Capt Mercaptan - 11-12-2017 at 08:15
Hi,
In the process of making potassium dichromate (using Cr2O3 + KOH + KClO3) I made the mistake of filtering the solution through my glass vacuum filter
and it is now clogged with the unreacted Cr2O3 (I should have used filter paper).
Most sources list Cr2O3 as amphoteric however I have had no luck cleaning with HCl, H2SO4, or NaOH solutions.
A very small amount seems to dissolve in HCl but without a pump capable of recirculating HCl I would have to waste a lot of time and acid to clean it
manually.
I'm hoping someone here has solved this problem already and can point me in the right direction.
Sigmatropic - 11-12-2017 at 13:35
Perhaps it can be oxidized to the soluble Cr(VI) salt with bleach? I've been using decantation to get rid of the fine Cr2O3 particles as they made it
through the filter paper at first and then continued to clog it.
[Edited on 11-12-2017 by Sigmatropic]
Capt Mercaptan - 12-12-2017 at 13:20
ok. so i tried the bleach just to be sure, but it failed. From what I've read chromium metal can be oxidized by things like bleach or nitric acid to
Cr2O3 but they dont oxidize any further under normal conditions. I think thats why molten hydroxide is needed to convert it to chromate,
unfortunately those conditions would destroy the glassware.
So I found this thread http://www.sciencemadness.org/talk/viewthread.php?tid=12099 but im not sure I can easily adapt this procedure to cleaning a filter also I dont
have any bromate on hand.
At this point im wondering if mechanical cleaning is better than chemical. the Cr2O3 seems to be mostly stuck in the top surface of the frit so maybe
I can back-flush with a large volume of water at moderate pressure
JJay - 12-12-2017 at 14:04
Does piranha solution work?
hissingnoise - 13-12-2017 at 08:47
Strong alkali should never be used in a fritted funnel as it will damage the frit.
Capt Mercaptan - 13-12-2017 at 10:38
ok, tried hot piranha no luck yet. I'm going to let it sit for a few hours just to make sure nothing is happening slowly.
so a few hours later... it dissolved/reacted a little bit of something (a slight yellow color formed in the solution) but its either the acid
dissolving a tiny amount of Cr2O3 or maybe there was a little bit of some old gunk in the filter.
So i have one last idea. After reading that stainless steel is passivated by forming a thin layer or Cr2O3 I googled etching of SS and found people
recommend aqua regia. So in theory this should work, in practice... I'll get back to you.
JJay - 13-12-2017 at 14:09
Hmm... I hadn't read woelen's post until just now... interesting stuff. You can make bromate from bromide by electrolysis pretty easily... it may be
tricky to recover the chromium and bromine, though (perhaps the bromine could be acidified, oxidized, and distilled and the dichromate purified by
recrystallization).
Meltonium - 13-12-2017 at 18:01
I might try HCl mixed with some hydrogen peroxide. That mixture can dissolve copper metal slowly, and it might just work for Cr2O3.
chemplayer... - 14-12-2017 at 01:49
Did the same thing and found the same. Very hard to get rid of! Piranha worked a little bit but not much. It took a few washings with conc. nitric
acid to remove most of it and the residual green colour was there for a month or so. You can see the after-math in a few shots:
https://youtu.be/YyE9EQTSjZI?t=403
Good news is that it goes eventually...
Capt Mercaptan - 14-12-2017 at 04:52
Thats funny, I was inspired to try this by your video. So really this is all your fault.
I'm going to try 1. nitric acid 2. aqua regia 3. HCl + H2O2 and will report back which is most effective, hopefully later today.
JJay - 14-12-2017 at 05:12
I bought this 1L filter funnel specifically for filtering chromate solutions. It was a Yee Chen special, but it's one of my most used pieces of
glassware:
https://www.sciencemadness.org/whisper/files.php?pid=483316&...
Metacelsus - 14-12-2017 at 13:29
What about EDTA? That might help the chromium dissolve.
Chemvironment - 14-12-2017 at 16:54
I'm at work so I barely skimmed this, but it may help idk.
http://www.sciencemadness.org/talk/viewthread.php
[Edited on 15-12-2017 by Chemvironment]
[Edited on 15-12-2017 by Chemvironment]
Capt Mercaptan - 14-12-2017 at 18:03
So after running a few small scale tests with nitric acid (70%), aqua regia and HCl H2O2. none of them do very much I think the aqua regia maybe
slightly better than nitric acid (both seem to dissolve a very small amount). it seems like chemplayers experience that it just takes a long time to
get rid of the green color is holding true.
The EDTA is an interesting idea. I found a few articles about Cr(III) with EDTA but it was in the form of Cr(NO3)3 I have no idea if it will work with
other forms of Cr(III). Were you thinking of the EDTA on its own or in combination with something else to react with the Cr2O3 ?
after reading more of woelen's experiments it sounds like acidified sodium persulfate + AgNO3 with react with Cr2O3. too bad I dont have any sodium
persulfate on hand. I may have to buy some just to try.
JJay - 14-12-2017 at 18:35
You might also try a reducing acid with a strong acid.
Micha - 18-12-2017 at 14:59
EDTA will definetly not work, I think you dont even have to try it. I would suggest to try and carbonate + peroxide solution and heating it a little
bit, so oxygen is producet. It's maybe just a plain guess. The reaction producing chromate and CO2, which shifts the equilibrium.
Capt Mercaptan - 11-1-2018 at 10:26
ok, sorry for the long delay. but between holidays and waiting for bromate electrolysis to finish I havent had much chance to play with the Cr2O3
problem.
I just tested woelen's bromate method of converting Cr2O3 to dichromate and it was successful (test tube scale), so I'm going to scale it up and try
to clean the filter with some NaBromate H2SO4 and a heatgun. I will try post the results within the next couple of days.
Capt Mercaptan - 19-1-2018 at 12:55
So I got the frit mostly clean. I tried the bromate method first and didn't have much luck, I think my problem was mechanical rather than chemical
(getting the acidic bromate mixture to the right temp and passing it through the filter repeatedly was difficult).
I ended up soaking the filter in dilute KMnO4 which seems to convert the Cr2O3 to dichromate, of course is leaves behind MnO2 which needs to be
dissolved in H2SO4 but in the end I have it 95% cleaner. I may do one more washing. Pic below...
[Edited on 19-1-2018 by Capt Mercaptan]