laekkerBoy
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Synthesis of Potassium Dichromate
Hi everyone!
As I was looking around on the "Rhodium's Chemistry Page", I found a method on synthesizing Potassium Dichromate. When I first looked at it, I
realized the it wasn't perfectly stoichiometric. Thats why i decided to recalculate the amounts in moles. In this post, I will both tell you about the
reactions and also seek corrections. As you probably already know, I am quite new on this forum and in inorganic chemistry as well.
But anyways, I hope that you like it and can use it:
1mol. (52g.) of Chromium is first reacted with
3mol. (316ml.) of Hydrochloric acid 30%.
Add in the metal in small amounts and heat if the reaction is going to slowly.
Filter the solution to remove any impurities.
This will yield 1mol. of Chromium(III) Chloride and the Hydrogen will bubble off.
(2Cr + 6HCl ==> 2CrCl3 + 3H2).
In a new beaker, dissolve
3mol. (120g.) of Sodium Hydroxide in just enough water.
Let the solution cool to room temperature.
Slowly add this solution to the previously made Chromium(III) Chloride-solution.
Filter the solution and wash the precipitate three times with water to remove any unreacted chemicals. No need to dry, just transfer the precipitate
to a clean beaker.
This will yield 1mol. of Chromium(III) Hydroxide and the side reaction of Sodium Chloride have been washed away.
(CrCl3 + 3NaOH ==> Cr(OH)3 + 3NaCl).
In this new beaker, react the
1mol. of Chromium(III) Hydroxide with
2mol. (80g.) of Sodium Hydroxide in just enough water and
1.5mol. (121ml.) Hydrogen Peroxide 37%
This will yield 1mol. of Sodium Chromate and the water will just stay in the solution.
(2Cr(OH)3 + 4NaOH + 3H2O2 ==> 2Na2CrO4 + 8H2O).
To this solution add,
Hydrochloric acid 30% until acidic.
The Sodium Chromate will then turn into Sodium Dichromate because of the acidic environment.
Boil down the solution to about 50% of it's original volume and eventually filter off white crystals of Sodium Chloride that might have formed from
excess Sodium Hydroxide reacting with the Hydrochloric acid.
To the concentrated solution add,
2mol. Potassium Chloride dissolved in 80C hot water to make a saturated solution.
This will yield 1mol. of Potassium Dichromate and a side reaction of 2mol. of Sodium Chloride.
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If Ammonium Dichromate is wanted instead of Potassium Dichromate, follow this process:
1mol. (52g.) of Chromium is first reacted with
3mol. (316ml.) of Hydrochloric acid 30%.
Add in the metal in small amounts and heat if the reaction is going to slowly.
Filter the solution to remove any impurities.
This will yield 1mol. of Chromium(III) Chloride and the Hydrogen will bubble off.
(2Cr + 6HCl ==> 2CrCl3 + 3H2).
In a new beaker, dissolve
3mol. (120g.) of Sodium Hydroxide in just enough water.
Let the solution cool to room temperature.
Slowly add this solution to the previously made Chromium(III) Chloride-solution.
Filter the solution and wash the precipitate three times with water to remove any unreacted chemicals. No need to dry, just transfer the precipitate
to a clean beaker.
This will yield 1mol. of Chromium(III) Hydroxide and the side reaction of Sodium Chloride have been washed away.
(CrCl3 + 3NaOH ==> Cr(OH)3 + 3NaCl).
In this new beaker, react the
1mol. of Chromium(III) Hydroxide with
2mol. (119ml.) of Ammonium Hydroxide 32% and
0.5mol. (40ml.) Hydrogen Peroxide 37%
This will yield 1mol. of Ammonium Chromate and the water will just stay in the solution.
(2Cr(OH)3 + 4NH3OH + H2O2 ==> 2(NH4)2CrO4 + 4H2O).
To this solution add,
Hydrochloric acid 30% until acidic.
The Ammonium Chromate will then turn into Ammonium Dichromate because of the acidic environment.
Boil down the solution to about 50% of it's original volume and eventually filter off white crystals of Ammonium Chloride that might have formed from
excess Ammonium Hydroxide reacting with the Hydrochloric acid.
___________________________________________________________
I hope that this is helpful! it took me quite a lot of time to calculate and so on! But the result seems good to me.
Please leave a comment and any corrections for the above reactions.
The Kindest Regards,
Christian.
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woelen
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Did you really try these reactions or are these just calculations? I expect some problems in this process. Especially the reaction of hydrogen
peroxide with chromium(III) is not a smooth one. There are many side reactions, such as formation of dark brown peroxo complexes of chromium and the
decomposition of hydrogen peroxide. I'm afraid that you need much more than the stoichiometric amount.
Other common methods of making chromates and dichromates uses molten NaOH or KOH, combined with Cr2O3 and an oxidizing species like KNO3 or KClO3 (or
just air, bubbled through the melt for a prolonged time). After this reaction, the melt is dissolved in water, acidified and crystals of K2Cr2O7 are
separated.
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laekkerBoy
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I have read of all that already, but my problem is that I am not capable of achieving temperatures high enough to complete the reaction. I do also
think that it is a lot easier on the aqueous plan.
But thanks a lot!
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not_important
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Yet you want to make phenol in a vapour phase reaction at similar temperatures. I find myself confused.
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laekkerBoy
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As far as i know, it is unnecessary to have the products in vapor phase to make phenol.
But it isn't vapors that are my problem. It is more the containment of the molten chemicals.
Sorry for bad explanation!
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not_important
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you have to run the hydrolysis at 420-480 C, see U.S. patent 2035917 - the original patent on the Raschig process; previous versions used poorer
catalysts and ran at ~550 C. You can do it in the liquid phase, but at those temperatures that means high pressures. The Dow process does run in the
liquid phase, dilute NaOH at 350 C which means high pressure.
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12AX7
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The best way I've found is Cr2O3 + KClO3. This can be done on the stove, in a pinch, but you'd better do it outside as it releases Cl2 and other
nasties, not to mention the tendancy to run away. The product is K2Cr2O7 and KCl.
Tim
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laekkerBoy
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Thanks a lot! Now I don't see anymore excuses why not to make a chlorate cell 
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Taoiseach
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Cr2O3 is a catalyst in the thermal decomposition of chlorate. At just 200°C the yield of dichromate based on sodium chlorate is well above 90%.
Google for "Thermal decomposition of sodium chlorate and chromium(III) oxide mixtures" for more.
A good ratio is 36 parts (w) sodium chlorate to 20 parts chromium oxide.
I tried the reaction and it works wonderfully, except that it takes several hours to expel the last traces of chlorine (at least at such a low
temperature). Since chlorine has such an intensive smell its easy to tell wether the reaction is complete or not.
The sodium dichromate thus obtained can be used to make ammonium dichromate in a single step using metathesis reaction with NH4Cl.
I also made dichromates using nitrate but it requires much higher temperatures. Yield is somewhat lower, possibly due to the dichromate decomposing at
such high temperatures.
[Edited on 23-4-2010 by Taoiseach]
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Hamilton
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Thx for that method of using Cr2O3 and KClO3 just got it to work in micro size!.
Could it be possible to make K2FeO4 in a similar manner?
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Taoiseach
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Yes it is 
You can mix iron sulfate or iron powder with potassium chlorate. After fusing the mixture, cooling and dissolving in cold(!) water you get an intense
purple color of Fe(+VI). However I dunno how to purify the potassium ferrate. You could try to precipate barium ferrate tough, its very badly soluble.
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