Sciencemadness Discussion Board - Dichromate Synthesis from Chromium metal and NaClO3

Dichromate Synthesis from Chromium metal and NaClO3

Kloberth - 13-11-2024 at 00:40

Not sure if anyone tried or wrote about this at least i couldnt find it. As far as I can tell this should work when adding the NaClO3 in solution to a solution of CrCl3 after acidifying with H2SO4 i guess.

Am I overlooking something?
I will also try this this evening on small scale and report it because in the eu it is at least for me easier to get chromium metal than high % bleach(for the synthesis like apoptosis: https://youtu.be/H4_owUXUViI?si=EB1DUX59DEWNXg1g) and i dont want to deal with a Cr2O3 melt.
Sorry if I overlooked anyone posting about this.
Cheers

Keras - 13-11-2024 at 03:33

I’m sure you can turn Cr (III) from CrCl₃ into chromium (VI) just by adding 12% hydrogen peroxide that you can source easily.
Probably standard bleach would work, too. But I have no CrCl₃ to try out, unfortunately.

Bedlasky - 13-11-2024 at 07:09

Not sure if this works. I know that in gravimetry if you want to make really good crystaline precipitate of PbCrO4, you do it using Cr3+ and bromate in acidic solution. But I know that only from theory, never tried that. Chlorate is more reactive, so it should work in theory. Try that on test tube scale first and you'll see. But be careful, don't make your chlorate too concentrated. HClO3 is quite unstable above 30% concentration, it could potentialy explode. Reaction also almost certainly will produce ClO2, which is also explosive. So you need very good ventilation, ideally blow gently air above reaction mixture to keep ClO2 concentration in air low (ClO2 explosions usually happen in mixtures with < 10% V/V ClO2).

Btw. as Keras said, if you want easy method for chromate/dichromate, you need just some NaOH/KOH and dilute H2O2. You can even use dilute bleach (4% NaClO), it works too. But it introduce lot of NaCl, I think that dilute peroxide is the best choice (you can get 10-12% peroxide in drugstore). It's very quick and easy reaction, I honestly never understand why is someone bothering with melts.

[Edited on 13-11-2024 by Bedlasky]

[Edited on 13-11-2024 by Bedlasky]

bnull - 13-11-2024 at 07:16

You're changing chromium from a cation to an anion, so you need a base to take it up; sulfuric acid won't do. The reaction is $$2~CrCl_3(aq) + NaClO_3(aq) + 8~NaOH(aq) \rightarrow Na_2Cr_2O_7(aq) + 7~NaCl(aq) + 4~H_2O(aq).$$ With two more moles of NaOH per mole of NaClO₃ you get chromate:$$2~CrCl_3(aq) + NaClO_3(aq) + 10~NaOH(aq) \rightarrow 2~Na_2CrO_4(aq) + 7~NaCl(aq) + 5~H_2O(aq).$$

I don't know if there is a best order to mix the reagents (probably sodium hydroxide and sodium chlorate are mixed first, then chromium (iii) chloride is added to the solution; or first you make a slurry with sodium hydroxide and chromium (iii) chloride and then add sodium chlorate). I suspect sodium carbonate also works.

Boffis - 13-11-2024 at 10:03

Chlorate is not a good oxidising agent under aqueous alkaline conditions, the reaction formulated by bnull will not happen in aqueous solution. If you wish to use sodium chlorate to oxidise chromium 3+ to chromate will will need to use the hot fusion process in which case you might as well use cheap chrome green pigment, sodium hydroxide and sodium chlorate. This method has the addition advantage of generating less sodium chloride to remove by re-crystallisation.

To oxidise Cr3+ in aqueous solution you need to make the solution very alkaline. When I recover chromium from chromate oxidations I usually precipitate the hydroxide and wash by decanting since it is rather gelatinous and hard to filter. The hydroxide is then added to vigorously stirred sodium hypochlorite bleach until no more will dissolve and then filter. The clear filtrate is then evaporated down until it crystallises on cooling. Hydrogen peroxide may also work as a substitute for bleach, I have never tried it but it seems plausible. If it does work it would have the advantage of adding the least amount of extraneous ions.

Sodium chromate crystallises as several different hydrates. If crystallisation doesn't start until the solution is almost cold it will crystallise as the 10-hydrate like sodium sulphate. It can however, be readily dehydrated.

Kloberth - 13-11-2024 at 11:03

well I was completely wrong on the oxidation in acidic medium i guess also tried it nothing happens. will try the oxidation in alkaline solution with h2o2 would be the cleanest as boffis mentioned. when there are no side reactions this would be really nice.
So next plan is to percipitate the Chromium hydroxide and react that with h2o2 or when this fails or there are side reactions with low % bleach.
Ill report back on the peroxide route

bnull - 13-11-2024 at 12:23

Quote: Originally posted by Boffis

Chlorate is not a good oxidising agent under aqueous alkaline conditions, the reaction formulated by bnull will not happen in aqueous solution.

Ah, crap...

Quote:

Hydrogen peroxide may also work as a substitute for bleach, I have never tried it but it seems plausible.

Hydrogen peroxide will reduce whatever chromate/dicromate is formed.

Kloberth - 13-11-2024 at 12:29

bnull read that also that these conditions would destroy the chromate will try it on testtube scale anyway just to confirm. so the only viable way remains low % bleach?

Boffis - 13-11-2024 at 12:55

@ bnull, will hydrogen peroxide reduce chromate under strongly alkaline conditions? I don't know and I am not currently in a position to test it.

In alkaline conditions H2O2 oxidises Mn2+ to Mn4+ but the reverse is the case under even weakly acid conditions.

j_sum1 - 13-11-2024 at 16:45

H2O2 forms a complex with chromium. The complex is soluble and transient. I am not sure what it breaks down to but it sure ruins and chromate production.
Oxidation can be done with hypochlorite bleach in basic solution. Potassium chromate is the easiest species to get to precipitate out. (Ok, Ca or Ba are probably easier but less useful.) Sodium dichromate is a pain to make.

Admagistr - 13-11-2024 at 18:02

Quote: Originally posted by j_sum1

H2O2 forms a complex with chromium. The complex is soluble and transient. I am not sure what it breaks down to but it sure ruins and chromate production.
Oxidation can be done with hypochlorite bleach in basic solution. Potassium chromate is the easiest species to get to precipitate out. (Ok, Ca or Ba are probably easier but less useful.) Sodium dichromate is a pain to make.

Yes, I observed the decomposition of the chromium peroxide compound into chromate, I was not happy about it because I wanted to prepare the peroxide compound! It was probably because I let the compound crystallize in a place where there were traces of acid fumes for a long time. It definitely releases oxygen.

bnull - 13-11-2024 at 18:53

@Kloberth and @Boffis: It seems that hydrogen peroxide is decomposed by chromate in alkaline solution. The odd thing is that the bubbles of oxygen only appear when the solution is shaken, like carbonated water. I have to verify some sources to confirm this behavior, but it is possibly due to the low concentration of peroxide.

Bleach? Why not TCCA in alkaline solution? TCCA plus sodium carbonate, for example.

Kloberth - 14-11-2024 at 00:04

@bnull I dont see any reason for tcca not to work but i have a feeling this gets messy quick. and would be roughly the same as using bleach directly afaik just with a big organic rest. when tcca is an option then you could also directly chlorine gas, but i read that Chlorine ions in solution decompose the chromate.
Still havent tried the peroxide but have prepared the chromium chloride in solution gonna percipitate the hydroxide this evening and try the peroxide route.

Kloberth - 14-11-2024 at 00:21

when using tcca the problem also would be that cyanuric acid will crash out when disproportionating to dichromate

[Edited on 14-11-2024 by Kloberth]

Kloberth - 14-11-2024 at 02:26

Quote: Originally posted by j_sum1

Researched this a bit aas far as i can tell this complex can only form with Cr6+ so H2O2 should probably work to oxidise the Cr3+ in Alkaline solution, actually also found a source confirming this reaction (https://www.chemguide.co.uk/inorganic/transition/chromium.ht...)

Also balanced out the TCCA thing
should be: TCCA+7NaOH(+access to remain in basic solution) +2Cr(OH)3 = 5H2O +3NaCl+ 2Na2CrO4 + Cyanuric acid
right?

bnull - 14-11-2024 at 03:23

Here's one advantage of using TCCA: you can make the solution as strong as it gets.

Edit: Some observations:

decomposition of hydrogen peroxide by chromate is pH-dependent, the rate increasing with the alkalinity of the solution[1];
decomposition of hydrogen peroxide is catalyzed by chromium (vi) and copper (ii) separately but inhibited when both are in solution[2]. Copper seems to prevent the reduction of chromium from +6 to +3;
this one is really interesting. Chromium (vi) catalyses the decomposition of hydrogen peroxide with the formation of peroxo-complexes. In acidic solutions, we see a blue color (perchromic acid) that soon turns green (reduction to Cr³⁺); in basic solutions, the color is an orange yellow[3]. This is consistent with what I saw yesterday.

[1] J. Rynasiewicz, Hydrogen Peroxide Determination in the Presence of Chromate, Anal. Chem. 1954, 26, 2, 355–358 (https://pubs.acs.org/doi/10.1021/ac60086a023)
[2] J.F. Perez-Benito, C. Arias, Mutual catalyst inhibition in the chromium (VI)-copper (II)-hydrogen peroxide reacting system. New J. Chem., 1999,23, 945-949 (https://doi.org/10.1039/A903797G)
[3] M. L. Haggett, P. Jones, W. F. K. Wynne-Jones, Peroxy-complexes as Intermediates in the Catalytics Decomposition of Hydrogen Peroxide, Discuss. Faraday Soc., 1960,29, 153-162 (https://doi.org/10.1039/DF9602900153)

[Edited on 14-11-2024 by bnull]

Mateo_swe - 14-11-2024 at 07:13

I bought a big lump of dichromate from ukraine before they started fighting there.
I dont remember if it was sodium or potassium dichromate but its a big lump, almost a kilo.
Maybe there still is cheap dichromate avaliable on auction sites, worth a look unless you do this for the experience and learning.

Kloberth - 14-11-2024 at 09:28

Tried a few things now: H2O2 it seems promising on dilute and small scale couldnt get it to work in more concentrated solutions for some reason there it "decomposed" to a green solution and orange green percipitate (could be starting material am not sure).
then also tried it with tcca(actually with the monosodium salt but shouldnt make a difference) again when doing it on small and dilute with a gross excess of oxidant it turned yellow and the chromium hydroxide percipitate dissolved.
Seems promising but I am going to have to dial in the stöchiometry. Also i tried all with a suspension of Cr(OH)3 after adding naoh to CrCl3. The hydroxide filters badly and needs to run through over night.

any thoughts on what should be in excess or controlled in the reaction for trying it on gram scale to see if this is promising at all?

Kloberth - 14-11-2024 at 09:29

Quote: Originally posted by Mateo_swe

I bought a big lump of dichromate from ukraine before they started fighting there.
I dont remember if it was sodium or potassium dichromate but its a big lump, almost a kilo.
Maybe there still is cheap dichromate avaliable on auction sites, worth a look unless you do this for the experience and learning.

I am trying this for fun and also perhaps finding another route to dichromate.

Bedlasky - 14-11-2024 at 10:46

Quote: Originally posted by bnull

Quote:

Hydrogen peroxide may also work as a substitute for bleach, I have never tried it but it seems plausible.

Hydrogen peroxide will reduce whatever chromate/dicromate is formed.

Hydrogen peroxide will reduce dichromate, but not chromate. I made K2CrO4 from Cr(OH)3 and H2O2 in the first year in high school.

Quote: Originally posted by j_sum1

Tetraperoxochromate is only stable at low temperetures. It decompose back to chromate and peroxide/oxygen.

Quote: Originally posted by Kloberth

Tried a few things now: H2O2 it seems promising on dilute and small scale couldnt get it to work in more concentrated solutions for some reason there it "decomposed" to a green solution and orange green percipitate (could be starting material am not sure).

Did you add hydroxide into Cr(OH)3 suspension?

4KOH + 3H2O2 + 2Cr(OH)3 --> 2K2CrO4 + 8H2O

[Edited on 14-11-2024 by Bedlasky]

Kloberth - 14-11-2024 at 11:53

@Bedlasky yes added not KOH but NaOH perhaps too little and didnt really heat it as I said hadnt had enough time to really test. But with you writing it should work ill try it on a larger scale. Also boiling off the peroxide to not compose the dichromate or using chromium excess before disproportionating?

RU_KLO - 15-11-2024 at 09:36

Quote: Originally posted by j_sum1

Will chlorine oxidize a KOH (excess) + Cr(OH)3 (or Cr(III) oxalate)?

Because I tried (without luck) different oxidizers to get chromate/dichromate, and one of the tries was, to make potassium hypochlorite (KOH + Cl2) and then oxidizing the Cr(OH)3 - instead of using sodium hypochlorite.

How would it react with a mixture of iron hydroxide/chromium hydroxide/ maybe nickel hydroxide?
(the one you get from Stainless steel + HCl; then KOH to precipitate the hydroxides)

bnull - 15-11-2024 at 10:55

I don't know about oxalate, but hydroxide, yes. You would be making hypochlorite in situ instead of making first the hypochlorite then mixing with chromium (iii) hydroxide. It is even better because you can stop bubbling chlorine when the sludge has dissolved.

Bedlasky - 15-11-2024 at 14:36

Quote: Originally posted by Kloberth

@Bedlasky yes added not KOH but NaOH perhaps too little and didnt really heat it as I said hadnt had enough time to really test. But with you writing it should work ill try it on a larger scale. Also boiling off the peroxide to not compose the dichromate or using chromium excess before disproportionating?

Yes, definitely boil solution for a while to destroy peroxide. Otherwise when you add acid, it will reduce dichromate.

zed - 16-11-2024 at 17:03

I'm a man, but I can probably change, if I really have to.... I guess.

On a practical note. Recently, I left nasty bathwater festering in my bathtub for an extended period. Crud precipitated out and settled on the enamel at the bottom of the tub. It left dark, gruesome stains, that did not respond to bleaching with regular strength Clorox.

Since I did have some ~10% solution on hand, I resorted to trying that.

I brushed that concentrated bleach on my stained tub enamel, utilizing a poly-ester type paint brush, and let it sit for a while. Sure enough, in a short period of time, my tub enamel was once again white and beautiful. But, not so, my Chrome plated plumbing fixtures; they were completely trashed.

Without heat, or any other assistance, ~10% Bleach will dissolve the Chrome plating off of plumbing fixtures.

So, I would expect that it will likewise dissolve elemental Chromium under lab conditions. I'm not projecting what the product might be, in terms of formula or purity. But, I would suggest that you not expose your fancy plumbing fixtures to concentrated bleaching solutions. You will be sorry if you do.