Sciencemadness Discussion Board

Ammonium Dichromate from Stainless Steel

ElchScience - 10-1-2017 at 12:32

Hello everybody!

So in this video I attempted to make Ammonium Dichromate starting with stainless steel as my source of chromium.

This is the basic procedure that I followed:

First we are dissolving the stainless steel in Hydrochloric acid (HCl).
Those are the overall reactions going on. (Without chloride ions that don't actually do anything):

Fe + 2H+ ► Fe2+ + H2
Ni + 2H+ ► Ni2+ + H2
Cr + 3H+ ► Cr3+ + 1,5H2

Then we are precipitating out all the transition metal salts as their hydroxides/ carbonates while neutralising the acid. For this we will use sodium carbonate:

Na2CO3(aq) + 2HCl(aq) ► 2 NaCl(aq) + H2O(l) + CO2(g)

Fe2+(aq) + CO32-(aq) ► FeCO3(s)
1+x Ni2+(aq) + 1+x CO32-(aq) + x H2O ► NiCO3 * x Ni(OH)2 * x H2O(s) + x CO2(g) + xH2O(l)
2 Cr3+(aq) + 3 CO32-(aq) + 3 H2O(l) ► Cr(OH)3(s) + 3CO2(g)

I then tried to oxidise the Fe2+ to Fe3+ with oxygen from the air supplied by my aquarium pump but this didn't work. After 30 hours of bubbling air through the suspension it made a mess and only oxidised an insignificant portion so I went ahead and oxidised it with sodium dichloroisocyanurate which is just regular pool chlorine. You can buy it relatively cheap at any given hardware store.

Before I added this I washed out all the soluable impurities from the suspension of hydroxides carbonates using 40L of water which was a little crazy.

When I then added the dichloroisocyanurate (C3H2N3O3) the following reactions occured:
C3Cl2N3O3Na + 2 H2O ► C3H2N3O3Na + 2HClO
ClO- + 2Fe2+ + H2O ► 2Fe3+ + Cl- + 2OH-
2 Cr3+ + 3 ClO- + 10 OH- ► 2CrO42- + 3Cl- + 5H2O

First the dichloroisocyanurate reacts with the water to form hypochlorous acid. This then reacts with the iron to form iron 3+ ions. The rest of the hypochlorate reacts with the Chromium 3+ ions to from Chromate Ions (CrO4(2-))

I then boiled everything to dryness so the voluminous precipitate of the iron "dies". This ways I can go back to working with regular beakers. (If 5L Beakers are considers "regular". XD)
After filtering off all the insoluable iron and nickel compounds from the chromate which was really difficult I was left with about 15L of chromate solution that I then boiled down. When I added some more dichloroisocyanurate to the insoluable rest everything turned yellow again indicating that I didn't extract all the chromium yet. So after filtering everything out and boiling it down again, I was finally left with my chromate solution containing a whole lot of soluable impurities. To get rid of those I first acidified the solution with hydrochloric acid to destroy all the carbonate and to turn the chromate into dichromate:

Na2CO3(aq) + 2HCl(aq) ► 2 NaCl(aq) + H2O(l) + CO2(g)
2Na2Cr2O4 + 2HCl ► 2NaCl + 2NaCr2O7 + H20

After filtering this the dichromate was reduced down to chromium III again by adding some more hydrochloric acid and ethanol. This produced a lot of acetalaldehyde gas as a byproduct which might be carcinogenic:

Na2Cr2O7(aq) + 8H+(aq) + 3 CH3CH2OH(aq) ► 2 Cr3+(aq) + 3 CH3CHO(g) + 2 Na+(aq) + 7H2O(l)

I then converted the Cr3+ to insoluable chromium hydroxide again using sodium carbonate so I could wash out all the soluable impurities.

2 Cr3+(aq) + 3 CO32-(aq) + 3 H2O(l) ► Cr(OH)3(s) + 3CO2(g)

Because I added to much carbonate I redissolved some of the chromium hydroxide to the hexyhydroxocarbonate complex. So I had to wait a couple days until it decomposed again to hydroxide ions and chromium hydroxide.

Cr(OH)3(s) + 3 OH-(aq) ► [Cr(OH)6]3-(aq)
[Cr(OH)6]3-(aq) ► Cr(OH)3(s) + 3 OH-(aq)

After I was done washing this thoroughly I added some aqueous ammonia and hydrogen peroxide in order to get ammonium chromate.

2 Cr(OH)3(s) + 3 H2O2(aq) + 4 NH3(aq) ► 2 (NH4)2CrO4(aq) + 4 H2O(l)

This reaction was not very clean at all and even with a giant excess of hydrogen peroxide I was left with an insoluable rest. I can't say for sure what this is but my only guesses are that this is somehow still undissolved Chromium hydroxide or this is chromium chromate (Cr2(CrO4)3). Those are the only components that would make sense to me. But I still should be able to dissolve them with an acces of hydrogen peroxide so I have no real clue of what's going on there...
At the end I boiled down the Ammonium Chromate to get Ammonium Dichromate:

2 (NH4)2CrO4(aq) ► (NH4)2Cr2O7(aq) + 2 NH3(g) + H2O(l)

So if you watched the video you might be able to tell that I struggled quite a bit with this. This is the reason I would like to discuss some better ways of doing this so I can have a lot more sucess when I get around making part two.

First of all I would like to ask if anybody has a better way of seperating iron and chromium because my way is quite a lot of work and very messy.
Also does anybody have an idea what the interesting insoluable precipitate is when I am oxidising the chromium hydroxide with hydrogen peroxide in an ammonia solution?

(Also: If you are afraid of getting poisoned I heard that a good antidote is vitamin c (ascorbic acid) so make sure you are getting enough of that. You should pay attention to that anyway. :D


j_sum1 - 10-1-2017 at 13:20

Look up the series by extractions and ire on this. He did much the same as you. Oxidising agent wss NaOCl IIRC. Nice final product but like yours, wuit volumous and messy. Avoiding carbonayes at least eliminates a foaming problem.

You should also arm yourself with a pourbaix diagram for Cr and another for Cr-Fe-Ni. It will help you identify species and work systematically. Chromium chromate does.not exist as far as I know.

I'll watch your video proerly later.

DraconicAcid - 10-1-2017 at 13:24

I would avoid ammonia in solutions containing Cr(III). It can coordinate, and is difficult to remove. Use a real hydroxide to make the solution basic, and you should have no trouble getting chromate.

ElchScience - 11-1-2017 at 10:29

@j_sum1 Thank you for your feedback! I have already watched the video by extractions and fire and his process differs quite a lot from mine. The only thing that's the same is the initial dissolving in hydrochloric acid. The rest of the process is made up by me during months of experiments. He also used about five times as much stainless steel as me so he could recristalise at the end and still have enough left. In his video he also didn't film a lot of the process which makes it kind of hard to reproduce everything. The carbonate foaming problem was caused by carbonate contained in the bleach I added. He used Ca(OCl)2 which is more expensive for me then the NaOCl I used. (Next to sodium dichloroisocyanurate.)
There is an entire wikipedia artikel in german about chromium chromate so why should it not exist? It is even a classified hazardous substance.
Also I am not quite sure how the pourbaix diagram can help me because I pretty much know the conditions under which chromates etc. are formed. What additional info is this going to give me? But thanks for your input!

@DraconicAcid Oh I have never considerd a complex between chromium and ammonia. That might actually be part of my problem. So thank you very much for telling me about that! The difficulty for me however is not to make chromates. That's the easy part. What is hard is to make pure ammonium dichromate. So if you have a good process for that too this would be amazing. :D But thanks anyway!