Descaling copper boilers--something NOT to do!!!

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#1: Post by rpavlis »

I just saw advice on cleaning copper that if anyone followed it with a copper boiler they would destroy the boiler!! The advice said to use "muriatic acid" to clean copper.

Muriatic acid is a Medieval name for hydrochloric acid that is used by people who know nothing about chemistry. Concentrated HCl in the presence of air is amazingly destructive to copper once the reaction gets established, especially at elevated temperature.

The chemistry is somewhat complex.

It is, in fact, a bad idea to attempt to clean copper with any strong mineral acids. Nitric acid reacts almost violently with copper, as any student of general chemistry knows.

My advice is to use nothing but dilute acetic or citric acid for this purpose.

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#2: Post by kowalej »

Good tip, I've always heard citric acid is the best for descaling. Never heard someone suggest muriatic acid though.

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#3: Post by chopinhauer »

kowalej wrote:I've always heard citric acid is the best for descaling
I've always successfully descaled my old Europiccola and Cremina with diluted citric acid. I think both have brass boilers. What about machines with stainless steel boilers, such as the post-2002 Creminas? Also citric acid?
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#4: Post by gor »

Hydrochloric acid does not react with copper! All it will do to your boiler is remove the varnish.
This info is easily obtained on the net and is backed up by the chemistry in countless reference books and texts.
It certainly works fast when descaling!
Note: Concentrated HCl will react, but who can get their hands on that sort of stuff?

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rpavlis (original poster)

#5: Post by rpavlis (original poster) »

Copper does not react with hydrochloric acid when there is no air around, or when there are no salts of copper already present. However, there is a vicious auto catalytic reaction that will occur as Cu salts build up.

Cu2+ + Cu -> Cu1+

Cu1+ + O2 -> Cu2+

These two reactions repeat over and over until the copper is completely reacted. There are strong complexes between both Cu1+ and Cu2+ with chloride ion that make oxidation of the Cu1+ by O2 much more favourable.

From the chloride present the copper gets into a peculiar tetrameric structure (and some higher structures), it is not uncomplexed copper, but I cannot write the full structures here.

When I worked for a copper company whilst on Sabbatical leave we always made CuCl by dissolving copper in HCl! We took concentrated HCl, added some CuCl2, and copper, and heated the mixture until the copper dissolved. Then we added more copper turnings and put it in an air free environment and heated it. This produced our snow white CuCl which separated on cooling.

HCl concentrated enough to dissolve Cu readily is often sold in grocery stores and hardware stores often under the Mediaeval name muriatic acid, or as material to clean sinks.

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#6: Post by TomC »

I think I'll side with the chemist on this one :)

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#7: Post by sweaner »

Here is an interesting video. Copper does not seem to do much. Look at 2:20.
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#8: Post by donn »

TomC wrote:I think I'll side with the chemist on this one
I can't tell who's a chemist here and who isn't, but ... if there's anyone who's cleaned a copper boiler with muriatic acid, that's who I'd go with. I've got an old jug of the stuff around here somewhere, I'd chuck a penny in it for grins, but there's sure to be some explanation why that isn't a valid demonstration.

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rpavlis (original poster)

#9: Post by rpavlis (original poster) »

You can put pieces of copper into concentrated HCl for years and nothing is going to happen in the absence of oxygen. In the presence of oxygen and copper salts things begin to happen. Copper boilers quickly develop black or dark reddish coatings of copper oxides. When strong mineral acids are added these protective coatings dissolve with the oxides forming soluble copper salt solutions. (This is why it is a bad idea to use ANY strong acid routinely in a copper boiler.)

The danger to boilers with strong HCl is that not only is the oxide coat removed, the chloride complexed cupric ion is in the solution. It reacts rapidly with the bare Cu and each cupric ion picks off a Cu atom and becomes reduced to Cu1+, cuprous, and the Cu from the wall becomes oxidised to Cu1+ at the same time.

These Cu1+ ions react at a diffusion controlled rate with any oxygen in the system to oxidise both cuprous ions to Cu2+. Each of these goes off and reacts with the metal, so now we have four Cu1+. These oxidise and we have four Cu2+. They now go and react with 4 Cu metal atoms.

The process will continue and accelerate as more and more copper ions are formed. The solution will be dark brown during the serious part of the reaction, because there are present complex ions with both cuprous and cupric ions in them that are brown. Cuprous chloride and its complexes in solution are white.

We often have need of CuCl for laboratory purposes. The commercial material almost always is partly oxidised by the time it arrives. The best way to make it is to put some copper turnings in concentrated HCl and add a bit of previously made CuCl. One can heat it and bubble in air until the Cu is dissolved, then add more copper, and seal up the container (with some provision to release the pressure) so air does not reach it and heat. The solution turns dark brown, and then clear. On cooling crystals of CuCl form.

Thus there is no problem if one puts strong hydrochloric acid into a boiler if in a vacuum chamber or inert gas chamber.

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#10: Post by iginfect »

I have a BS in chemistry and was a TA after at Cal tech for 6 months 36 years ago and am working in another field with a doctorate. I'd say Robert R. Pavlis is right on the money.