It speaks pretty clearly to me....the redox potentials of the free ions are given at 1.57 and again at 3.04 and the flipped redox potentials for the complex ions at 3.27. Visually you can see the reduction of tin onto the copper at 2.45 and the reaction is really rapid so the copper need only be exposed to the SnCl2:N2NCSNH2 solution for a few seconds at most...nowhere near long enough to etch the copper even on half ounce board.
HCl does not etch copper because there is no oxidizing agent to raise the copper metal to Copper-I or Copper-II. If copper is placed into a solution of HCl, (of any concentration) for a VERY LONG TIME...something like a week or more, CuCl (Cu2Cl2), copper-I chloride will start to form as a pale greeny white gug using atmospheric oxygen as the oxidant and the rate being determined by how fast it dissolves into the solution. As CuCl is insoluble in most aqueous solutions it coasts the metal and slows the process down further. At high concentrations the tea-coloured CuCl3- ion forms and the reaction proceeds slowly determined by the diffusion of oxygen, but as the HCl is consumed by the reaction CuCl eventually "gugs up" the surface again. If you tip a little hydrogen peroxide in it will etch quite fast, but you risk oxidizing the chloride ions to free chlorine and gassing yourself.
H2SO4 does not really react with copper either for much the same reason, however if it is really concentrated and hot the sulfate ions will be reduced by the copper to sulfite...and expelled as SO2...and consequently the copper will be oxidized to at least Cu-I.
HNO3 does attack copper rapidly and in almost any concentration because the nitrate ions are readily reduced to NO2 by the Copper and subsequently oxidize it all the way to blue Cu-II.
The commercial polish he used contained supfamic acid (as well as the wanted thiourea), this acid is the monoamide of H2SO4, (HSO3NH2) and its pKa is somewhat less than the pK1 of H2SO4, but, like H2SO4 it is not a strong enough oxidizing agent to oxidize Copper alone...some other oxidant like H2O2 or Bromine would be needed.
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I have considerable experience in recycling Ferric Chloride, (Iron III Chloride) from spent PCB etchant...in fact, if you understand your chemistry you will actually make MORE FeCl3 from the spent solution than you started with before you did any etching....so if you are silly enough to pay something like $15 for 200g at an electronics supplier rather than get kilogram quantities for a few dollars by dissolving rust in hydrochloric acid..gotten cheap at swimming pool chemical places or hardware outlets, then let your ignorance fleece you blind!
To recycle "spent" Ferric Etchant...one method, "the acidic method"...
1a: Add enough concentrated HCl to dissolve all the rust that has formed, if none has formed proceed to the next step...
2a: Add steel wool to react with the CuCl2 from the etched boards and displace the Copper.
CuCl2 + Fe ------> FeCl2 + Cu
The Steel Wool will also react with any Excess HCl too according to the next equation:
2HCl + Fe---------->FeCl2 + H2
So do it in a ventilated area so the hydrogen does not build up.
Once no more Copper sponge forms on the steel wool decant the solution...it is a mixture of Ferrous and Ferric Chlorides, (Iron-II and Iron-III Chlorides).
3a: Let it stand in the same tray used to etch the boards in...atmospheric oxygen will gradually oxidize the FeCl2 to Fe-III according to the next equation:
12FeCl2 + 3O2 + 6H2O -------> 8FeCl3 + 4Fe(OH)3
Fe(OH)3 is effectively rust, so the tray will get rusty over time...about a week. Alternatively you can slosh some hydrogen peroxide in and it will all happen instantly, but the hydrogen peroxide is expensive and the whole idea here is to keep it CHEAP.
12FeCl2 + 6H2O2 --------> 8FeCl3 + 4Fe(OH)3
4a: Add just enough conc HCl to dissolve all the rust.
Fe(OH)3 + 3HCl -----> FeCl3 + 3H2O
If you add excess conc HCl it will react with the FeCl3 already there to make HFeCl4, (Therachloroferric acid), which etches boards even better and faster!
Disadvantages of this method....it is slow...determined by how long the oxygen in the air will oxidize all the Fe-II back to Fe-III...this is why it is done in the etching tray, for maximum surface exposure to the air. People that are likely to disturb it must be kept away...like children and the missus.
Advantages: Copper comes out as the metal. You end up with more Ferric Chloride than you started with because you added Iron (in the from of Steel Wool) and HCl.
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Second method. (The basic/alkaline carbonate method)
Your Spent echant will contain CuCl2 from the copper you etched off the boards, it will also contain FeCl2, ferrous chloride from where the copper metal reduced the ferric chloride...according to the etching reaction
2FeCl3 + Cu -------> 2FeCl2 + CuCl2
There will inevitably be some unreacted FeCl3 in it too.
So it will initially be a mixture of FeCl3, FeCl2 and CuCl2 and a sort of dark greeny-brown colour.
CuCl2 is Emerald Green when concentrated and pure...it is blue when dilute.
FeCl2 is Absynthe Green when it is concentrated and pure.
FeCl3 is dark brown like beer bottle glass.
If it has been left sitting around for a while some of the FeCl2 will have been oxidized to a mixture of FeCl3 and rust by the oxygen in the air...this does not matter. Green emerald-likre crystals of CuCl2:2H2O might even form in the brown rust if it has sat for a really long time..this still matters little. If it has been left this long, you might need to add some water.
1b: Carefully add Bicarbonate of Soda (Baking Soda), NaHCO3 to your spent echant in small lots. A lot of carbon dioxide will be produced and it will froth up...be aware of this.
The NaHCO3 will react with the FeCl3 and FeCl2 first before it reacts with the CuCl2 so if you add it slowly the copper will initially remain in solution. The NaHCO3 reacts with the two Iron Chlorides thus:
With FeCl3:
FeCl3 + 3NaHCO3 ----> Fe(OH)3 + 3NaCl + 3CO2 (Rust, Salt and Carbon Dioxide)
With FeCl2:
FeCl2 + 2NaHCO3 -----> FeCO3 + 2NaCl + H2O + CO2
The grey-green insoluble Ferrous Carbonate, FeCO3, formed reacts almost instantly with the oxygen in the air to make more rust after the carbon Dioxide has dissipated.
12FeCO3 + 3O2 + 18H2O -------> 12Fe(OH)3 + 12CO2 (More Rust and Carbon Dioxide)
This reaction is very quick and you do not need to leave it in a tray for a week, like above.
Eventually all the Iron compounds will have converted to rust...it should appear as a deep red brown the instant the NaHCO3 touches the dark solution. Once the Iron has gone to insoluble rust a new reaction with the copper will start...and we don't want it to! It will produce green Cupric Carbonate or Malachite, which, like the rust is insoluble in the aqueous mix. As the Iron is removed as rust the solution should be getting greener and greener, this is the CuCl2 concentrating.
This is the unwanted reaction between the BiCarb and CuCl2 to make green Malachite, Salt, Carbon Dioxide and Water.
3CuCl2 + 6NaHCO3 ------> CuCO3:2Cu(OH)2 + 6NaCl + 5CO2 + 3H2O
If the weather is really cold, pale blue Azurite will form instead:
3CuCl2 + 6NaHCO3 -------->2CuCO3:Cu(OH)2 + 6NaCl + 4CO2 + 3H2O
If the mixture is subsequently warmed the Azurite will slowly decompose to Malachite and Carbon Dioxide, which will annoyingly bubble slowly out for ages and ages.
2CuCO3:Cu(OH)2 Azurite + H2O------->CuCO3:2Cu(OH)2 Malachite + CO2
So eventually you will add some NaHCO3 and instead of brown rust, you will get something pale green or light blue...this is the time to stop. Pour it all into a large plastic bucket and toss a couple of kettles of boiling water into it...this will bring the rust down quickly as a brown sludge at the bottom of the bucket...it will drive off any remaining CO2. The solution above the rust should now be blue, it will contain CuCl2 and NaCl.
Carefully decant the blue solution off and retain the rust in the bucket. Decant the blue solution into another bucket and add more NaHCO3 (until it stops fizzing) to it there to bring down insoluble green Malachite, (CuCO3:2Cu(OH)2. This will be the way the copper leaves the system. To both buckets add a few more kettles of boiling water...it will wash the rust in one bucket and bring down the Malachite in the other. When all has settled pour off as much of the liquid as possible...the rust bucket liquid will be mostly water with a tiny bit of CuCl2 and salt in it, the Malachite bucket will contain just salty water...so tip em down the sink. Wash the Malachite one more time with a kettle of boiling water and drain it off. Leave it to dry out, crush it up and store it in jars. Chemistry people will take it to make other copper compounds from.
The rust is your prize...it is Ferric Hydroxide. Like the Malachite, try to get it into a thick mud and get rid of as much of the water as possible...because the thicker and drier you get it, the more concentrated your new batch of Ferric Chloride will be.
1c: Dissolve the Rust in a minimum of concentrated HCl (Hydrochloric Acid or "Muriatic" Acid).
Fe(OH)3 + 2HCl ------->FeCl3 + 3H2O
This is your reconstituted echant ready to use again.
If you can procure rust directly, you can jump directly to this step...but you will still have to work up the old stuf as it is irresponsible to tip it down the sink.
Advantages of this method, Quite Fast, can be done in a few hours.
Disadvantages of this method, does not make any more FeCl3 than you stared with, generates Malachite you must store and lots of Carbon Dioxide which can froth things over of you are not careful.
The BiCarb Soda, NaHCO3 can be purchased at the Supermarket, but if you go to a Swimming Pool place it can be purchased a hell of a lot cheaper and you get a hell of a lot more.
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Ammonium Peroxydisulfate...that other "go to" etchant for electronics blokes can also be made on the cheap from ammonium sulfate (fertiliser) and dilute sulfuric acid from car batteries...but you do need an expensive platinum anode for the cell in which the peroxydisulfate ions are formed.
(NH4)2SO4 + H2SO4 -------> 2(NH4)HSO4
In a High Current Density Electrolytic cell with a Platinum Anode and Graphite Cathode:
2(NH4)HSO4 -----------> (NH4)2S2O8 + H2