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Author: Subject: Explaining the Cleaning of Silver with Salt, NaHCO3 (or Na2CO3) and Aluminum Foil
AJKOER
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[*] posted on 5-11-2014 at 17:04
Explaining the Cleaning of Silver with Salt, NaHCO3 (or Na2CO3) and Aluminum Foil


So what is the precise mechanics of using NaCl, hot aqueous NaHCO3 and Aluminum foil in removing Silver tarnish?

Here is a typical reference http://www.madsci.org/posts/archives/2002-12/1041253726.Ch.r... , which, to quote, simply presents the net reaction:

"3Ag2S + 2Al --> 6Ag + 2Al+3 + 3S- 2

or including the water

3Ag2S + 2Al + 3H2O --> 6Ag +Al2O3 + 3H2S"

This source also claims the purpose of the baking soda is "reacts with water to make a basic solution--one having more OH- ions.

NaHCO3 (baking soda) + H2O --> H2CO3 + Na+ + OH-"

If this is, indeed, the pupose of the warm NaHCO3, then Na2CO3 should be even better and this seems to be verified by this comment, to quote:

"After several tries, I found that it is WASHING SODA (SODIUM CARBONATE) which is very effective in removing silver tarnish.
My experimentation showed that Baking Soda (SODIUM BICARBONATE) was, at best, only minimally effective, while Washing Soda was dramatically effective within a very short time (two minutes) with associated aluminum discoloration. "

which can be found at http://www.creativekidsathome.com/activities/science_experim... .

Another comment on the whole process presented at http://www.finishing.com/145/02.shtml cites the same net equation, but also introduces sodium hydrosulfide, to quote:

"Aluminum has a lower ionization energy (energy required to remove electrons from an atom of the element) than silver. As a result, aluminum is oxidized (loses electrons and oxidation number increases), and silver is reduced (gains electrons and oxidation number is reduced). Depending on the amount of tarnish, the silver will be bright and the aluminum foil may be brown with tarnish (aluminum oxide), in a short while. The silver tarnish is "transferred" to the aluminum via reactions, which occur instantaneously, as follows:

3Ag2S(s) + 2Al(s)+ 3H2O(l) => 6Ag(s) + Al2O3(s) + 3H2S(aq)

silver sulfide + aluminum + water => silver + aluminum oxide + hydrogen sulfide
(* Note, this reaction can be done without the baking soda, but it takes longer to see results).
The baking soda (sodium bicarbonate) reacts with the (sulfur-smelling) H2S:

3 NaHCO3(aq) + 3 H2S(aq) => 3 NaHS(aq)+3 H2O(l)+ 3 CO2(g)

baking soda + hydrogen sulfide => sodium hydrosulfide + water + carbon dioxide. The CO2 gas can be observed escaping from the most tarnished parts of the silver.
The silver and aluminum must be in contact with each other because a small electric current flows between them during the reaction.This type of reaction, which involves an electric current (because atoms are charged), is called an electrochemical reaction, and is used in batteries to produce electricity."

This last sentence is, of course, not entirely correct, but the electrochemical reference explains the use of salt to serve as a good electrolyte. Here is an educational reference (of hopefully higher repute) presenting half reactions (see http://lecturedemos.chem.umass.edu/electrochemistry19_4.html ) albeit with the same net reaction:

OXIDATION: 2 Al(s) + 6 OH– (aq) –––> Al2O3(s) + 3 H2O (l) + 6 e–

REDUCTION: Ag2S(s) + 2 H2O (l) + 2 e– –––> 2 Ag(s) + H2S (aq) + 2 OH– (aq)

3 Ag2S(s) + 2 Al(s) + 3 H2O (l) –––> 6 Ag(s) + 3 H2S (aq) + Al2O3(s)

Interestingly, per Wikipedia on H2S (http://en.m.wikipedia.org/wiki/Hydrogen_sulfide ), to quote:

"The sulfide dianion S2− exists only in strongly alkaline aqueous solutions; it is exceptionally basic with a pKa > 14."

And, we have on the hydrolysis of a sulfide:

6 H2O + Al2S3 → 3 H2S + 2 Al(OH)3

Also, perhaps some valuable background on the related Silver oxide/Zinc battery, to quote from Wikipedia (see http://en.m.wikipedia.org/wiki/Silver-oxide_battery ):

"A silver-oxide battery uses silver oxide as the positive electrode (cathode), zinc as the negative electrode (anode) plus an alkaline electrolyte, usually sodium hydroxide (NaOH) or potassium hydroxide (KOH). The silver is reduced at the cathode from Ag(I) to Ag and the zinc is oxidized from Zn to Zn(II). The chemical reaction that takes place inside the battery is the following:

Zn + Ag2O --KOH/NaOH→ ZnO + 2 Ag

....During the charging process, silver is first oxidized to silver(I) oxide: 2Ag(s) + 2OH– → Ag2O + H2O + 2e– and then to silver(II) oxide: Ag2O + 2OH– → 2AgO + H2O + 2e–, while the zinc oxide is reduced to metallic zinc: 2Zn(OH)2 + 4e– = 2Zn + 4OH–. The process is continued until the cell potential reaches a level where the decomposition of the electrolyte is possible at about 1.55 Volts."

From which I surmise that the half reactions are:

Ag2O + H2O + 2e– → 2Ag(s) + 2OH-

Zn + 2OH– = Zn(OH)2 + 2e–

For a net reaction of:

Ag2O + H2O + Zn --KOH/NaOH→ 2Ag(s) + Zn(OH)2
--------------------------------

Based on all of the above and the unlikely nature of the creation of sulfide dianion S2− under only mild alkaline conditions and the likely reason for the addition of the hot alkaline Na2CO3 to promote the silver cleaning, I would suggest for the electrochemical part of the reaction only involving salt, Ag2S and aluminum foil, the following half reactions:

3 Ag2S + 3 H2O + 6 e– –––> 6 Ag(s) + 3 OH- + 3 HS-

Al(s) + 3 OH– (aq) –––> Al(OH)3 + 3 e–

Al(s) + 3 HS– (aq) –––> Al(HS)3 + 3 e–

For a proposed net electrochemical part of the Silver cleaning of:

3 Ag2S (s) + 3 H2O + 2 Al = 6 Ag(s) + Al(OH)3 + Al(HS)3

Note, per this source (see http://www.allreactions.com/index.php/group-1a/natrium/sodiu... ), to quote:

"NaHS (solution) = Na2S + H2S↑ (boiling)"

absence the addition of a basic salt, strong boiling may still be a able to effect the removal of some tarnish. But, in the presence of say heated aqueous Na2CO3, we could increased OH- for the non-electrochemical based (or standard chemical reaction) part of the cleaning operation via:

3/2 Na2CO3 + 3 H2O = 3 NaOH + 3/2 H2O + 3/2 CO2 (g)

(note, the above reaction is just not theoretical as I have dissolved Al foil by boiling it in a concentrated solution of Washing Soda.)

Further, we have the reaction:

Al(HS)3 + 3 NaOH = Al(OH)3 + 3 NaHS

and, in the presence of hot/boiling water again:

3 NaHS + 3 H2O –––> 3 NaOH + 3 H2S (g)

( [Edit] Or, a possible amphoteric action by Al(OH)3, itself, as per the last reference given the cited reaction, NaHS + НСl (diluted) = NaCl + H2S↑, the possible reaction:

2 NaHS + 2 Н3AlO3 = 2 NaAlO2 + 2 H2S↑ + 2 H2O )

where the formation of gases may promote my proposed overall net reaction of:

3 Ag2S + 7.5 H2O + 2 Al + 1.5 Na2CO3 –Boiling–> 6 Ag(s) + 3 NaOH + 2 Al(OH)3 + 3/2 CO2(g) + 3 H2S (g)

[Edit] With no boiling, possible net overall reaction:

3 Ag2S + 2.5 H2O + 2 Al + 1.5 Na2CO3 ––> 6 Ag(s) + NaHS + 2 NaAlO2 + 3/2 CO2(g) + 2 H2S (g)

where both overall reactions are basically the same assuming also the formation of an aluminate and a hydrosulfide.

Clearly, this presentation is not pretty or simple, but it may do a better job at explaining the very presence and roles of hot Na2CO3 (or NaHCO3), NaCl and Aluminum in promoting the removal of the tarnish from the Silver and the observed formation of the gases CO2 and H2S.

[Edited on 6-11-2014 by AJKOER]
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AJKOER
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[*] posted on 7-11-2014 at 09:05


On my discourse on the chemistry/electrochemistry of cleaning Silver above, I should have also mentioned that my proposed modified electrochemical reaction has the following favorable points per some of the observations cited during actual Silver tarnish removal over the widely promulgated single electrochemical equation answer (although my implied overall net reaction would be in basic agreement, assuming heating/boiling to breakdown the hydrosulfide, without the addition of a basic salt to expedite the reaction into a more workable cleaning exercise):

1. Explains why the reaction proceeds better with added NaCl being, in part, electrochemical in nature, with the salt serving as an effective electrolyte. Salt apparently also interacts well with the Aluminum oxide coating on the underlying Aluminum.

2. Explains why the reaction nevertheless proceeds, but apparently much more slowly, absent a basic salt.

3. Explains why the more basic Washing Soda (Na2CO3) is more effective than Baking Soda (NaHCO3).

4. Explains the positive effect of heating on driving the overall reaction with the liberation of CO2 and H2S gases.

[Edited on 7-11-2014 by AJKOER]
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[*] posted on 8-11-2014 at 14:01


Far more effective as a silver tarnish remover is thiourea in dilute H2SO4. That's what most commercial silver tarnish remover dipping solutions are based on and they work in seconds, even on heavily tarnished items, mostly without any rubbing.



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[*] posted on 8-11-2014 at 15:59


That is quite a bit more expensive, though.



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[*] posted on 9-11-2014 at 06:03


Quote: Originally posted by blogfast25  
Far more effective as a silver tarnish remover is thiourea in dilute H2SO4. That's what most commercial silver tarnish remover dipping solutions are based on and they work in seconds, even on heavily tarnished items, mostly without any rubbing.


The Al foil and baking soda method isn't exactly slow or ineffective..
http://www.youtube.com/watch?v=vGCx9HZwYBo
it looks like about 5 seconds to me, but I didn't time it exactly.
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[*] posted on 9-11-2014 at 06:28


Quote: Originally posted by unionised  
The Al foil and baking soda method isn't exactly slow or ineffective..
http://www.youtube.com/watch?v=vGCx9HZwYBo
it looks like about 5 seconds to me, but I didn't time it exactly.


Yep. That was remarkably quick.

I guess commercial solutions (thiourea/H2SO4) have the advantage that only dipping is needed, no aluminium foil.

Quote: Originally posted by TheChemiKid  
That is quite a bit more expensive, though.


Dipping solutions can be used over and over again, almost at infinitum. Cost is thus almost zero.

[Edited on 9-11-2014 by blogfast25]




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[*] posted on 2-8-2021 at 12:23


I'm researching this reaction for an upcoming video, and am having a hard time finding the exact reaction of the baking soda reaction. I know it reacts with and disrupts the aluminum's passivation layer, but what exactly is the equation? I was able to balance this one out:
Al2O3 + 2 NaHCO3 + 3 H2O == 2 NaAl(OH)4 + 2 CO2

But just because an equation balances doesn't mean it's actually happening. Any ideas or citations anyone has on this reaction?


The full tarnish-removal process:
1. Baking soda disrupts passivation layer: [?]
2. Bare aluminum reacts with silver sulfide: 3 Ag2S + 2 Al == 6 Ag + Al2S3
And then a few side reactions to produce gaseous products:
3. Baking soda decomposition in hot solution: 2 NaHCO3 == Na2CO3 + H2O + CO2
4. Aluminum sulfide reaction with water: Al2S3 + 6 H2O == 2 Al(OH)3 + 3 H2S
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