Cleaning precious metals - dubious method

[jason_recliner]

I've seen this posted on several friends' Facebook pages recently and have been giving it some thought. I'm interested in what's going on.

NEVER BUY JEWELLERY CLEANER AGAIN!
1 tablespoon salt
1 tablespoon baking soda
1 tablespoon dish detergent
1 cup water
1 piece aluminium foil

Directions:
1. Heat water in the microwave for 1 or 2 minutes.
2. Cut a piece of aluminium foil that roughly covers the bottom of a small bowl (like a cereal bowl).
3. Pour hot water into bowl. Place salt, soda, and dish-washing liquid into bowl. Place jewellery on top of foil and let it sit for 5 to 10 minutes. Rinse jewellery in cool water and dry jewellery completely with soft cloth. Discard solution after use and make a new batch next time.
4. According to wire-sculpture.com, "this works well for gold-filled, brass, German (nickel) silver, and sterling silver. I have even cleaned jewellery with freshwater pearls, shell cameos and mother of pearl with no problem.".

As I look at it, I think the process is to generate weak HCl. I'm trying to follow the chemistry. Numbers are not exact. I'd welcome any comments on my assumptions and errors.

NaHCO3 + NaCl -> Na2CO3 + HCl.

Straight HCl won't attack silver or gold, but while there is still a reaction taking place there may be free chlorine in solution for a short time. This can allow a small amount of gold at least (I'm not so sure about silver yet) to be dissolved. Therefore one should not do this on plated items.

Eventually any free HCl will be consumed up by the aluminium foil.
In hot water, any free chlorine would be released pretty quickly as toxic gas.
Any PMs that happened to dissolve would be cemented onto the foil as a black or brown powder. This gives a wonderful feeling of "ooh, look how much dirt came off", though it's probably partly your precious metal as well as dirt and sulphate.
It looks like way too much detergent.

Only I think you also wouldn't want 1:1 bicarb/salt by volume, but 84:58 by weight (according to their molar masses). I'm working on a tablespoon being roughly 15 grams. It's a bit hard to say. Their densities I'm calling as close enough (in my industry, pi = 3).

This would be about 0.18 mol of NaHCO3, 0.25 mol of NaCl. Because they're unbalanced, only 0.18 mol will convert to HCl and the rest of the salt will stay salt. In 13.88 mol (250g) of water, 0.18 mol would give about 1.29% strength HCl. ??

How did I do?

I also wonder if a boil in water (or maybe weak soda) afterwards would be a good idea to remove any leftover HCl, which might react with sulfates in your skin and turn silver black again quickly.

 

[g_axelsson]

Think electrolytic cell, or else the jewellery doesn't need to sit on top of the aluminum foil.

Göran

 

[jason_recliner]

The forum needs an icon for gobsmacked, vacant stare.
You've thrown me, Göran. I've just spent the last half hour trying to find any reference to an electrolytic cell without applied current. And apart from seeing Billy's internet dating picture in his party dress again, I'm no better off.

 

[Shark]

I know I can't explain this very well, but here goes.

Metals in salt water will produce varying amounts of electrical current. By using certain metals you can produce a narrow range of current, ( amps, I think it is). The way I learned this, is not for the use in a cell so I cannot translate over very well. Sharks have receptors in the nose area that is sensitive to electrical currents. It is used in their search for food. Like wise the wrong metal (electricity) can repel them. It is one of the main reasons for the discussions (elsewhere) of using an insulating coating on hooks and metal leaders when fishing for sharks. Some believed the wrong metal would repel the targeted fish, (sharks). The University of Hawaii did some studies years ago, including video footage, showing the reactions of the scalloped hammerhead in tanks with very small amounts of current in specific spots. Likewise they seemed to avoid certain spots until an insulating coating was applied to exposed metal in the holding tanks. Their conclusion was the receptors in the sharks nose was sensitive enough to detect electrical current produced by metal being exposed to salt water, as well as the electrical current that the food source needed to sustain life. So basically, the items mentioned, are forming a type of battery/cell.

 

[butcher]

A battery (or multiple cells), or galvanic cell basically two dissimilar metals in an acid or alkaline solution.
This can be as simple as an zinc wire and a copper wire stuck inside an acidic lemon.

Completing the circuit with an external circuit, where current can flow.
One metal is oxidized (anode), giving up electrons and goes into solution, and at the cathode which becomes the place of reduction as electrons are gained.

 

[lanfear]

In Breaking bad season 1, the chem professor (heisenberg) made electricity with various metals divided in different cells. I don't know if it was feasible, but I would think the theory of it is correct.


Jon

 

[g_axelsson]

Two different metals in an electrolyte will form a battery, if the metals is connected with a conductor a current will flow. In this case we have aluminum and some alloy in the jewellery. As the jewellery and aluminum is touching a current will flow, dissolving (oxidizing) the aluminum and reducing the oxides on the jewellery.

The same effect is used when you put sacrificial anodes on a ship, the more reactive metal is dissolved while the less reactive is protected.

Göran

 

[nickvc]

There is an old method for cleaning silver that uses bicarbonate of soda, hot water and a piece of aluminium foil it works very well.

 

[rickbb]

That's how my mom always cleaned the silverware because it is just plated silver and she didn't want to rub it off with polishing cloths.

 

[jason_recliner]

Just when you think you're getting a handle on the basics... :lol:

 

[Lou]

Really quick way to clean sterling silverware (NOT weighted things like knife handles!) is to put them in the oven on clean.

Ag2S + exces O2 --> AgO--> Ag +SO2

 

[geedigity]

I think the way this works for the gold and silver is as follows:

Soil, grease, dried food, etc on gold + detergent + hot water = clean gold

Silver sulfide (Ag2S) is the tarnish on the silver, so Ag in direct contact with the Al in the presence of an electrolyte (NaCl and NaHCO3) creates the reaction where the Al (more reactive) displaces the Ag from the Ag2S, and plates itself back onto the Ag surface. Heat just makes the reaction occur quicker. The detergent is not required for the reaction, but may be there to help clean away greases and possibly the sulfide?

So basically: 3Ag(+) + Al => 3Ag + Al(3+)

Not sure about the brass, but it would seem that copper in the alloy would be involved with a displacement reaction similar as above, but the tarnish on brass is more than likely a mixture of compounds. Not sure about how this would work on yellow brass verses red brass and the various other metals present in them.