Gooferking Science
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Mysterious gas formation
Hello! Recently I was trying to convert stainless steel to potassium chromate. I had dissolved some stainless steel in HCl, which makes chromium
chloride and some other metal chlorides. Then I added NaOH to make chromium hydroxide and to convert the other metal chlorides to hydroxides. I was
supposed to see a precipitate, and I didn't. I decided to continue with the process and add hydrogen peroxide. When this was done, a gas started to
vigorously bubble from the mixture. This is very frightening when you don't know what it is. It wasn't chlorine. Could it be oxygen? I didn't think to
try a burning splint test which was stupid of me... I just want to make sure I wasn't breathing anything dangerous.
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DraconicAcid
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If you didn't see a precipitate, then you didn't add enough hydroxide to neutralize all the leftover hydrochloric acid.
Any bubbles that you get from adding hydrogen peroxide to a solution are 99% likely to be oxygen from the decomposition of the
H2O2.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Bot0nist
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My guess would be the decomposition of the hydrogen peroxide. What concentration of peroxide did you add?
Oops, beat me to it DA.
[Edited on 10-1-2014 by Bot0nist]
U.T.F.S.E. and learn the joys of autodidacticism!
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woelen
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How much NaOH did you add? If you did not add enough, then you did not yet neutralize all the acid. If you add too much, then a precipitate of
chromium hydroxide redissolves again, giving chromite ion. Chromium in oxidation state +3 is amphoteric and this means that its hydroxide can act as
acid and as base.
The bubbles you had are oxygen. Many metal ions (including chromium and iron) react with peroxides, giving complex intermediates and finally giving
oxygen and plain metal ions. At high pH, chromium(III) can be oxidized to chromium(VI) by peroxide.
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DraconicAcid
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Quote: Originally posted by woelen  | | How much NaOH did you add? If you did not add enough, then you did not yet neutralize all the acid. If you add too much, then a precipitate of
chromium hydroxide redissolves again, giving chromite ion. Chromium in oxidation state +3 is amphoteric and this means that its hydroxide can act as
acid and as base. |
Since he wanted to make chromate, then he would want to add enough base to redissolve the chromium. There would still have been precipitate present,
because the iron and other components of steel won't redissolve in base.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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blogfast25
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DA:
You need a LOT of alkali to redissolve the Cr to chromite and later you have to get rid of that. Not very practical...
I've done this as follows. Dissolve a known amount of SS in a calculated excess of 37 % HCl.
Precipitate all iron (as Fe(II)) and all Cr(III) as hydroxides using a calculated amount of NaOH. Filter off and wash precipitates to get rid of
chloride.
Treat washed hydroxides with a calculated amount of KOH and strong peroxide, on an ice bath (adding the peroxide slowly, one aliquot at a time, cool
intermittently if needed). Fe oxidises to insoluble Fe(OH)3, Cr oxidises to soluble K2CrO4. Filter to separate.
CALCULATION of stoichiometries is key here. Failing to plan = planning to fail.
[Edited on 10-1-2014 by blogfast25]
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