woelen
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Easy preparation of hexavalent chromium for electrolysis in chlorate and bromate cells
Making chlorates and bromates from chlorides and bromides by means of electrolysis is an interesting process, which can be performed at a simple home
lab very easily. There is one main problem though and that is that cell efficiency sharply decreases at increasing concentration of chlorate or
bromate in the cell. The chlorate or bromate is reduced back to chloride or bromide at the cathode. This undesirable process occurs both with graphite
cathodes and with metal cathodes.
A common remedy is to add some hexavalent chromium to the cell, usually some potassium (di)chromate, but this chemical unfortunately becomes harder
and harder to obtain, due to the toxicity of hexavalent chromium.
I now discovered a very easy way of making hexavalent chromium, suitable for use in an electrolytic cell. With a little more effort one can also
isolate chromate or dichromate, but main emphasis I have put on making hexavalent chromium from easy to obtain chemicals with trivalent chromium. I
found that hypochlorite is capable of oxidizing chromium(III) in alkaline solution at an amazing speed. It works MUCH better than hydrogen peroxide,
without side reactions in which peroxo complexes are formed. In just a few minutes you can make hexavalent chromium, just using a few test tubes and
using slight heat.
Suitable chemicals are: chromium sulfate: http://www.ebay.co.uk/itm/Chromium-Sulphate-Hydrate-Cr2-SO4-...
Basic chromium sulfate: http://www.ebay.co.uk/itm/500g-Chromium-Sulphate-top-quality...
chrome alum: frequently on eBay, not now though.
Any of these is useful. All of them are soluble in water. The basic chrome sulfate is marketed as Cr2(SO4)3.H2O, but that is not correct. It is
produced by leading excess SO2 gas through a solution of Na2Cr2O7 until this solution is purely green and then excess SO2 is driven off by boiling and
the green liquid allowed to evaporate, such that green crystals are formed. A much better formula is Cr2(OH)2(SO4)2.Na2SO4 (Cr and Na are in a 1 : 1
ratio in this compound). This compound is soluble in water very well and has a beautiful green color. The other compounds have a purplish/bluie/gray
color.
For making hexavalent chromium you just have to dissolve some of one of these chemicals in water and add some bleach and a small amount of solid NaOH
or KOH. Normal 6% bleach does the job. This mix must be gently heated and after a minute or so you have a beautiful bright yellow solution.
This yellow solution still contains hypochlorite. You then have to add drops of moderately concentrated HCl (e.g. 15% to 20%) and shake after each
drop and you have to continue, until the liquid just becomes golden yellow to orange. During this process some Cl2 is produced. Finally, this liquid
must be heated somewhat to drive off any Cl2. The resulting liquid is more or less neutral, has a golden/orange color and can be used directly as an
additive to an electrolysis cell.
I tested with such a liquid, making KClO3, using MMO anode and titanium cathode. It works like a charm. There is some slight sulfate contamination,
but this does not affect the cell at all. For each half a liter of cell liquid one only needs a few 100's of mg of chromium(III) salt.
I post this, because I was suprised at the speed and ease of this reaction. I read about making hexavalent chromium from melts of NaOH, some oxidizer
and Cr2O3, but the method I found is much easier and safer, especially if you do not need to isolate the compound but just want to do some simple
experiments in aqueous solution, or want to use it as additive for an electrolysis cell. This brings home-made chlorate and bromate again somewhat
closer to the masses.
[Edited on 3-9-12 by woelen]
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phlogiston
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Great find that I am sure many will find useful.
I have not tried this myself but a friend reported that adding permangate to his chlorate cells had a similarly beneficial effect, substantially
improving his yields. Permangate, while also become more scarce, seems to be easier to acquire for now.
Conceivably, it is able to lower the rate of cathodic reduction of chlorate by a similar mechanism, i.e. the formation of a layer on the cathode with
a low permeability to chlorate.
[Edited on 3-9-2012 by phlogiston]
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"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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LanthanumK
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I used to electrolyze stainless steel in salt solution, dry the precipitate produced in air, and react it with sodium hypochlorite to generate a
chromate solution.
hibernating...
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vmelkon
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Quote: Originally posted by LanthanumK  | | I used to electrolyze stainless steel in salt solution, dry the precipitate produced in air, and react it with sodium hypochlorite to generate a
chromate solution. |
Did you purify and crystallize the chromate? How did you do it?
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elementcollector1
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Well, I just did basically the same thing, and it's heavily contaminated with bleach, salt and sodium chlorate.
I would guess at recrystallization or precipitation of potassium dichromate if one wanted a purer product.
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Poppy
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I would like to do th other way around, reduce Cr IV back to Cr III via SO2,
Thanks for the valuable information!
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elementcollector1
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How do you plan to generate SO2?
This could be useful information to me, as I do have a large stock of chromate I wish to turn back to Cr(III) simply. An alternative route is
acidification to dichromate, add ethanol to reduce to Cr (III), and then precipitate out Cr(OH)3 with sodium carbonate.
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woelen
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| Quote: Originally posted by elementcollector1 | Well, I just did basically the same thing, and it's heavily contaminated with bleach, salt and sodium chlorate.
I would guess at recrystallization or precipitation of potassium dichromate if one wanted a purer product. |
I
posted this method of making chromate with electrolysis in mind. In that application, the presence of the hypochlorite, chloride and some chlorate is
not a problem at all, just dump in all of it in the solution to be electrolysed. If you want to isolate the chromate, then you will have a harder job.
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platedish29
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| Quote: Originally posted by elementcollector1 | How do you plan to generate SO2?
This could be useful information to me, as I do have a large stock of chromate I wish to turn back to Cr(III) simply. An alternative route is
acidification to dichromate, add ethanol to reduce to Cr (III), and then precipitate out Cr(OH)3 with sodium carbonate. |
ERr hmm , hum
This equation seems to provide a good source for SO2:
2 FeSO4 → Fe2O3 + SO2 + SO3
But , just dunno.
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elementcollector1
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Decomposition for that reaction starts at 480 C, which is doable with a good hotplate and good quality glass. I suppose this would work,
ironically, in a steel vessel, but I don't think the fumes of sulfur trioxide would be too nice to metal (although this is speculation, so
feel free to prove me wrong).
So, for the use of chromate in electrolysis cells, why not start with stainless steel and dissolve it in acid instead of ordering chemicals? This
works for me, and the iron will just precipitate out as hydroxide anyway. Just a thought, for those real DIY chemists...
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tetrahedron
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| Quote: Originally posted by LanthanumK | | I used to electrolyze stainless steel in salt solution, dry the precipitate produced in air, and react it with sodium hypochlorite to generate a
chromate solution. |
here's a possible variation: once your steel anode has dissolved, switch to an inert one (graphite, MMO, PbO2 etc). dilute the electrolyte
as needed, without filtering out the precipitate. the hypochlorite produced in situ then oxidizes the Cr3+ in suspension to chromate. could
this work in theory?
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Fantasma4500
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im currently attempting to make Na2Cr2O7 by a greenbluish solution i got from electrolysing HCl i believe with iron cathode and steel anode..
i found this video before this thread, possibly this video has been made after this thread, perhaps he even read this thread..
http://www.youtube.com/watch?v=hRGJn7x3Qyw
but i suppose if you can somehow convert sodium dichromate into chromic acid you would be able to make potassium dichromate, as i think its for some
reason harder to make potassium hypochlorite than getting sodium hypochlorite
also.. what about tri-chromate, would that be better for electrolysis than dichromate?
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woelen
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If you did electrolysis of hydrochloric acid with a steel anode, then you will have some Cr(3+) in solution, but also a lot of Fe(2+) and Fe(3+).
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Dichromate, chromic acid, trichromate... All of these are very interchangeable and what you really have in solution depends on pH and concentration.
For electrolysis, any soluble form of hexavalent chromium does the job. Either CrO3, Cr2O7(2-), Cr3O10(2-) or CrO4(2-) work equally well. In the
solution and after some running time, all will end up mainly as CrO4(2-) and some HCrO4(-) and Cr2O7(2-).
If you want to make potassium dichromate, then there is no need to convert to chromic acid first.
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Mixell
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Not exactly on the topic, but I might as well ask it here since someone has brought up this thread.
Is it possible to use a MMO coated titanium cathode instead of a regular titanium one in electrolysis experiments (including chlorate production)?
Also, how well do MMO electrodes resist corrosion in an acidic medium?
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blogfast25
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Chromium Alum on eBay. I sell it there and it's a bit 'patchy' but it's been on for quite a while now.
Also, I don't doubt your method, woelen, but the REAL challenge is to make quite pure, solid and more or less anhydrous K2Cr2O7. Oxidising Cr(III) to
chromate/dichromate in solution/small amounts isn't hard and peroxide works well, IMHO.
Commerical NaClO solutions have the drawback of being weak and thus only fairly weak dichromate solutions can be created with it.
[Edited on 9-3-2013 by blogfast25]
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woelen
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@Mixell: MMO resists chlorate and bromate very well, but the titanium substrate does not withstand the bromine-environment. I did experiments with MMO
on titanium mesh for making chlorate and that works very well, but when the same anode is used for making bromate, then the titanium which is not
covered by MMO quickly erodes and a pale grey flocculent precipitate is produced.
@Blogfast25: A real challenge indeed is isolating pure crystalline K2Cr2O7 from the dilute solutions, but this is not the subject of this thread. Here
the only goal is to obtain a small quantity in solution, such that it can be used for improving cell efficiency in electrolysis. Bleach solutions
being dilute is not of a problem at all for this application. E.g. 100 mg dichromate is enough for liters of electrolysis solution and then it is no
problem if only a 1% or so concentration can be reached (then you still only have 10 ml, which is very small compared to the 100's of ml in a medium
size electrolysis cell).
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Mixell
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Thank you for the information, but I was asking if MMO coated titanium can be used as a cathode (probably yes, if the MMO coating does not greatly
impart the conductivity of the electrode)?
Also, how well do you think MMO electrodes will resist corrosion in a very acidic medium (pH=0)?
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woelen
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Sorry the wrong answer. In my mind I was reading the word 'anode' instead of 'cathode', because MMO normally always is used as anode.
Why would you want to use MMO as cathode. A plain titanium strip works well, stainless steel also works well. I used a titanium strip as cathode and
MMO-coated titanium mesh as anode in my cell. I do not know whether MMO can be used as cathode or not, because I never considered doing so. Just every
plain metal can be used as cathode.
There is one important thing to keep in mind. FIRST apply power to the electrodes, then immerse them in the liquid. As long as there is power on the
electrodes, the negative voltage at the cathode protects it. When the power is switched off, then the corrosive hot liquid quickly corrodes the
cathode, especially if this is made of iron or copper. So, I always connect the power supply to my electrodes and then immerse them in the liquid.
Low pH should not be a problem for an MMO anode. MMO is suitable for making chlorine and that should be done at low pH (otherwise you get hypochlorite
and chlorate). I have used my MMO anode for electrolysis experiments at fairly low pH (I estimate around 2) and had no problems with that.
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Mixell
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Well, I got 10 1"x10" MMO electrodes for 50$ including shipping, so I don't really see a reason to invest in a titanium strip in addition to that.
I assume the conductivity of the MMO electrode is pretty much the same as that of a plain titanium one, so it probably can be used as a cathode.
on another subject, how well does a silver electrode can operate as an anode in various environments?
I also thought about using gold, although it tends to form pretty stable complexes with many ligands, but is it favorable to platinum in some cases?
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