morsagh
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Coordination bond in chrome alum
When solution of chrome alum KCr(SO4)2 is heated above 50C soluton turns green, can be this green color caused by trans diaquadisulfato (sulfate as
bidentate ligand) trans-K[Cr(H2O)2(SO4)2]? Are there any works about this? If not should i try measure
free sulfate concentration to confirm my theory or is it just waist of time...
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blogfast25
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Quote: Originally posted by morsagh  | | When solution of chrome alum KCr(SO4)2 is heated above 50C soluton turns green, can be this green color caused by (sulfate as bidentate ligand)
trans-K[Cr(H2O)2(SO4)2]? Are there any works about this? If not should i try measure free sulfate
concentration to confirm my theory or is it just waist of time... |
I don't know if it is exactly trans diaquadisulfato Cr(+3) but it's definitely a sulfato complex. It's quite stable and returns to 'normal
crystallisable Cr alum' only after weeks at RT.
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Hegi
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Yes it can be possible. And it is damn hard to go back and crystallize chrom alum again... 
Our webpage has been shut down forever cause nobody was willing to contribute. Shame on you all!!!
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morsagh
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I know that trans-diaquadisulfato complex of chromium isn´t described but according to existence of green trans dichloro-tetraaqua complex why not,
because that i want to titrate concentrations of free sulfate anion in heated solution of Cr-alum to know if i am right (maybe try Al-alum too, maybe
in hot solution will exist tetraedric disulfato ion, but this is just prediction according to sidgwick theory, in Cr-alum there´s green color typical
for trans chromium complex, in hot solution, trans coordination is prefered etc. so i am more sure). I am asking do you find it interesting? Should I
try it?
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morsagh
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Or i can think of tetraaquadisulfato chromium complex (sulfate as monodentate), not sure what of these two, depending on stability of bidentate
sulfate ligand, but according to sulfate geometry i think it is more likely bidentate as i mentioned before.
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Sulaiman
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I do not know if it helps but the Wikipedia entry for Chrome Alum
https://en.wikipedia.org/wiki/Chrome_alum
mentions the change from violet to green of aqueous solutions at 50C
with a reference that I can not access
Holleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (1985). "Chromium".
Lehrbuch der Anorganischen Chemie (in German) (91–100 ed.).
Walter de Gruyter. pp. 1082–1095. ISBN 3-11-007511-3.
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morsagh
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I am writing this because in all this books it is written that alums are just mixed salts, not real coordination compounds...
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DraconicAcid
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| Quote: Originally posted by morsagh | | I am writing this because in all this books it is written that alums are just mixed salts, not real coordination compounds... |
Generally true, but chromium(III) is always coordinated to something. In the regular dodecahydrate, it's got six water molecules as ligands. If you
get rid of them, you have to replace them with something.
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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morsagh
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I know but in books it´s meant like it isn´t K[Cr(SO4)2(H2O)2] but K2SO4*[Cr(H2O)6]2(SO4)3*6H2O, no anionic complex of chromium, so just "mixed
salts".
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