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Author: Subject: making potassium chromate from dichromate
jamit
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[*] posted on 27-9-2012 at 23:15
making potassium chromate from dichromate


I want to convert potassium dichromate into potassium chromate. Now I know this can be done by adding a base like potassium hydroxide to the solution of potassium dichromate.

My question is, how much is needed to fully convert all dichromate into chromate?

How much base will bring about the equilibrium?

Once I have added enough base (KOH) to bring about the equilibrium, can I just evaporate the solution to get "pure" yellow potassium chromate?

Has anyone tried this at home, that is, make potassium chromate?

Any suggestions? Or point me in the right direction. thanks.
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weiming1998
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[*] posted on 28-9-2012 at 00:45


Quote: Originally posted by jamit  
I want to convert potassium dichromate into potassium chromate. Now I know this can be done by adding a base like potassium hydroxide to the solution of potassium dichromate.

My question is, how much is needed to fully convert all dichromate into chromate?

How much base will bring about the equilibrium?

Once I have added enough base (KOH) to bring about the equilibrium, can I just evaporate the solution to get "pure" yellow potassium chromate?

Has anyone tried this at home, that is, make potassium chromate?

Any suggestions? Or point me in the right direction. thanks.


How much KOH is needed to convert a certain amount of K2Cr2O7 into K2CrO4 can be calculated with stoichiometry. Basically, the Cr2O7(-2) ion partially hydrolyses as:
Cr2O7(-2)+H2O<--->2CrO4(-2)+2H+. Adding KOH shifts the equilibrium to the right and consumes the produced, extra H+ ions.

Using a balanced equation of the reaction of KOH and K2Cr2O7 (K2Cr2O7+2KOH---->2K2CrO4+H2O), you can calculate the amount of KOH needed. If 30 grams of K2Cr2O7 (approx 1/10 mol) needs to be converted into K2CrO4, then 2/10 mol of KOH, which is 11.2 grams are needed to convert all K2Cr2O7. If the stoichiometry is correct, there should be no other waste products apart from water, so (assuming your K2Cr2O7 is relatively pure) pure K2CrO4 can be obtained via evaporation.
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Mildronate
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[*] posted on 28-9-2012 at 04:40


This story is not only about stoichiometry, its also about pH :P
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Hexavalent
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[*] posted on 28-9-2012 at 07:48


Quote: Originally posted by Mildronate  
This story is not only about stoichiometry, its also about pH :P


Yes, adding base will automatically generate the chromate from aqueous dichromate.




"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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Mildronate
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[*] posted on 28-9-2012 at 11:06


jamit can draw distrubition diagram, and will see how depends molpart fom pH. Its clasic in analitycal chemstry in universitys.

[Edited on 28-9-2012 by Mildronate]
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Lion850
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[*] posted on 13-2-2021 at 15:35


Jamit - did you perform this experiment? Did you get a complete conversion to potassium chromate?
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artemov
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[*] posted on 13-2-2021 at 20:15


Quote: Originally posted by Lion850  
Jamit - did you perform this experiment? Did you get a complete conversion to potassium chromate?


Did the conversion recently, currently drying in a drybox (hence the blurry pic).

Used a stoichiometric amount of KOH (assume 100% purity). Not sure if the conversion is complete, but it's quite yellow now, probably with some K2CO3.

20210214_120409.jpg - 30kB
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Lion850
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[*] posted on 13-2-2021 at 22:34


Artemov thanks for the feedback. Did you use potassium hydroxide or carbonate? (to add to the dichromate solution).
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artemov
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[*] posted on 13-2-2021 at 22:42


Quote: Originally posted by Lion850  
Artemov thanks for the feedback. Did you use potassium hydroxide or carbonate? (to add to the dichromate solution).


Potassium hydroxide, but it's supposedly 94% pure only, so I guess there will be water as well as potassium carbonate.

The original potassium dichromate is also contaminated with sodium, sulfate and chloride, as I made it from chromium sulfate, bleach and potassium chloride.
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Lion850
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[*] posted on 13-2-2021 at 23:52


I plan to covert the dichromate to chromate by adding potassium carbonate as per the attached from Prepchem.

FF071455-628B-4A31-878A-B81236A0C111.png - 433kB
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artemov
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[*] posted on 14-2-2021 at 00:52


Quote: Originally posted by Lion850  
I plan to covert the dichromate to chromate by adding potassium carbonate as per the attached from Prepchem.



Do take pics! And protect yourself from the inevitable chromate/dichromate aerosol created!
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