With the objective of determining the Sn content of synthesised tin (IV) compounds I’ve been looking at producing some fairly pure tin as a standard
material.
Several routes are available such as reduction with either C or H, or even Al (Goldschmidt reaction) but also electrolysis of either aqueous or molten
anhydrous stannous chloride (SnCl2).
Before elaborating on the method of electrolysis, just a few words on the reduction of SnO2 with carbon. Many moons ago I carried out several of these
reductions using a mixture of SnO2 and BBQ charcoal in a steel crucible on a charcoal fired furnace w/o much problems.
But the day before yesterday I tried to reproduce this on a small scales using a propane Bunsen burner and a test tube and obtained very little actual
Sn. I used a stoichio + 20 % mix acc. SnO2 + C Sn + CO2 (as before), 6 g mix.
Test tube being fired:
Close up:
After cooling:
The bottom of the test tube does contain small pearls of tin metal, as evidenced by their vigorous reaction with nitric acid (the test tube could even
be recovered without much damage apart from a little sagging…)
In short: this needs higher temperatures to proceed at an appreciable rate…
After that I made two other attempts with a ceramic and steel crucible respectively but got no significant reaction.
So, on to electrolysis:
I’m very tempted to try the electrolysis of anhydrous SnCl2, molten: the MP of the salt is just above the MP of the metal which is kind of ideal.
Danger spot is the fact that hot Sn and Cl2 unite with great vigour, so cell design must be good to avoid the two from joining up. Also, I’ve very
little SnCl2 at the ready. This first attempt thus focused on aqueous electrolysis.
So a small amount of SnCl2.2H2O was synthesised by dissolving about 7 g of (mostly lead free) pewter in an excess of hot strong HCl, filtering,
precipitating as Sn(OH)2, careful washing with DIW, re-dissolving in strong HCl and finally carefully evaporating the solvent from the (clear)
filtrate. I overshot at first: reducing the solution to almost nothing and putting it on ice bath yielded no product (it’s very highly soluble in
water). Then evaporating further and hydrolysis occurred. But the hydrolysed mass dissolved back into strong HCl effortlessly. Then evaporating again,
this time even more carefully and the small pond of saturated SnCl2 solution crystallised over without any hydrolysis.
The electrocell is pictured below. Two graphite (battery) electrodes, each just under 2” long, connected to a disused computer (or such like)
adaptor power unit, 12 V of stabilised DC, max. 4 A (max. 48 W) and a standard multimeter switched to 10 A scale:
Close up:
After loading the solution of SnCl2 (7 g pewter as SnCl2 in about 200 ml of DIW) and after just a couple of minutes a ‘tin tree’ is forming and
gas evolves at the other electrode:
The photo really doesn’t do it justice: the “tree” is quite wonderful to behold: fast growing needles of glistening tin metal. The current
started off at about 2.5 A and gradually crept up to about 4.0 A.
A better view of the ‘electro-tin’ on the right hand electrode, after lifting the electrodes from the electrolyte the tree collapses:
But the run wasn’t entirely w/o problems either:
1. the tin tree grows so quickly that it starts touching the other electrode soon. I then used a wider cell receptacle so the electrodes were further
apart.
2. I could not smell any chlorine: the gas bubbles stubbornly refused to burst and formed a foam, expanding also towards the other electrode.
3. where chlorine (assumed) evolved the graphite electrode seemed to suffer, with bits of it coming off. I tried using a copper lectrode but that just
got covered in what I presume is Cu(I)Cl and current was very low. CuCl formed (?):
After a few runs, collecting the tin and washing it with acetone, here’s the yield:
It now needs weighing and possibly meting to a small ingot.
There’s room for improvement but that wasn’t too bad for a first run. I’m thinking of using a titanium anode and a much larger steel cathode
sheet to collect more tin in one run.
Thoughts are much appreciated…
[Edited on 4-11-2010 by blogfast25]kuro96inlaila - 4-11-2010 at 08:31
I would love to see the tin crystal forming......
It is beautiful!
Maybe electrolysing tin compound is my future experiment!bquirky - 4-11-2010 at 08:38
If you run your cell at a lower voltage and for a longer period of time.
is the cathode deposit less spongy ?
I dont know about tin. but copper will only form 'solid' deposits at about 2V without aggressive electrolyte agitationblogfast25 - 4-11-2010 at 09:41
If you run your cell at a lower voltage and for a longer period of time.
is the cathode deposit less spongy ?
I dont know about tin. but copper will only form 'solid' deposits at about 2V without aggressive electrolyte agitation
What I noticed is that with successive runs using the same solution, the crop seems to become less needlelike. The solution must have been
substantially depleted of tin by now because each run deposits about 0.4 g of it (from amps, duration and Faraday’s Law).
What I also noticed is that there is gas evolution too, besides tin, although that seems to cease after a short while. Hydrogen?
Bquirky, I agree the set potential of 12 V is a limitation. I either need a fully regulatable source or cobble a rheostat together. There is no
electrolyte agitation here whatsoever…
A few more pix:
Here’s the tin collected yesterday and carefully heated. The stuff was still slightly moist and most of it reverts to oxide upon heating. Hard to
see but easy to see are two clearly shiny patches of tin metal.
After cooling and adding nitric to it: vigorous evolution of NOx:
Another run after a minute or so: the flash brings out the metallic nature of the tin dendrites:
After about 4 minutes (same run):
The crop of that same run (calculated to be about 0.4 g of tin), being dissolved in hot, 22 % HCl. It dissolves completely leaving only he slightest
touch of turbidity.
