a_bab
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SrCrO4. What to do with it?
I've got some SrCrO4 and I'm fiddling with the idea of extracting the Sr from it as a salt.
As this is a pigment, that means it'a a fairly tough chemical too, meaning it's kind of inert due to the low solubility in the first place.
I know about the reaction with sodium bisulfate in boiling water, which would end up with strontium sulphate.
Any other ideas?
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Jor
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I think this material dissolves in nitric acid and hydrochloric acid, as heavy-metal dichromates are generally more soluble (at least if I remember
right) than chromates.
This would give dichromate in solution. Make sure you add excess acid. Next would be adding a reducing agent, I'm not sure yet wich, sulphites are not
suitable, as SrSO4 will be formed. This would reduce the dichromate to trivalent chromium. You would then have a solution containing Cr(III)-ions and
Sr-ions, and some other stuff, depending on the reducing agent.
The reducing agent is not so easy for me to think of right at this point.
I think the best possibility I can currently think of is either formic acid or oxalic acid, as when they are oxidised they produce CO2, so no side
products. Both are pretty cheap.
Maybe there is a lot easier method, and I am just looking over it, or I might be forgetting a reducing agent....
Oh, and if following this method, don't use nitric acid. This is oxidising by itself, and will oxidise the reducing agent you add, with formation of
toxic nitrogen oxides.
After, you're left with a solution of CrCl3 and SrCl2. Add excess sodium hydroxide, untill all chromium has precitipated (no more green color). The
solution you are left with, should be colorless, and adding carbonate should precitipate SrCO3, wich can be dissolved in acids. The Cr2O3 prectipate
might still contain quite some Sr(OH)2, but not a lot , as Sr(OH)2 is moderately soluble (1,77g/100mL), so precitipation of Cr2O3 is highly favored
over Sr(OH)2, especially if you dilute the solution before adding hydroxide dropwise.
If you want any remains of strontium out of the Cr2O3 precitipate, shake with a small amount of water, filter, and add carbonate.
Maybe the Cr2O3 you are left with is interesting for preparing Cr(III)-salts. The pottery version is quite intert, but this one if freshly precitpated
so it might dissolve nicely in acids, I have no experience though.
But isn't it all a lot of hassle? As far as I know, most pottery suppliers sell SrCO3 , very cheap. I would be more interested in extracting
chromates/dichromates, as there would be harder to get...
Or it might be just the fun of it.
[Edited on 25-11-2008 by Jor]
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a_bab
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Well,
It's difficult for me to get the SrCO3, and I can get plenty of SrCrO4.
Then, there is the fun of doing it...
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blogfast25
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You could "thermite" it:
SrCrO4 + 8/3 Al ---> SrO + Cr + 4/3 Al2O3 (eq. 1)
I'm not entirely sure whether the SrO would react with the Al2O3 to form Sr(AlO2)2 (as calcium does) but chances are that it does...
If it does, the slag, a mixture of Sr(AlO2)2 and residual Al2O3, might be soluble in strong HCl (as is Ca(AlO2)2), yielding SrCl2 and AlCl3 which can
be separated (Al is amphoteric, Sr isn't) easily.
Please note that the reaction according to eq. 1 is likely to be extremely exothermic, off the top of my head in the order of magnitude of -
2000 kJ/mol (of chromate): that's veering towards flash powder. Start with small amounts/safe distances, preferably by "taming" the mix with
a good dollop of heat sink like CaF2 and see how it goes. All "being well" you'd end up with some metallic chromium and a soluble Sr salt...
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Jor
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I would be very wary of performing thermites with chromates. There's always SOME unreacted chromate, that will go in the air as fine dust, and you
might inhale it. Cr(VI) is a proven carcinogen, especially by inhalation (it's classified as 'very toxic by inhalation')
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a_bab
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Also, I would need lots of Al powder good for other things.
What about "roasting it"? Prolonged - heating it? Will it decompose to some oxides?
Or maybe mixing it with some cheap oxidants (KNO3; NH4NO3)?
Digesting it in strong mineral acids?
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blogfast25
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I've used small amounts of potassium dichromate as a booster in Cr2O3 thermites, with success.
As with ALL open crucible thermites, good ventilation is crucial. And because the reaction of chromate/dichromate is so exothermic it's also very
complete, assuming the formulation is well designed, e.g. by using a small stoichiometric excess of Al, leaving very little unreacted chromate. I
can't see residual Cr (VI) in the air being much of a problem at all: it's too unstable at these temperatures to survive...
Roasting it would potentially be more dangerous, actually because at relatively low temperatures air-borne chromate would survive...
[Edited on 25-11-2008 by blogfast25]
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JohnWW
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As well as paint pigments, insoluble chromates are also used in anti-rust primers for painting steel and galvanized iron. ZnCrO4, which has an intense
deep red color, is used for this purpose, along with PbCrO4. I presume that SrCrO4 has a similar color and properties. Insoluble plumbates (especially
CaPbO3), molybdates, tungstates, and (experimentally) pertechnates and perrhenates have been similarly used in paints.
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woelen
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Jor, what you are proposing in your first post contains at least one error, and probably a problem as well.
The error is that a mixed solution of Sr(2+) and Cr(3+) will not be separated by means of precipitation with excess NaOH. Sr(OH)2 simply is too
soluble, it won't precipitate that easily, but Cr(OH)3 also won't precipitate. Cr(3+) is amphoteric, if you have a chromium(III) salt in solution and
add NaOH, then a precipitate is formed, which mostly is Cr(OH)3, but adding more hydroxide redissolves this precipitate, forming chromite, CrO3(3-) or
partially protonated variations of this, such as CrO(OH)2(-) and CrO2(OH)(2-).
There is not a single pH, in which all chromium is precipitated, there is wide bands of pH, in which there are cations, in which there is partial
precipitate and in which there are anions and these bands overlap.
One way to precipitate a large part of the chromium (but unfortunately not all) is adding excess ammonia. Part of the chromium remains in solution, as
explained above, and that part slowly will form pink/purple ammine-complexes.
Then there is a problem as well. Sr-oxalate is exceedingly insoluble and even with oxalic acid you'll precipitate this solid and it will be hard to
get this in solution again (maybe with hot nitric acid). With formic acid, the reaction most likely is way too slow. Formic acid is not that easily
reduced by chromate, certainly if this is trapped in a marginally soluble compound. So, this reaction will proceed very slowly (if at all). Maybe
fairly strong heating will help, but I have no personal experience with this in this particular reaction.
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Jor
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I already knew Cr(III) is amfoteric, but I though that if you dilute well enough and add sodium hydroxide slowly until no more precitipate, it would
work fine. Im probably wrong here then.
I also knew Sr-oxalate is extremely insoluble, just as Ca-oxalate. But I though the oxalic acid would react before precitipating, if the solution is
hot enough.
What would you recommend then woelen?
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woelen
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I hardly can recommend something on this. My experience with this kind of pigments is that at ordinary temperatures they are very inert and hard to
get dissolved. One possible option could be to use moderately concentrated hydrochloric acid, mixed with ethanol (denatured is OK, but preferrably
without dyes and other crap). As a start one could try 3 volumes of 30% HCl, mixed with 3 volumes of water and 1 volume of ethanol. Adding the solid
to this, and careful boiling may lead to reduction of the chromate and subsequent dissolving of the solid. The oxidation products are volatile
(acetaldehyde and acetic acid) and the liquid will contain soluble Sr(2+) and Cr(3+) species. The liquid will turn deep green and clear when all this
succeeds.
I just would suggest to give it a try with small quantities first. If the SrCrO4 is so inert that it does not dissolve at all, then only the thermite
route or glowing with carbon powder are left as options to get the chromium out of this compound.
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