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Clean Nickel Silver

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Enthalpy:
The hot solution of bicarbonate and some carbonate does nothing to the scale.

I put aluminium foil in the solution, it fizzled without any nickel silver. A small tube of aluminium foil was to release more concentrated gas. With a lighter, no explosion. Maybe the gas burns faintly with a purple flame resembling hydrogen, but the amount is too small for clear observation, and the lighter's propane could also have reignited further away. Near to the solution, in the vicinity of aluminium, something extinguishes the lighter, while this doesn't happpen clearly away from the fizzle. Vapour? Dioxide?

Heat is paramount to obtain bubbles, and the aluminium foil is inactivated once corroded. A more corrodable alloy could improve.

I let an aluminium foil react for some time in the solution, then I stirred and removed the foil. The new solution did nothing or very little to the scale.

I put a new foil at one side of the solution, and the scaled nickel silver part at the other side, and nothing happened, or very little.

The I put the foil at the bath's bottom. In the air over the bubbles, nothing happened to the scale. But the bubbles in the solution descale nickel silver instantly and very efficiently. Contact with the aluminium changes nothing, neither for the removal of scale, nor at the amount of bubbles.

The composition of the magic bubbles remains obscure to me. Aluminium may react with the bicarbonate, carbonate, or hydroxide ions, and with water. The bubbles could contain hydrogen, carbon dioxide, monoxide, or some reaction product with water, or something else. Suggestions?

Marc Schaefer, aka Enthalpy

Enthalpy:
It works with sodium hydroxide instead of bicarbonate. So I believe the cleaning gas is hydrogen, or at least its useful component is, without any carbon compound. Opinions welcome!

The hydroxide is sold to clean drains. Starting from pH~12 to make beetroot juice yellow according to Wiki, I estimated 0.4g/L NaOH, but the beetroot juice was a bit orange. 1g/L gave true yellow as for the boiled bicarbonate solution. Heat didn't change the colour.

The cold alkali did nothing to aluminium nor nickel silver. When boiling hot, it left nickel silver nearly untouched, but it did make bubbles from an aluminium foil, and the bubbles cleaned the nickel silver. The aluminium foil corroded to brown as with bicarbonate. Here too, cooling down stops the reaction soon, and corroded aluminium slows it down.

Cleaning seemed less impressive with hydroxide, so I tried bicarbonate on the same joint end (left on the picture), which didn't clean further. The nickel silver part is more dirty there, including from rests of adhesive, which supposedly prevents cleaning.

Marc Schaefer, aka Enthalpy

pcm81:
Hydroxide part
Few years ago i played with black iron oxide bath, which is a boiling bath of 2lb sodium nitrate ad 5lb sodium hydroxide per gallon of water. This hot solution will bubble very violently when aluminum is added.I suspect you were seeing a similar chemistry play out in your bath but on a much smaller / slower scale. Hot hydroxide can dissolve metal, so i suspect your flute got cleaned as the result of hydroxide dissolving everything, including top layer of metal.

Aluminum + baking soda part
This may be of interest: https://en.wikipedia.org/wiki/Carbon_dioxide_cleaning
I think in your case the aluminum foil and baking soda produce CO2, which is what is cleaning the flute.

Enthalpy:
Hi pcm81, thanks for your interest!

To my understanding, CO2 ice cleans by mechanical processes:

* The impact of the solid, where CO2 advantageously leaves no solid nor liquid residue;
* the explosion of CO2 ice in contact with the warmer surface.
CO2 as bubbles or solution doesn't bring these effects. Nor do I see what cleaning reaction could occur with the corroded metal.

Consistently, warmed bicarbonate alone releases CO2 but doesn't clean. It needs the aluminium foil, which releases hydrogen rather than CO2.

pcm81:

--- Quote from: Enthalpy on September 01, 2020, 08:09:22 AM ---Hi pcm81, thanks for your interest!

To my understanding, CO2 ice cleans by mechanical processes:

* The impact of the solid, where CO2 advantageously leaves no solid nor liquid residue;
* the explosion of CO2 ice in contact with the warmer surface.
CO2 as bubbles or solution doesn't bring these effects. Nor do I see what cleaning reaction could occur with the corroded metal.

Consistently, warmed bicarbonate alone releases CO2 but doesn't clean. It needs the aluminium foil, which releases hydrogen rather than CO2.
--- End quote ---

So, what you saying is that Al + Sobium bicarb gives off H2. Which then probably oxidizes silver and other metals to make the relevant salt soluble in water and cleaning is achieved. Silver bicarb is 15 more soluble in water at 100*C than at 15*c, so that kinda makes sense that you see better result in hot bath...

Sounds to me you are just replicating results of using a weak acid to clean metal oxides.
Personally, I'd just use vinegar.

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