Sciencemadness Discussion Board

Ammonium Dichromate to Potassium Dichromate ?

Sulaiman - 1-8-2017 at 02:00

I wanted some potassium dichromate for electrolytic cell experiments but I bought Ammonium Dichromate instead intending to also make a volcano.
I have decided to not waste any of my precious 25g by making toxic fumes,
so I intend to use it just as a source of the dichromate ion.
Which raises two questions;

1) What are the things that I should consider if I use ammonium dichromate instead of the often cited potassium dichromate ?
(stoichiometry of course, but otherwise ?)

2) How difficult would it be to produce pure potassium dichromate from ammonium dichromate ?
potassium nitrate + ammonium dichromate ... how to separate the products ?
Potassium Hydroxide + Ammonium Dichromate ... could the ammonia just be boiled off ?
any other suggestions ?

[Edited on 1-8-2017 by Sulaiman]

theAngryLittleBunny - 1-8-2017 at 02:07

If you wanna make a chlorate cell, I don't think using ammonium dichromate would be a good idea, because the hypochlorite in the cell will oxidize the ammonium ion. You can easily convert any ammonium salt into an alkali salt by adding an alkali hydroxide, so you could just add the stocheometric amount of KOH to the ammonium dichromate and boil it down.

(NH4)2Cr2O7 + 2KOH -> K2Cr2O7 + 2NH3 + 2H2O

As you can see, this reaction produces ammonia, so do this outside or in a fumehood. I would suggest you to use NaOH instead though, to make Na2Cr2O7, because commercial KOH contains 10 to 15% water, so that's a little differcult with the stocheometry.

Sulaiman - 1-8-2017 at 02:13

Thanks !

I may try the KOH instead of NaOH just because of the purity of my reagents, my NaOH just being drain unblocker with unknown 'extras'.

P.S. I no longer fear NH3, Cl2,, NOx, SO2 etc. as my eyes, lungs and nose (combined with hasty retreats) protect me :D
(plus, I expect my body to decrepitate long before chemistry hobby side-effects get me :P

[Edited on 1-8-2017 by Sulaiman]

ave369 - 1-8-2017 at 15:56

You can definitely boil off ammonia with KOH, but this will give you the yellow chromate. Acidify the resulting solution to convert it back to dichromate and crystallize after that. An alternate method is burning down ammonium dichromate and synthesizing potassium dichromate through melting KOH in a crucible with the resulting green Cr2O3 ash and saltpeter. This reaction yields potassium chromate which can be converted to dichromate through acidifying and recrystallizing.