Growing large silver crystals

[Harold_V]

Lou & Peter,

Speaking as the novice I am, I have a question about your statements. Bare in mind, I do not understand chemical terms, so I'll talk in common language, and would appreciate an explanation that will make sense to someone of my level of understanding.

I'll begin with the fact that I've dissolved a large amount of pure silver in my days. It was used as electrolyte for my silver parting cell, and was usually dissolved, diluted and immediately put to use. On rare occasion, however, that was not the case. I can recall on no less than a couple occasions, I had dissolved all of the 30 troy ounces of silver that I used for the cell, but not diluted the solution, which was roughly two liters in volume. I could rely on a liter of nitric to dissolve the bulk of the silver, but I had a policy of using 50% (distilled) water, which made a huge difference in the dissolution rate, plus, from what I've read, greater service from the nitric (some talk of the hydronium ion, which I do not fully understand).

At any rate, when this highly concentrated solution was permitted to cool, it, almost immediately, started growing silver nitrate crystals. The entire lot became a solid mass, more or less.

Armed with the above information, are you folks telling me that if a concentrated solution of gold chloride was permitted to cool to room temperature, that no crystals would form? Seems a bit strange, but then I do not understand chemistry as you two do. I'd certainly appreciate anything you might be able to tell me in that regard. I had long assumed that's all it would take to grow the crystals.

Harold

 

[Lou]

What you had occur Harold was saturation of your solution--it couldn't hold any more silver nitrate at that temperature. You did this all while the solution was hot and is it cooled down, the situation got worse: the solubility of silver nitrate in your water decreased and left you with an unstable supersaturated solution, which crashed out your AgNO3. As an aside, by dilution of your concentrated nitric with distilled water, you increased the amount of hydronium ion (H3O+, what makes an acid an acid, at least inorganically). The reason for that lies in equilibrium--more solvent means the acid dissociates more completely. The greater service from the nitric is again due to equilibrium--more water means the NO2 produced can redissolve in water and make more nitric.

Now the difference with the AuCl3 lies in the fact that it is unstable in solution. Pure gold chloride forms red, monoclinic crystals. It is also hygroscopic (loves water), when this water gets into it, it forms a new compound, a hydrated one, with a different crystal structure. So it's going to hold onto its water; it won't form a neat, readily discerned crystal lattice which means no nice crystals--what you'll get is a very viscous syrup. It is very difficult to remove water from AuCl3, even more so than HAuCl4 (add HCl to AuCl3 essentially) which is a tetrahydrate.

The real, detailed answer is very complex and deals with the interaction of the solvent with the salt.

Another more extreme example is that of Aluminum (III) chloride, a potent Lewis acid eager to form AlCl4-. Any moisture that gets ahold of this immediately reacts with it, to give HCl gas and hydrous aluminum oxides.

Lou

 

[Harold_V]

So then, it's safe to conclude that the vial of gold chloride crystals I had at one point in time, were not just the result of a concentrated solution as I assumed.

Thanks, Lou.

I pretty well understood the silver situation----and assumed that gold would react similarly.

Harold

 

[Lou]

If you have crystals of ''gold chloride'' coming out of an aqueous system, chances are, it's not gold chloride, but rather HAuCl4, which will be a dark orange red. I have posted pictures of HAuCl4 conc. solution, and freshly made AuCl3 in (IIRC, ether?)


Lou

 

[peter i]

I can add nothing of value to Lou’s explanation, but will return to the original topic.

After a week of low current in thin silver nitrate, it looked like the photo below.

There were a single crystal of about 2 by 7 millimetres (just visible in the lover left corner), the rest was a very pretty little trident of approximately 1 mm crystals. Unfortunately, they were not bonded together, but came apart with a gentle pressure.
My parting cell still grows larger crystals!

My next experiment will be even lover voltage in a concentrated solution.

 

[Lino1406]

This is called: electroforming

 

[peter i]

This is called: electroforming

And I dare say it is not!

This is wild and uncontrolled growth, whereas electroforming is the very controlled homogenous deposition of metal on a mandrel in order to achieve a desired shape or object.
It is possible to work with rather extreme tolerances, so this is definitely not electroforming.

It is electrodepositing, but we all knew that already.

And it still leaves me with the challenge to grow large single crystals. :wink:

 

[Lino1406]

and not "wild" crystals, use electroforming
principles, e.g. no brighteners which cause
embrittleness, very narrow voltage range,
and if I'll recall something else, I'll inform

 

[austexjwlry]

Peter i,

Thank you for posting you're experiments, I'm enjoying them very much!

To duplicate my crystals;
1. Dissolve nearly pure gold in aqua regia
2. Dilute with a few times its volume of water (this may work without diluting with water)
3. Add a large excess of SMB, enough to make the smell of chlorine really rude & offensive upon removing a lid from you're container
4. Boil down until crystals form upon cooling.

This should work in a test tube. I believe Lou is absolutely right about the AuCl3 but for lack of a better term at this point on my part, these do seem to be just AuCl salt type crystals. It seems you may have to boil down enough to remove most of the nitric from solution before they will form.

The general procedure in refining is to boil down first to remove nitric, then precipitate. I added the SMB first then boiled down the very dilute solution which might explain why most refiners haven't created these particular crystals in their boildown pans.

Wayne

 

[peter i]

Thanks, then it could be sodium sulphate (or the SMB itself!)

Can you smell sulfur dioxide (the pungent smell you describe as chlorine) if you dissolve these crystals in a bit of boiling water?

 

[peter i]

I refuse to give up!
Especially after the parting cell made larger crystals! :twisted:

#4
I’m going to try with 0.35 V, the concentrated AgNO3 solution and a larger anode.
I cast the anode from refined fine silver in a sand mould, the fine silver behaving much better than I had expected. When used to casting sterling, the look of metal solidifying while staying bright is quite funny.

The micro cell is running now with #4, and if it runs away, #5 will be made with very dilute silver nitrate solution.

 

[peter i]

#4 was a stinker!
Lots of small cubic crystals.

#5 is now running with the same parameters, except of a 1:20 dilution of the silver solution.

 

[draftinu]

Peter, got to love your persistence!!!!!!!!!!!!!!!! :wink:

 

[peter i]

”A persistent nerd is one of the scariest creatures in the forest”

#5 went sour too
It grew a single thin long strain of crystals from minus to plus, shorting out the cell.

#6 is running:
The dilute solution from #5, but this time, “minus” is not a single point, but a large single crystal placed perpendicular to the electric field. This seems to make the crystals less eager to “reach” for the anode. (but so far looks like a lot of small crystals)

In #7, I’ll minimize the area of the anode, and make the cathode as wide as possible.


Slow nucleation and slow growth should be the key to succes.

 

[Oz]

Peter,

Progress ???

 

[peter i]

#6 very slowly grew a lot of small crystals
:evil:

#7 has not launched yet

 

[Oz]

Peter,

I have been watching this with great interest, thanks for the update! Off the path you are taking, you had made the comment “Slow nucleation and slow growth should be the key to succes”. It made me wonder if it might be possible to start with a saturated solution at a relatively warm temperature in a shallow glass pan. If you place it in an insulated cabinet with a reasonably accurate thermostatically controlled heating element. Then lower the temperature a half degree every other day or so. A few chunks of lead or even rock in the cabinet would even out temperature swings in the cabinet. Then again the temperature swings if not too severe may even help the growing of large crystals on the warmer end of the temperature swing re-dissolving the smaller crystals back into solution. The larger crystals would not completely re-dissolve but the smaller ones would.

 

[peter i]

I was thinking about that too.

The “normal way” to achieve super saturation in crystal growing is to lower the temperature (if the wanted substance has lower solubility at lower temp), remove the solvent by evaporation or change the solubility of the substance in the solvent (adding alcohol to an aqueous solution is a classic).
Another approach could be formation of the substance. E.g. A+B-> AB .
If A and B are soluble, and AB has a lower solubility slow addition of a solution of A to a solution of B could give a slight super saturation and a crystal would grow.

There is an energy barrier to nucleation, and the general trick in growing large crystals is to supersaturate enough to make the crystal grow, but not that much as to allow other crystals to nucleate. That’s why slow cooling of a warm saturated solution is normally so successful in growing nice large crystals.

In galvanic processes the supersaturation is caused by the current. And I think one of the problems is, that metallic silver is very insoluble in water. The moment a few silver atoms are reduced, they will clump together to form a crystal.
The growth characteristics depend on the voltage (fine powders of many metals are made by electrochemical means, normally at relatively high voltage), the concentration and the shape of the electric field. Temperature would probably have some effect too, not on saturation but on the way the field shapes and the conductivity of the solution.

I think that the main problem is to avoid unwanted nucleation. Presently it seems like the crystals grow to some size, then stop and others nucleate and grow instead.


… there is quite a bit of black magic in electroplating and electrodepositing!

 

[calgoldrecyclers]

peter,
try this:
suspend a copper wire into a testube of .1m silver nitrate overnight.
keep in a cool dark place with low noise and movement. by morning you will see crystals!
hint: if you coil the wire or bend in odd shapes, you will get a bigger crystal.

 

[Gotrek]

peter,
try this:
suspend a copper wire into a testube of .1m silver nitrate overnight.
keep in a cool dark place with low noise and movement. by morning you will see crystals!
hint: if you coil the wire or bend in odd shapes, you will get a bigger crystal.

Experiment 15 on http://alex.edfac.usyd.edu.au/Methods/Science/Chemistry%20Demonstrations