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Name: Richard
Status:
Grade: other
Location: WI
Date: July 2008


Question:
We poured vinegar into our coffee maker to clean and a clear gel formed on the top of the stainer. The coffee maker was on when the vinegar was poured in and it was of the white distilled type (5%) and not diluted at all. Any idea what could have caused this reaction? The water supply we use is filtered through reverse osmosis from the Milwaukee, WI water supply.



Replies:
Richard,

Vinegar is a dilute solution (in your case 5%) of acetic acid and water. Some of the acetic acid breaks into ions (negatively charged anions, acetate -- C2H3O2 - and positively charged cations, hydrogen, H+, or sometimes called hydronium, H3O+). Your Milwaukee water (presumably from Lake Michigan) contains dissolved ions that cause the water to be called "hard water." Your coffee make probably had a layer of these ions dried onto its surface. The hot vinegar (really the acetate ions) reacted with some of the dried ions to form gel-like products. Most likely, magnesium (Mg 2+) and calcium (Ca 2+) ions reacted with the acetate to form Mg(C2H3O2)2 and Ca(C2H3O2)2.

Warren Young


Is your coffee maker contain any aluminum? The vinegar (acetic acid) could attack the aluminum.

Vince Calder


Richard -

Not sure, but it is possible the gel is a very low-density hydrated form of aluminum oxide. I think that your having used only purified water (having no dissolved silicates) makes that more likely. Aluminum silicates are less soluble than hydrated aluminum oxide. Aluminum in pure water tends to self-protect by forming a soft layer of aluminum hydroxide that gradually densifies and crystalizes into a harder layer of aluminum oxide. But the process is not extremely sure, and the water in the aluminum percolating-pipe is hot, which makes resisting corrosion tougher. Relatively likely to build up a thick soft layer and keep it.

Most people, using tap water with maybe >50ppm's dissolved alkali silicates, probably build a quicker thinner denser layer of aluminum silicates, which can be thought of as mixed aluminum oxide and silicon dioxide, certainly less soluble than aluminum oxide alone.

When you added vinegar it promptly dissolved all of your thick, loose, pure-alumina build-up and maybe eroded some freshly exposed aluminum metal too, and carried it to the filter as aluminum acetate in water. There it moved slower and was not sealed in, so acetic acid evaporated away, leaving aluminum oxides and hydroxides, which can easily deposit from water in a gel-like form. It's even conceivable to make aluminum metal-organic substances that way, non-ionic compounds joining aluminum with alcohol or ketone, which are clear liquids that readily make "sol-gel" oxides when exposed to air. Sol-gels are a branch of chemistry useful for electronic and optical stuff.

If I recall correctly, diluting the vinegar is usually recommended. Sorry, I forget whether 1:1 or 1:10... Probably better not to do this cleaning too often.

Might help to run some actual hard water through the machine: when new, and immediately after this acid cleaning, and occasional other times. You might drink a little less dissolved aluminum in the long-run that way.

Just as an example of the principle, if you read the label on a jug of automotive anti-freeze, you will probably find one of the listed ingredients is a silicate. It is used as a corrosion inhibitor for aluminum in the hot-water environment of the radiator and engine block. Maybe it helps out iron a little too, I do not know. (Of course anti-freeze has poisonous ethylene glycol, do not ever consider putting that in your coffee-maker.)

If you have a water-softener, that usually adds chloride ion to the water. Small amounts of chloride make aluminum even less likely to self-protect with a thin oxide layer, more likely to shed gel. Because of the sodium chloride generally used, Ion-exchange household water-softening is distinctly different than reverse osmosis, as regards corrosion.

Jim Swenson



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