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blogfast25
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[*] posted on 17-9-2013 at 12:30


AJ:

The notation ‘Na2O.Al2O3’ is just that: an antiquated notation. Look at older texts: the formulas of many double salts that are known now to be coordination complexes were written in that format because at least it reflects molar ratio. But it says nothing about structure or bonding.

Mixing a solution of sodium aluminate and ammonium chloride solution precipitates flocculant Al(OH)3.nH2O. The aluminate ion in solution is likely more accurately described as:

[Al(OH)<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>]<sup>-</sup>, which may lose that water quickly to become the tetrahedral [Al(OH)4]- anion.

It’s likely to be surrounded by more loosely bound water (solvation), causing a highly hydrated gel to precipitate when a proton is absorbed.

Anhydrous sodium aluminate would react with such an ammonium chloride solution much in the same way, in the sense that it would have to re-hydrate and solubilise at the surface first. The results would be gel-like Al(OH)3 too. Nowhere does Al2O3 come into it.

‘Concise Enyclopedias’ have their uses but are generally not so great in dealing with ‘the devil that’s in the detail’.



[Edited on 17-9-2013 by blogfast25]




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[*] posted on 17-9-2013 at 12:38


Unionised:

That's very interesting. I assume the molar ratio CuSO4.5H2O:Na2SiO3.5H2O is 1:1 (don't have time to check right now)?

Those solutions are fairly strong (0.4 M), did the whole mixture 'solidify'?

It's quite intriguing that a bit of the gel in water doesn't change appearance on boiling.

Interesting result with the ammonia too!

I want to replicate your experiment, perhaps with some photos too...

[Edited on 17-9-2013 by blogfast25]




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[*] posted on 17-9-2013 at 13:07


I was aiming for 1:1 though the silicate isn't exactly "analytical reagent".
http://www.amazon.co.uk/CHILTERN-CONNECTIONS-DEVELOPER-UNIVE...
The mixture was viscous, but not solid.
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[*] posted on 17-9-2013 at 13:15


Quote: Originally posted by unionised  
God knows what the structure of the blue gel is, but it's not copper hydroxide and I think it has as good a claim to be called a silicate as my windows have.

Did you miss the part where I had my precipitate analyzed and it was brochantite? I don't know if this forms ammonia complexes, but it is stable up to 250°C.

Why all the mumbo-jumbo about the precipitate not being Cu(OH)2, when it's not expected to be Cu(OH)2?
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[*] posted on 17-9-2013 at 16:19


OK, I was hoping that the chemistry surrounding silicates wasn't that complex or extensive.

However, after reviewing the material at Atomistry,com (link: http://silicon.atomistry.com/silicates.html ), I am clearly way off the mark.
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[*] posted on 17-9-2013 at 18:19


YES! should we be surprised, though? (since Si is in the same group as C)
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[*] posted on 18-9-2013 at 05:07



Quote: Originally posted by turd  
Why all the mumbo-jumbo about the precipitate not being Cu(OH)2, when it's not expected to be Cu(OH)2?


Quote: Originally posted by turd  

This seems to support the more intuitive scenario: copper hydroxide is precipitated and then due to lowered pH, there is slow gelling. Due to quick filtration I seem to have gotten not much silica. If you insist I can ask for an analysis of the precipitate as well as the calcinated powder. Whether a putative gel includes copper in the framework or only as inclusion - who cares. It is irrelevant for a subsequent hydrothermal treatment / calcination.


Hmmm...






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[*] posted on 18-9-2013 at 07:17


Hey now, that was posted before I got the analysis. Expectation can change...

But your criticism is right - I was an idiot for suggesting copper hydroxide, since Nicodem's paper said the precipitate is a basic nitrate, so one would likewise expect a basic sulfate. I sugarcoat it by pretending that I meant a copper hydroxide, not copper hydroxide. :P

Anyway I made a further experiment which convinced me even more that PinkHippo didn't actually obtain a silicate. The first time I tried to simulate PinkHippo's experiment by pouring silicate in an excess copper sulfate. This time I wanted to do it unionised-style: copper sulfate in an excess/stoichiometric amount of silicate.

The silicate solution was made by dissolving 3 g NaSiO3 in 30 ml warm H2O. This was stirred for a few h at ~50°C for complete hydrolysis. Then 3 g CuSO4.5H2O in 30 ml H2O was added (still at ~50°C in the hope that this would speed up gelling). Immediate precipitation of a nasty colloid (note: PH11 got a filterable precipitate). The blue "gel" was suction filtered (took quite long), washed with H2O and dried at 60°C. Filtrate clear, colorless.

A part of the precipitate was heated to red heat over a Bunsen burner. It turned first green, then black (as I had expected for a copper silicate, but see below). Contrast PH11: white/gray.

Digestion with 25% NH3 gave the characteristic copper complex (in contrast to unionised)?

Analysis of the precipitate: completely amorphous.

Analysis of the heated precipitate: not well defined, but definitely contains CuO. Also very likely tridymite/SiO2 and at least one other unidentified phase, presumably a copper silicate.

Conclusion 1: just because your gel is completely amorphous doesn't mean that it's a copper silicate.

Conclusion 2: PH's experiment resembled more closely my first run where I got well defined basic copper sulfate. (If it isn't completely fake.)

Outlook: I'm tempering the precipitate for 18 h at 900°C - let's see if the finely dispersed SiO2 and the CuO react to a silicate and whether the other phases are better defined to allow for a definite assignment. Unfortunately I cannot do this much longer, since I don't have infinite access to the analytics.
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[*] posted on 18-9-2013 at 09:15


Turd:

I still think it could be worth looking at the filtrate too. Does it contain the sodium sulphate one would expect? Any 'free ' silica', perhaps?

On 'digestion with 25 % NH3', what precisely happened?

I'm preparing a 'Na2SiO3.5H2O' 0.4 M solution tonight for an experiment tomorrow.

Lucky you to have at least finite access to the analytics! It'd be great if you could spend a few words on the type of analysis that were performed.

[Edited on 18-9-2013 by blogfast25]




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[*] posted on 18-9-2013 at 10:40


Quote: Originally posted by turd  
Quote: Originally posted by unionised  
God knows what the structure of the blue gel is, but it's not copper hydroxide and I think it has as good a claim to be called a silicate as my windows have.

Did you miss the part where I had my precipitate analyzed and it was brochantite? I don't know if this forms ammonia complexes, but it is stable up to 250°C.

Why all the mumbo-jumbo about the precipitate not being Cu(OH)2, when it's not expected to be Cu(OH)2?


"I don't know if this forms ammonia complexes"
Well, you should.
Practically every kid who has done high school chemistry has taken a solution of copper sulphate and added ammonia solution gradually. The initial reaction produces a ghastly mess of "basic copper sulphate" and / or copper hydroxide.
That material then dissolves when an excess of ammonia is added.
If brochantite is formed by the reaction of copper sulphate and an alkali then it's definitely soluble in ammonia solution. If it isn't produced then nobody cares.

Incidentally, re "Analysis of the precipitate: completely amorphous." What sort of analysis?

[Edited on 18-9-13 by unionised]
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[*] posted on 18-9-2013 at 11:08


I should know a bazillion things that I have forgotten. Fortunately I don't have to - I just ask smartasses on the internet.
Quote: Originally posted by unionised  
Incidentally, re "Analysis of the precipitate: completely amorphous." What sort of analysis?

XRD, I thought that was completely obvious.

So, are you still sure that PH11 got a "copper silicate" after I am able to crash out crystalline Cu salts and grow amorphous gels which seem not to be copper silicates?

blogfast: On digestion with NH3, the precipitate became blue and slowly migrated into the liquid phase.

[Edited on 18-9-2013 by turd]
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[*] posted on 18-9-2013 at 12:27


I meant elemental analysis.

I've never been convinced that PH11's was anything like a copper silicate and expressed much scepticism about that. Now I'm not sure about anything with regards to copper silicates anymore...




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[*] posted on 18-9-2013 at 13:09


Well, I still have a test tube with a blue precipitate at the bottom of a nearly colourless solution that smells strongly of ammonia.
It's clearly not copper hydroxide and it's not basic copper sulphate.
It was prepared by reaction of a solution of a silicate with a solution of copper sulphate.
What do you think it is?
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[*] posted on 18-9-2013 at 13:14


Hm? I think we're talking at cross-purposes...

I asked unionised who seemed to be dead set that PH11 got a silicate and yet couldn't give any convincing argument. To be honest I had from the beginning the suspicion that PH11 was fake - why dig up such an old thread? And the turning white on heating thing is dubious. OTOH I don't want to insinuate anything.

You are right that I should have had a closer look at the filtrate, but my time is very limited at the moment. And I can't do elemental analysis, at least not with an acceptable time effort.

Quote:
Now I'm not sure about anything with regards to copper silicates anymore...

Then simply don't call these undefined and uncharacterized gels copper silicates. ;) It's certainly not what the original poster had in mind. And I object to the comparison with glasses - both are amorphous but glasses are single-phase (in principle) and can be perfectly defined (think silica or some perfectly pure organics that form glasses).
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[*] posted on 18-9-2013 at 13:21


Quote: Originally posted by unionised  
Well, I still have a test tube with a blue precipitate at the bottom of a nearly colourless solution that smells strongly of ammonia.
It's clearly not copper hydroxide and it's not basic copper sulphate.
It was prepared by reaction of a solution of a silicate with a solution of copper sulphate.
What do you think it is?

I have no idea what this. But why do you insist that PH11 got exactly what you got and not what I got in two different experiments? What happens when you heat it to ~700°C? For all I know, a copper silicate should be dark green/black.

What do you think was the perfectly amorphous gel, that did dissolve in aq. NH3 and calcinated into SiO2 and CuO (+ other phases)?

[Edited on 18-9-2013 by turd]
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[*] posted on 19-9-2013 at 01:23


Quote: Originally posted by unionised  
What do you think it is?

Now that I managed to get the same thing I'm sure that this is a gel containing CuO and SiO2. It is certainly not anything like the Na/Cu silicate of the Acta Crystallogr. paper. This would not form such a nice tetramine complex. Relating the two would be like relating diamond and graphite because they're both composed of carbon.

[Edited on 19-9-2013 by turd]
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[*] posted on 19-9-2013 at 04:52


Quote: Originally posted by turd  
Quote:
Now I'm not sure about anything with regards to copper silicates anymore...

Then simply don't call these undefined and uncharacterized gels copper silicates.


Nowhere have I done that. I was very sceptical about PH11's results (I think he did actually do an experiment) and thought the precipitate to be an undefined mixture of Cu(OH)2 and hydrated silica gel.

It's unionised's result that had me wondering, when on heating his precipitate didn't turn black so easily. You claim it's a basic copper sulphate but other than your assertion we have precious little evidence for that either. Also, why would a basic copper sulphate form in these conditions? Right now we don't even know where the sulphate anions have gone because neither of you checked the filtrate for anything.

I'm very short of time too but hope to run a replica of unionised's run this week end, looking also at the filtrate if I can get some.




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[*] posted on 19-9-2013 at 05:08


When quoting you removed the smiley. I was kidding.

I admit the last posts where a bit chaotic because I'm posting besides real work. :o

To summarize:

- I tried to simulate PH11 conditions - I got basic copper sulfate. 100% certain
- unionised got a mixed CuO/SiO2 gel. He heated to <100°C, where basic copper sulfate does not decompose. But the ammonia test proves that there is none.
- I tried to reproduce unionised gel and got a 100% amorphous precipitate that *did* dissolve to good parts in NH3. I think this is remarkable. Google gives a few hits on amorphous Cu(OH)2, but so far I haven't had time to check them out.
- In another try I did get the CuO/SiO2 gel. But not as cleanly. I still get a distinctly blue solution on treatment with 25% NH3 but also a very nice amine-complex precipitate (or maybe blue solution in the gel?). To check I will have to wash multiple times with aq. NH3.
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[*] posted on 19-9-2013 at 11:43


Puts on suit of armour.
Quote: Originally posted by unionised  
Well, I still have a test tube with a blue precipitate at the bottom of a nearly colourless solution that smells strongly of ammonia.
It's clearly not copper hydroxide and it's not basic copper sulphate.
It was prepared by reaction of a solution of a silicate with a solution of copper sulphate.
What do you think it is?


Well, it's possible that the answer is copper phosphate.
:mad:
I got suspicious when the results I got weren't repeatable. I'd not expect a perfect match because silicates are often a bit (well, a lot) variable.
But I thought I'd better check the sodium "silicate" I was using.
It doesn't give a precipitate with acid so, whatever it is, it's not a silicate.
It's a strong base, it was fairly cheap and it's not hygroscopic so it's not NaOH. It doesn't fizz with acid so it's not carbonate.
My guess is phosphate.
I have been ripped off.

Now, just as soon as someone answers my point about hydroxide being a strong enough base to react with silica ( to produce silicate ions) and the fact that copper hydroxide, while not very soluble, is a source of hydroxide ions to the extent that it dissolves and, given that there is at least one copper silicate species that has a very low solubility- lower than that of the hydroxide since it's formed in solution where hydroxide would be an alternative, then they will have ruled out the idea that you get a copper silicate by reacting copper sulphate solution with sodium silicate solution.

The amorphous product proves that it's not the crystalline one, but it doesn't rule out a silicate.

Raman spectrum anyone?
That should show up any Cu-O-Si groups


[Edited on 19-9-13 by unionised]
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[*] posted on 19-9-2013 at 18:58


Maybe PH11 got the same Na2SiO3 batch? That would explain the (green)grey color on heating?
Quote: Originally posted by unionised  
Now, just as soon as someone answers my point about hydroxide being a strong enough base to react with silica ( to produce silicate ions) and the fact that copper hydroxide, while not very soluble, is a source of hydroxide ions to the extent that it dissolves and, given that there is at least one copper silicate species that has a very low solubility- lower than that of the hydroxide since it's formed in solution where hydroxide would be an alternative, then they will have ruled out the idea that you get a copper silicate by reacting copper sulphate solution with sodium silicate solution.

The amorphous product proves that it's not the crystalline one, but it doesn't rule out a silicate.

You will find I never ruled out such a thing. On the contrary, I'm convinced such gels exist and if you don't get them by precipitation/ageing of water glass, then you will probably get them by the classical alkoxide route.

I object to your logic that because a highly crystalline silicate is obtained under hydrothermal condition you will get a homogenous SiO2/CuO gel (As opposed to a SiO2 gel with a few Si-O-Cu bonds at the surface) by precipitation. The hydrothermal experiment is no proof since autoclaving solid Na2SiO3, Cu(OH)2 and water will provide the same/similar silicates.

And I object to the equation of SiO2/CuO gels with the Na/Cu-silicate in the Acta paper. It's not about amorphous/crystalline, but about degree of condensation.

PS: What's up with the double posting?

[Edited on 20-9-2013 by turd]

[Edited on 20-9-2013 by turd]
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[*] posted on 20-9-2013 at 09:39


He might, but I doubt it, for a start he said he was using water glass. The stuff I was using was a solid.

Since I have always accepted that the gel is amorphous it's hard to see why you think that I believe"you will get a homogenous SiO2/CuO gel (As opposed to a SiO2 gel with a few Si-O-Cu bonds at the surface) by precipitation. "

"The hydrothermal experiment is no proof since autoclaving solid Na2SiO3, Cu(OH)2 and water will provide the same/similar silicates. "
All it needs to prove is that there is a silicate with a low solubility.

As far as I can tell, the double posting is a glitch with the editing.

Do you agree that silica gel and Cu(OH)2 would form a silicate on autoclaving in water?

Anyway, as I said, Raman would be really helpful here.
Any takers?
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[*] posted on 20-9-2013 at 10:03


Accidentally found article, may be interesting for some:
Compositional analysis of copper–silica precipitation tubes

Code:
http://www.chem.fsu.edu/steinbock/papers/pccp07.pdf




Klaszczę w dłonie, by było mnie więcej....
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[*] posted on 20-9-2013 at 12:28


Unionised:

Are you sure your sodium metasilicate is not a silicate? You see, mine (advertised as ‘Na2SiO3.5H2O’) also didn’t give a precipitate with acid. But I’m trying that again tomorrow because I’m sure I’m doing something wrong.

In the mean time I replicated unionised’s experiment, with some add-ons. 0.01 mole of sodium metasilicate was dissolved in 25 ml and stood overnight. 0.01 mole of CuSO4.5H2O dissolved in 25 ml of water was then added to it. A blue, gelatinous precipitate formed.

Boiling this in a test tube caused no change in appearance whatsoever. By contrast some freshly prepared Cu(OH)2 quickly turned black on boiling. Precipitate after boiling test:



Adding 33 % NH3 to the gel caused it to dissolve and the copper diammine complex blue colour to appear.

The precipitate was then filtered off on Buchner and washed once with 25 ml of deionised water (DIW). The filtrate ran clear and colourless, at a paper pH of about 5. To the 50 ml of filtrate was added 0.01 mole of Ba(NO3)2 dissolved in 75 ml of DIW. A lot of white precipitate formed, this is it after about 30 min of standing:



Presumably this is BaSO4 and visually speaking in a quantity that would be consistent with about 0.01 mole of BaSO4. This is strong evidence against a basic copper sulphate being the blue precipitate.

A small teaspoon of the Buchnered and washed precipitate was then loaded into a nickel crucible and heated on a 10 inch hot plate on maximum setting, for about 1 h. The filter cake and crucible:




Some whitish material seemed to seep out and the blue substance gradually turned green. After a bit of drying, first formation of green material:



After about 1 h of drying:



The crucible was then further heated on a high, blue Bunsen flame for about 20 minutes with lid on. After cooling it was clear that the green material had turned black.



[Edited on 20-9-2013 by blogfast25]




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[*] posted on 20-9-2013 at 13:13


blogfast25: Your experiment is very consistent with my second run. And probably also my third run. There I probably just had a more condensed gel making leaching with ammonia harder.

Quote: Originally posted by blogfast25  
Are you sure your sodium metasilicate is not a silicate? You see, mine (advertised as ‘Na2SiO3.5H2O’) also didn’t give a precipitate with acid. But I’m trying that again tomorrow because I’m sure I’m doing something wrong.

Drop the silicate solution in HCl. That's apparently how those drying beads are made.

Quote:
Adding 33 % NH3 to the gel caused it to dissolve and the copper diammine complex blue colour to appear.

Exactly what I got: amorphous copper salt dispersed in amorphous silica gel. :(

Quote:
Presumably this is BaSO4 and visually speaking in a quantity that would be consistent with about 0.01 mole of BaSO4. This is strong evidence against a basic copper sulphate being the blue precipitate.

You can visually distinguish 0.010 and 0.0075 mol BaSO4? Respect.

Quote:
A small teaspoon of the Buchnered and washed precipitate was then loaded into a nickel crucible and heated on a 10 inch hot plate on maximum setting, for about 1 h. [...]Some whitish material seemed to seep out and the blue substance gradually turned green. After a bit of drying, first formation of green material:
[...]
The crucible was then further heated on a high Bunsen flame for about 20 minutes with lid on. After cooling it was clear that the green material had turned black.

Exactly the sequence that I have seen, just with a Bunsen burner it took around a minute. The black residue, as I stated above, was CuO, SiO2 and an unidentified phase. Presumably a copper silicate, as one would expect from heating finely dispersed silica and copper to red heat.

Meanwhile I calcinated the residue over night. Got a hard shiny graphite-y residue. So far I couldn't be assed to stem it out of the crucible.

Seriously: I think it would be much more fruitful to do some literature work. The last two decades had a huge sol-gel fad (can you call it a fad if it lasts so long?). If there are mixed CuO/SiO2/H2O gels, there must be some literature - this is just too obvious a target. So far I only found things like Cu or CuO nano-particles in silica gel, Cu immobilized with organics grafted on silica gel, Cu immobilized with amines grafted on silica gel, etc.
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[*] posted on 21-9-2013 at 00:47


Quote: Originally posted by kmno4  
Accidentally found article, may be interesting for some:
Compositional analysis of copper–silica precipitation tubes

Code:
http://www.chem.fsu.edu/steinbock/papers/pccp07.pdf


Great!
Now we can check to see if there are bands in the raman spectra of the gel that are not due to silica gel or to copper hydroxide.
As far as I can see, they didn't run the raman spectrum of silica gel- they used Na silicate as their reference.
I found a spectrum for silica gel here
http://www.ias.ac.in/matersci/bmsapr2011/299.pdf
Fig2- the line labeled x=0
It has sharp bands at about 600 and 900 /cm and a broad band near 800 /cm
Those may arise specifically as a result of the method of preparation but, in any event, they are missing from the spectra recorded for the copper containing gel in the paper KMnO4 found.

It looks to em like further work is needed in this field.

Anyway, to answer Blogfast's question, I can't see how the silicate wouldn't give silica gel on adding acid. It might be colloidal (particularly at first) but this stuff didn't ppt even when the solution was left to dry/ settle in a shallow dish overnight.
I will get some batteries for my balance and do a titration on it to see where that gets me.
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