chromium trioxide, Chromium(VI) oxide, trioxochromium, Monochromium trioxide, Chromtrioxid, CHROMIC OXIDE, CHROMIC TRIOXIDE, Chromsaeureanhydrid, Anhydride chromique, Chromium(6+) oxide, CrO3, Chromium(VI) trioxide, Chromium oxide (CrO3), Chromium anhydride, trioxyde de chrome, Chrom(VI)-oxid, Chromia (CrO3), Chromic acid, solid, Red oxide of chromium, Chromtrioxid [German], Chroomtrioxyde [Dutch], Chromium(6+) trioxide, Sintered chromium trioxide, Anidride cromica [Italian], Chromium trioxide, sintered, Chroomzuuranhydride [Dutch], Chromium oxide (Cr4O12), Puratronic chromium trioxide, [CrO3], Chromsaeureanhydrid [German], CCRIS 160, Chromium(VI) oxide (1:3), Cromo(triossido di) [Italian], HSDB 518, Chromic acid, polymer-supported, Chrome (trioxyde de) [French], CHEBI:48240, EINECS 215-607-8, UN1463, Chromium trioxide [Chromium and chromium compounds], anhydride, Chroomtrioxyde, chromic acie, Anidride cromica, Chroomzuuranhydride, Cromo(triossido di), Chrome (trioxyde de), chromic acid (misnomer), Epitope ID:158535, AC1L24SL, Chromium trioxide, anhydrous, 07404_ALDRICH, 07404_RIEDEL, 232653_ALDRICH, 359823_ALDRICH, 523577_ALDRICH, 675644_ALDRICH, 27081_FLUKA, MolPort-003-925-559, 27081_SIAL, 232653_SIAL, 236470_SIAL, 523577_SIAL, Chromic acid on Amberlyst(R) A-26, LS-769, AKOS005259906, Chromic acid on Amberlyst® A-26, Sulfuric acid-chromium trioxide mixture, Jone inverted exclamation markas reagent, AN-49108, BP-30107, Chromium trioxide solution in sulfuric acid, FT-0694904, CHROMIUM TRIOXIDE (CHEMICAL MIXTURE STUDY), 3B4-1941, Chromium trioxide, anhydrous [UN1463] [Oxidizer], Chromium trioxide, anhydrous [UN1463] [Oxidizer], I14-18129, 12324-05-9, 12324-08-2, 65272-70-0
Chromic anhydride, Chromium oxide, Chromic(VI) acid, Chromosulfuric acid, Anhydride chromique [French], UNII-8LV49809UC, CID14915, 1333-82-0, C028801