Dissolution of Chromium by conc. HNO3

Blogfast25:
Guess how even higher concentrations of hydrochloric acid would behave on the purpose? Any means on trying that?
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A method dealing with sulfuric acid only has been tried again. The passivation layer absolutely is the major problem here. Suppose a stainless stell plate, as for sulphuric acid, I could break the layer at ~120°C IF the whole piece of ss could hold this temperature, i.e., applying the solution locally will not corrode the plate. Once broken the corrosion starts and keep until 90°C. Temperature however should not cool below 110°C to prevent the passivation from forming again. The sulfuric acid solution was no less than 10 molar (10 mol/ L), yes, 850g/ L H2SO4 (50% by volume).
Dissolving then turns even more problematic with this acid because as acid molarities goes down the reaction stops
But a cool and well-known effect was observed after cooling the part: the common ion effect. Salts of ironII and chromium III could be retrieved in anhydrous form from the slurry, thus being that the supernatant can be reused to dissolve the plate and so on. But this as presented requires expendure of energy sources and a heat resistent vessel to be put on oil baths or directly upon flame, the later being so a 1L borosilicate glass to ensure cracking will not occur.


[Edited on 3-14-2012 by Poppy]

Quote: Originally posted by Poppy

Blogfast25: Guess how even higher concentrations of hydrochloric acid would behave on the purpose? Any means on trying that? [Edited on 3-14-2012 by Poppy]

Quote: Originally posted by Poppy

Strange I think I've seem HCl concentrations up to 90%, [...] [Edited on 3-14-2012 by Poppy]

No, sorry poppy but you're 'seeing things'. Concentrated HCl is basically 37 w% HCl.

Careful breathing in those chemfumes!

[Edited on 14-3-2012 by blogfast25]

Excellent! Thanks so much, but still a few questions:
1) How do I get the sodium dichromate made? (I know it's a double salt, but I also don't know anything about double salts.)
2) Can the ammonium sulfate be bought pure as fertilizer, or made from a sulfate salt (epsom?) and an ammonium compound?
3) Are you sure Cr(OH)3 forms? From what I know, the hydroxides of transition metals tend to turn into some oxide over time and Cr(OH)3 isn't well documented.
Also, I do have quite a bit of oxalic on hand. Time to experiment!

Quote: Originally posted by Poppy

That is exactly the point. The passivation layer regenarates rapdly once the dissolving bath goes trought saturation. Twice excess acid is generally required to dissolve a given ammount of the ss, ant that with heating. The trick here we might guess is to create a bath wherein a catalyst constantly breaks passivation, allowing the acid to keep doing its job longer. Chromium ore seems more fruitful than dissolving the own ss IMHO, because Cr2O3 is expected to react easier. Once dissolved chromium can be pushed into Na2Cr2O7, decanted and put to react with (NH3)2SO4, where ammonium dichromate shall coprecipitate with sodium sulfate. Then both are placed in a heated bowl and thermal decomposition of the ammonium dichromate takes place, leaving Cr2O3 together with soluble impurities. The Cr oxide is then washed and, as wiki says, its hygroscopic, which means it probably turns into Cr(OH)3 once it contacts water. A mass verification will probe the reversibility of the hygroscopicity upon heating. Might oxalic acid be this accelerator?

Sorry folks,
A little reconsideration on the making of Cr2O3 by this method. I dont think it works perfectly. Last time I checked on this reaction a dark green powder was obtained, which is similar to the so told pure Cr2O3, but this time now I obtained a black powder probably because I fried the mixture of Na2Cr2O7+(NH)3SO4 with a aluminium pan, which almost surely released Al3+ ions in product. Darn it!
Gotta use glass inside my oven, but I am a bit afraid of the gases going off and increasing the pressure in the oven making the expected worst to come by.
I will try with a smal ammount. I am just uncertain as maybe chromium III sulfate may be depositing after the reaction

About its color, Wiki says:
" It is not readily attacked by acids or bases, although molten alkali gives chromites (salts with the Cr
2O2−
4 anion, not to be confused with the related mineral chromite). It turns brown when heated, but reverts to its dark green color when cooled. It is also hygroscopic."
Between this is the picture Wiki throw for it:



Also Wiki says:
"The conversion of chromite to chromia proceeds via Na2Cr2O7, which is reduced with sulfur at high temperatures:[5]
Na2Cr2O7 + S → Na2SO4 + Cr2O3"
So we could at least expect better results with just a sulfate than with sulfur itself.
As long as we talking about ion trapping and reactiojn yield.


EdIt: april 03
Checking on the mass of the product it can be reported the double salt decomposition method is worthy although care must be taken to avoid contamination, as the highly oxydising dichromates are likely to inpart stains from metallic surfaces.


[Edited on 4-3-2012 by Poppy]

Here the doublesalt can be seen just prior to thermal decomposition and the resulting blackpowder thus formed.


[Edited on 4-3-2012 by Poppy]



Washing the black powder yields the expected ammount of Cr2O3, but because of contamination the best I could get until now is a very dark green powder.

[Edited on 4-3-2012 by Poppy]

Quote: Originally posted by AJKOER

For curiosity, first make some HOCl by adding vinegar to Bleach (NaClO/NaCl/NaOH). This produces dilute HOCl in a solution of Sodium acetate.