bolbol
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Dissolution of d-block metals in HCl vs HF
Hi all,
I am looking to compile information on how d-block metals react with the following acids in atmospheric pressure on temp scales of 0-100C:
-HCl
-HF
-HNO3
-HCl+HNO3
-HF+HNO3
I know some like Pd/Ag/Cu/Zn/V/Mn in HNO3, or Zn in HCl, etc. I would think that someone may have tried this and published but I have been
unsuccessful in finding anything covering most of the d block metals, especially the left half. Anyone with experience on this? I predict metals like
Ti, Zr, W, Hf, Mo to dissolve in HF but I am curious if HNO3 is required for oxidizing power.
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Bedlasky
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Ti dissolves well in HCl and HF. Zr and Hf dissolves in aqua regia and HF+HNO3.
I experimented quite a lot with dissolving Mo powder in various acids. It dissolves very well in H2O2 and basic/acidic mixtures with H2O2. Reaction
with 30% H2O2 is pretty violent. In 1+1 HNO3 it dissolves to form Mo2O5, in 65% acid it dissolves to form MoO3. 99% HNO3 passivates Mo, but if you add
little bit of water, violent reaction occurs and lots of MoO3 is formed. 15-36% HCl reacts very slowly with Mo to form green [MoOCl5]2- ion with Mo in
V oxidation state. 65% HNO3+NH4F reacts with Mo to form soluble Mo(VI) oxo-fluoro complexes. 1+1 HNO3+H3PO4 forms soluble Mo(V) oxo-phosphato
complexes.
I want to make similar test with W powder in the future. H2O2 and mixtures with it surely dissolve it. I suspect that HNO3+HF will dissolve it pretty
well with formation of soluble W(VI) oxo-fluoro complexes. I also think that aqua regia will dissolve it to form WO3.
Nb and Ta are quite resistant, they can be only slowly dissolved in HNO3+HF mixtures.
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chornedsnorkack
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Electrode potentials, from
https://en.wikipedia.org/wiki/Standard_electrode_potential_(data_page)
Sc Sc3+ + 3 e− ⇌ Sc(s) -2.077
Y Y3+ + 3 e− ⇌ Y(s) -2.372
La La3+ + 3 e− ⇌ La(s) -2.379
Lu Lu3+ + 3 e− ⇌ Lu(s) -2.28
Ti Ti3+ + 3 e− ⇌ Ti(s) -1.37
Zr Zr4+ + 4 e− ⇌ Zr(s) -1.45
Hf HfO2+ + 2 H+ + 4 e− ⇌ Hf(s) + H2O -1.724
V V2+ + 2 e− ⇌ V(s) -1.13
Nb Nb3+ + 3 e− ⇌ Nb(s) -1.099
Ta Ta2O5(s) + 10 H+ + 10 e− ⇌ 2Ta(s) + 5H2O -0.75
Cr not expressly there but
Cr Cr3+ + 3 e− ⇌ Cr(s) -0.744
Cr Cr3+ + e− ⇌ Cr2+ -0.407
I get
Cr Cr2+ + 2 e− ⇌ Cr(s) -0.912
Mo MoO2(s) + 4 H+ + 4 e− ⇌ Mo(s) + 2H2O -0.15
W WO2(s) + 4 H+ + 4 e− ⇌ W(s) + 2H2O -0.12
Mn Mn2+ + 2 e− ⇌ Mn(s) -1.185
Tc not there
Re Re3+ + 3 e− ⇌ Re(s) 0.300
Fe Fe2+ + 2 e− ⇌ Fe(s) -0.44
Ru not expressly there but
Ru Ru2+(aq) + 2 e− ⇌ Ru 0.455
Ru Ru3+(aq) + e− ⇌ Ru2+(aq) 0.249
I get
Ru Ru3+(aq) + 3e− ⇌ Ru 0.386
Os not there
Co Co2+ + 2 e− ⇌ Co(s) -0.28
Rh not there
Ir not there
Ni Ni2+ + 2 e− ⇌ Ni(s) -0.257
Pd Pd2+ + 2 e− ⇌ Pd(s) 0.915
Pt Pt2+ + 2 e− ⇌ Pt(s) 1.188
Cu Cu2+ + 2 e− ⇌ Cu(s) 0.337
Ag Ag+ + e− ⇌ Ag(s) 0.7996
Au Au3+ + 3 e− ⇌ Au(s) 1.52
Zn Zn2+ + 2 e− ⇌ Zn(s) -0.7618
Cd Cd2+ + 2 e− ⇌ Cd(s) -0.4
Hg Hg2+2 + 2 e− ⇌ 2Hg(l) 0.7973
Note that Ti, Zr and Hf are far in the negative. Which means that they should react with nonoxidizing acids...unless they, like the even more negative
Al, get passivated. Which they could well be.
If HF can break passivation of Zr and Hf, would an oxidant help or would the metal just evolve H2?
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Bedlasky
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Zr and Hf are quite resistant to acids. Maybe HF alone can attack them, but I never read about it. HCl don't react. Only two acids mixtures I know can
surely dissolve these metals - aqua regia and HF+HNO3. Periodic videos make recently video about Hf and they dissolved it in aqua regia - the reaction
was really slow, they compared it with dissolution of gold and gold dissolved much quicker.
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chornedsnorkack
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Quote: Originally posted by Bedlasky  | | Zr and Hf are quite resistant to acids. Maybe HF alone can attack them, but I never read about it. HCl don't react. Only two acids mixtures I know can
surely dissolve these metals - aqua regia and HF+HNO3. Periodic videos make recently video about Hf and they dissolved it in aqua regia - the reaction
was really slow, they compared it with dissolution of gold and gold dissolved much quicker. |
This
https://scholarsmine.mst.edu/cgi/viewcontent.cgi?article=653...
Page 5 (13 of PDF) refers to a work that states nitric acid has no effect. Page 8 (16 of PDF) - same conclusion, between 0 and 13 N of HNO3. Figure 6
(page 23, 31 of PDF) - the speed of dissolution in aqueous HF was followed to 3 N, in stronger HF it got too fast to measure.
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bolbol
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Thank you all for the contributions. The Mo information is very useful and I definitely saw the Hf periodic video and I kept screaming behind the
screen that they test with HF.
I tend to stay away from thermodynamic data as I'm interested in knowing what one would observe mixing these metals with acids. Not so interested in
the microscopic liquid-solid interface. The products must be soluble for the reaction to evolve and the kinetics are also not accounted for when
looking purely at thermodynamic data. One example I had issues with was nickel metal. I tried different mixtures of HCl, nitric, and sulfuric. Started
very violently but I could never achieve full dissolution.
I think there should be metals that dissolve in HF without needing oxidizers, similar to how Zn reacts with HCl. I might do these tests myself later
on this year if I find time. Any data I can gather until then will guide my work and make it more efficient.
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Bedlasky
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Oh, thanks for info, this is interesting!
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chornedsnorkack
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| Quote: Originally posted by bolbol |
I tend to stay away from thermodynamic data as I'm interested in knowing what one would observe mixing these metals with acids.
I think there should be metals that dissolve in HF without needing oxidizers, similar to how Zn reacts with HCl. I might do these tests myself later
on this year if I find time. Any data I can gather until then will guide my work and make it more efficient. |
The thermodynamic data there are a constraint. A reaction which is thermodynamically allowed may or may not be passivated. A reaction which is
thermodynamically unfavourable is blocked regardless of passivation. So you can guide the work and make it more efficient by looking at the
electrochemical series and identifying metals to rule out.
Another source:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&...
For the missing metals I get
Tc Tc3+(aq) + 3e− ⇌ Tc 0.31
Os OsO2+4H+ + 4e- ⇌ Os + 2H2O 0.712
Rh Rh3+(aq) + 3e− ⇌ Rh 0.76
Ir IrO2+4H+ + 4e- ⇌ Ir + 2H2O 0.73
This makes the electrochemical series of d-block metals - note, still missing p, s and f blocks:
La La3+ + 3 e− ⇌ La(s) -2.379
Y Y3+ + 3 e− ⇌ Y(s) -2.372
Lu Lu3+ + 3 e− ⇌ Lu(s) -2.28
Sc Sc3+ + 3 e− ⇌ Sc(s) -2.077
Hf HfO2+ + 2 H+ + 4 e− ⇌ Hf(s) + H2O -1.724
Zr Zr4+ + 4 e− ⇌ Zr(s) -1.45
Ti Ti3+ + 3 e− ⇌ Ti(s) -1.37
Mn Mn2+ + 2 e− ⇌ Mn(s) -1.185
V V2+ + 2 e− ⇌ V(s) -1.13
Nb Nb3+ + 3 e− ⇌ Nb(s) -1.099
Cr Cr2+ + 2 e− ⇌ Cr(s) -0.912
Zn Zn2+ + 2 e− ⇌ Zn(s) -0.7618
Ta Ta2O5(s) + 10 H+ + 10 e− ⇌ 2Ta(s) + 5H2O -0.75
Fe Fe2+ + 2 e− ⇌ Fe(s) -0.44
Cd Cd2+ + 2 e− ⇌ Cd(s) -0.4
Co Co2+ + 2 e− ⇌ Co(s) -0.28
Ni Ni2+ + 2 e− ⇌ Ni(s) -0.257
Mo MoO2(s) + 4 H+ + 4 e− ⇌ Mo(s) + 2H2O -0.15
W WO2(s) + 4 H+ + 4 e− ⇌ W(s) + 2H2O -0.12
H 2H+ +2e- ⇌ H2 0
Re Re3+ + 3 e− ⇌ Re(s) 0.300
Tc Tc3+(aq) + 3e− ⇌ Tc 0.31
Cu Cu2+ + 2 e− ⇌ Cu(s) 0.337
Ru Ru3+(aq) + 3e− ⇌ Ru 0.386
Os OsO2+4H+ + 4e- ⇌ Os + 2H2O 0.712
Ir IrO2+4H+ + 4e- ⇌ Ir + 2H2O 0.73
Rh Rh3+(aq) + 3e− ⇌ Rh 0.76
Hg Hg2+2 + 2 e− ⇌ 2Hg(l) 0.7973
Ag Ag+ + e− ⇌ Ag(s) 0.7996
Pd Pd2+ + 2 e− ⇌ Pd(s) 0.915
Pt Pt2+ + 2 e− ⇌ Pt(s) 1.188
Au Au3+ + 3 e− ⇌ Au(s) 1.52
A suggestion here: only elements more negative than H can be expected to react with nonoxidizing acids. And Mo and W are close to limit, and HF a weak
acid, so they would have problems, like Pb and Sn.
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bolbol
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I have started this experiment already and am compiling data. I wanted to share some brief obersvations:
Zn, Cd, Co, Fe, Mn, Cr all dissolved in both 20% HCl and in 67% Nitric acid. Sometimes diluted, sometimes heated but they fully dissolved rapidly for
the most part.
-Vanadium did not dissolve in boiling 20% HCl. Dissolved rapidly in nitric
-Titanium dissolved by a very little amount ~0.03 g in HCl to give a very faint purple solution. Did not dissolve in nitric.
-Zr, Nb, Ta, Hf, no reactions were observed and no mass loss was measured when boiling in 20% HCl. Same with nitric acid.
I don't understand why Vanadium doesn't dissolve in HCl. It's chlorides appear stable in aqueous solutions. Any insights?
All experiments done on metal pellets, not powder. 99.9% purity and above.
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clearly_not_atara
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I am also surprised by the observations of V in hydrochloric acid. Generally V3+ will dissolve as the chloride while V5+ forms vanadates as you
observed. So I think the passivating layer must be V4+, i.e. VO2, which is oxidized by nitric acid but resistant to hydrochloric acid.
An interesting problem for anyone who wants to use vanadium.
[Edited on 04-20-1969 by clearly_not_atara]
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Bedlasky
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Try to use 30% HCl for dissolving vanadium and titanium. Titanium dissolve quite well in hot 30% HCl forming dark blue solution. I never try to
dissolve vanadium, but it should react with HCl as well.
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bolbol
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Worth noting that the V pellets I used had a very thin oxide layer (rainbow color). This immediately vanished when adding the acid. Also, it reacted
rapdily with the nitric acid and once degassed, solution was blue.
I have thought about redoing the metals that didn't dissolve with 37% HCl(or whatever it turns out to lower to at boiling temperatures). However,
wouldn't just rerunning the experiment with 20% HCl for a longer period of time do the same thing? Unless this is not purely related to kinetics....
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clearly_not_atara
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What did the V pellet look like after it came out of the HCl?
[Edited on 04-20-1969 by clearly_not_atara]
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Bedlasky
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| Quote: Originally posted by bolbol | Worth noting that the V pellets I used had a very thin oxide layer (rainbow color). This immediately vanished when adding the acid. Also, it reacted
rapdily with the nitric acid and once degassed, solution was blue.
I have thought about redoing the metals that didn't dissolve with 37% HCl(or whatever it turns out to lower to at boiling temperatures). However,
wouldn't just rerunning the experiment with 20% HCl for a longer period of time do the same thing? Unless this is not purely related to kinetics....
|
It's not just about kinetics, but also about complex formation. Ti dissolves in conc. HCl to form blue [TiCl5]2- ion, which upon dilution with water
decompose to purple [Ti(H2O)6]3+ and Cl-. You already observed [Ti(H2O)6]3+ when you tried to dissolve titanium in 20% HCl. Complex formation usually
makes dissolving easier, because it lowers electrode potential.
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unionised
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A lot of the first row of d group element difluorides have rather low solubilities.
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chornedsnorkack
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Some of them don´t have difluorides.
Sc: just ScF3. Said to be insoluble in water but soluble in excess fluoride
Ti: Ti(II) is unstable. TiF3 said to be "soluble"
V: VF2 is described as soluble, VF3 as insoluble
Cr: CrF2 quoted 76,7 g/100 ml
Mn: MnF2 1,02 g/100 ml
Fe: FeF2 called "slightly soluble"
Co: CoF2 1,4 g/100 ml
Ni: NiF2 4 g/100 ml
Cu: CuF2 "slightly soluble"
Zn: ZnF2 1,5 g/100 ml
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