"Tin Metal Crystal Experiment Lab Book Notes" - Views: 206 · Hits: 206 - Type: Public

Tin Crystals by Electrolysis

Tin chloride electrolyte needs mixing with HCl (same molarity or thereabouts?  or just some?) to prevent precipitation
Also needs tin metal lumps stored in it to prevent areal oxidation
Tin does react with HCl, but apparently very very slowly.  Keep solution cool and try to not have concentrated HCl
May not need SnCl2 at all to start - 7.8%HCl or 90ml concentrated diluted to 550ml might also work

Start!  4-29, 1200.  Using 13.1-14.6g SnCl2*2H2O in 140-150ml water, acidified with a few drops of HCl.  Tin anode, tin cathode.  Anode on bottom, cathode protruding down into electrolyte.  LM317 adjustable voltage regulator driven by 7.5VDC wall wart, also has 6 diodes on regulator output to drop lowest achievable voltage below LM317's 1.25V.

4-29,1200: 85mV at 10.6mA (voltage higher without current measurement)
1500: 72mV at 10.6mA, crystals grown too much already but seem sturdy.  Adjusted to 34mV at 4.6mA.  There seems to be a jump in current from 6mA to 10mA, or thereabouts.  Why?  
4-30,0900: Crystal has no additional visible growth, seems grey (not shiny).  Maybe no tin deposited?  Adjusted back up to 72mV at 9.7mA; shiny tin is now depositing.  Swirling jar disturbs higher concentration of electrolyte near anode, causing schlieren
5-1,1530: Crystal now looks like broccoli (shrub like grey thing) with some shiny tin growths protruding.  Current at 9.7mA.  Swirled again.  Why won't this work?!?!
5-3,1730: very good large flat shiny crystal forming, broccoli grey thing larger.  Schlieren still happening.  Current at 12mA.
5-4, solution slightly cloudy.  Removed crystal and washed in water.  Quite beautiful, and it doesn't tarnish!  The grey stuff fell apart, but the crystals themselves are adequately sturdy.

5-14,1800: Began new experiment.  Anode washed off and scrubbed (electrochemical etching of crystal growth revealed crystal domains below surface of anode blob).  Same cathode setup.  Electrolyte processed by adding HCl till clear then neutralizing a bit with washed tin chloride-baking soda precipitate.  Still acidic - I would guess more than last time.  Also added small plastic tube protruding ~3/4 way into solution with flattened end (paddle shape).  Tube is rotated at low RPM using LM317-driven D.C. hobby motor.  Am hoping this will very gently cause agitation - need to make sure crystal always has available Sn++ near it, but swirling by hand is infrequent and too vigorous - it stirs up debris from anode which may be additional nucleation sites.  11.6mA with 155mV.  Without ammeter 45mV.  Why?
5-14,2000: 170mV at 11.5mA.  35mV without ammeter.  Crystals visible (growing too darn fast!) and anode has blackened considerably.  Motor still spinning agitator, and no schlieren immediately visible.
5-15,1430: lots more crystals, smaller though - kind of look like eBay vanadium samples.  40mV or 163mV still 12mA.  Oldest regions on crystal blackened slightly?  Anode definitely blackened.
5-16,0830: 35mV or 165mV at 11.2mA.  Motor noticeably louder in revolutions.  Crystal still looks good - still  like vanadium crystals.  Solution clear and anode black/gray.
5-16,2200: 38mV or 169mV at 12.3mA.  Crystal looking good!
5-17: motor was found to have stalled in morning and evening checkup.  Voltage was increased slightly to restart motor stirring.
5-18,0900: 32mV or 109mV at 7mA.  Crystal looks very good, with long arm reaching down toward anode.  Long arm has multiple exceedingly flat and shiny large crystal faces.  Arm within 2cm of anode.
5-18, 1900: removed crystal.  Has ok crystals, some of large ultra flat faces seem to have been covered with small crystals.  Crystal apparently has an extremely fragile connection between initial growth and long arm.  There are a lot of smaller (1mm diameter at most) crystals along crystal tree arms.

Third try:
5-24,1300: removed 30mL solution and replaced with tap water.  Added 4 drops HCl to ensure solubility of tin and scraped anode off a bit (it had been left in the solution since the last run).  New tin cathode - same idea, a U shape poking into solution.  11.3mA at 170mV or 70mV.  11.3mA is about the lowest I can get before the current drops to 6-7mA.  It seems like a discontinuity.  Why?
5-24,1345: adjusted to 9.05mA at 134mA.  Leaving ammeter on in circuit to monitor.
5-24,1530: ammeter off and disconnected, voltage at 41mV.  Large shiny long crystals forming!  Anticipating result.
5-25,1730: HUGE, long, shiny flat crystals.  Grown quite a bit down in cell - probably will short out after less than a day.  44.4mV.  Did not measure amperage due to theory that breaks in electrodeposition (moving wires to ammeter connection) lead to nucleation on large shiny faces (undesired).
5-26, 0900: Crystal has shorted out cell, but everything seems fine.  Absolutely huge crystal faces, ultra shiny and flat.  Removed and washed with water.  Still looks good - just needed a bit of hot glue to short up after rough handling.