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Author: Subject: K2Cr2O7 from (NH4)2Cr2O7
ChemistryForever
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[*] posted on 12-12-2018 at 06:11
K2Cr2O7 from (NH4)2Cr2O7


I have some of ammonium dichromate and i want to convert some of it to potassium dichromate. How could i do this?
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[*] posted on 12-12-2018 at 06:17


Easiest way would be to make saturated solutions of KCl and ammonium dichromate and then combine them. After chilling the resulting solution to 0 C, potassium dichromate should precipitate. The residual liquid will still contain some hexavalent Cr so dispose of it properly (i.e not down the drain). You should reduce it to Cr III with acidified sulfites or ascorbic acid.

2KCl+(NH4)2Cr2O7=2NH4Cl+K2Cr2O7
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woelen
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[*] posted on 12-12-2018 at 06:24


Add KOH to the compound. Use a little excess (NH4)2Cr2O7. Take 1.8 moles of KOH for each mole of (NH4)2Cr2O7. If you use weights of both compounds, according to molecular weights and a 2 : 1 molar ratio, then you are quite sure that you use a little excess (NH4)2Cr2O7. KOH never is 100%, it contains several percents of water (appr. 10% is not uncommon).

Use concentrated solutions of both chemicals. Mix them slowly (be careful, a lot of NH3 will escape and the liquid may become hot). The color will become golden yellow. Next, heat the liquid and boil off most of the water (and a lot of NH3), until solid matter separates from the liquid. Then add, while stirring, one or two drops of 50% H2SO4 (or 50% HNO3 and if that is not available, two to three drops of 30% HCl) to assure that all hexavalent chromium is converted to dichromate. Much more solid K2Cr2O7 will settle on cooling down. Allow to cool down to room temperature. Next, cool down in a fridge to freezing cold in order to obtain most of the K2Cr2O7. Filter the solid (use a glass fritte, filter paper will be attacked and may lead to contamination with chromium(III)), if no fritte is available, slowly decant the liquid from the solid and then put the wet solid on a coffee filter on top of a bunch of paper tissues. Allow most of the liquid to soak into the tissue and then scrape the damp solid from the coffee filter.

Recrystallize the damp solid from as little as possible of boiling hot distilled water, which is allowed to cool down to freezing cold. This gives very pure K2Cr2O7. The compound K2Cr2O7 crystallizes very well and it is easy to obtain this in pure form.

[Edited on 12-12-18 by woelen]




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[*] posted on 12-12-2018 at 08:27


Quote: Originally posted by ChemistryForever  
I have some of ammonium dichromate and i want to convert some of it to potassium dichromate...


I once wanted to convert a little of my ammonium dichromate to potassium dichromate so that I could follow a particular procedure.
If your planned reaction specifically needs the potassium ion, or it is just because you want to, then go ahead.

Otherwise it is likely that a similar molar quantity of ammonium dichromate will work.
If you post your intended use then someone here will probably give good advice.




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