BJ68
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Chromium trioxide
Dear All,
I made research how to produce chromium trioxide from potassium dichromate and found this description:
| Quote: |
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*.
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See https://www.prepchem.com/synthesis-of-chromium-trioxide/
*= ISBN 1-973245-37-X
Does this method work?
I am a little bit sceptical, because from that what I have read, you can not air-evaporate a aqueous solution of "chromic acid" (CrO3 in
water) to get beautiful dark red crystals of pure chromium trioxide.
bj68
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bnull
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It was certainly lifted by Ledgard from a patent or someone else's book.
Did he keep a recipe for porridge in the third edition? There was one in the second.
Sìdgwick (The Chemical Elements and Their Compounds, vol. 2, p. 1003): | Quote: | Chromium Trioxide
A strong solution of potassium dichromate (or the much more soluble sodium salt) is treated with excess of strong sulphuric acid, and the trioxide,
which is only slightly soluble in the concentrated acid, crystallizes out; it is then washed free from sulphuric acid by means of nitric acid, which
must not contain any oxides of nitrogen, as they would reduce the CrO3; the HNO3 can then be removed by warming. If necessary,
the CrO3 can be recrystallized from a small quantity of water. |
Faith, Keyes & Clark (Industrial Chemicals, p. 237-238): | Quote: | | True chromic acid, H2CrO4, is reputed to be formed by heating the anhydride with a small amount of water and cooling. However,
if it does exist, it is very unstable and is not isolated commercially. Aqueous solutions of chromium trioxide are strongly acid and are believed to
contain several polychromic acids such as dichromic (H2Cr2O7) and trichromic acid
(H2Cr3O10), although chromic acid (H2CrO4) is no doubt present. |
Walton (Inorganic Preparations, p. 125): | Quote: | | Chromic acid is a rather weak acid an is also unstable, losing water easily to form its anhydride, CrO3. This oxide is extremely soluble in
water, but the solubility is greatly reduced by adding sulfuric acid, as will be seen in the phase diagram in Fig.21. |
The gist of all that is you can make chromium trioxide by evaporation from a solution in water but you cannot make chromic acid itself by the same
process. Some persons misuse the name chromic acid as synonym for the trioxide.
Edit: Et tu autocorrect.
[Edited on 23-12-2025 by bnull]
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Hexabromobenzene
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This is similar to my method. If you electrolyze a weak acid or base compound, or a salt of an acid or base that is poorly soluble in water, you can
achieve excellent conversion and high current efficiency.
Examples:
Obtaining organic acids from salts or amines, producing sulfuric acid from iron sulfate, magnesium sulfate, and producing ferric chloride from salt
and scrap iron.
This method also produces sulfuric acid. Hydroxide ions don't accumulate in the cathode chamber but are precipitated by magnesium salts. However, you
won't be able to completely remove potassium from the dichromate. This is what sulfuric acid from magnesium sulfate is for. Yes, the method is well
thought out.
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Varungh
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Quote: Originally posted by Hexabromobenzene  | This is similar to my method. If you electrolyze a weak acid or base compound, or a salt of an acid or base that is poorly soluble in water, you can
achieve excellent conversion and high current efficiency.
Examples:
Obtaining organic acids from salts or amines, producing sulfuric acid from iron sulfate, magnesium sulfate, and producing ferric chloride from salt
and scrap iron.
This method also produces sulfuric acid. Hydroxide ions don't accumulate in the cathode chamber but are precipitated by magnesium salts. However, you
won't be able to completely remove potassium from the dichromate. This is what sulfuric acid from magnesium sulfate is for. Yes, the method is well
thought out. |
Well this looks nice. I am gearing up for diaphram electrolyser 2.0 (the main problems with your setup seems to be the leaking). I will try modifying
your setup using the polypropylene diaphram, possibly coated with something as a diaphram for a H Cell. Besides the obvious that this needs some
really nice annodes(Pb).
Any more info on making H2CrO4 or CrO3 will be added to the Making chromate thread.
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Hexabromobenzene
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| Quote: Originally posted by Varungh | | Quote: Originally posted by Hexabromobenzene | This is similar to my method. If you electrolyze a weak acid or base compound, or a salt of an acid or base that is poorly soluble in water, you can
achieve excellent conversion and high current efficiency.
Examples:
Obtaining organic acids from salts or amines, producing sulfuric acid from iron sulfate, magnesium sulfate, and producing ferric chloride from salt
and scrap iron.
This method also produces sulfuric acid. Hydroxide ions don't accumulate in the cathode chamber but are precipitated by magnesium salts. However, you
won't be able to completely remove potassium from the dichromate. This is what sulfuric acid from magnesium sulfate is for. Yes, the method is well
thought out. |
Well this looks nice. I am gearing up for diaphram electrolyser 2.0 (the main problems with your setup seems to be the leaking). I will try modifying
your setup using the polypropylene diaphram, possibly coated with something as a diaphram for a H Cell. Besides the obvious that this needs some
really nice annodes(Pb).
Any more info on making H2CrO4 or CrO3 will be added to the Making chromate thread. |
You can try this diaphragm PP/PP. This is the best I've gotten so far
https://www.sciencemadness.org/whisper/viewthread.php?tid=16...
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