May 15, 2005

Aggressive Cleaning 

Now, something completely different: how to clean stubborn dirt...

Today I tried to remove the limescale left by this hard London water on my kitchen's sink using a commercial cleaning fluid. I was only partially succesfull, given that the limescale grew through years of superficial celaning.

The best thing to remove limescale is to use diluted hydrochloric or nitric acid (say 5 - 10%), but there are serious drawbacks. First, those solutions surely aren't delicate on the skin. Second, they tend to corrode anything that isn't stainless steel or good quality ceramic (and plastic). Chrome-plated brass taps can be irremediably ruined byt this kind of acid cleaning. It's a shame, really, because you should see how diluted HCl eats away at limescale.

And how can you clean residues from the inside of alimentary plants? If foodstuff residues remain, bacteria and/or fungi will grow, and at least you'll have to discard the first batch of product when production restarts. In the tomato processing plant where I worked, they cleaned the sterile parts of the plants (of course, upstream the sterilizers bacterial contamination is a lesser problem) using first hot nitric acid, 30% in water and then 30% hot caustic soda. If you get hot soda solution on your hand, it may corrode its way through it before you can do anything about that. A pretty aggressive cleaning, but one must keep in mind that avoiding bacterial contamination is of paramount importance for canned foods. And the plants are all built with stainless steel. Alternatively, also a rahter diluted solution of hydrogen peroxide and peracetic acid in water can be used - or this goes after the acid/soda wash in order to perfect the cleaning. In those days, I was there only as a labourer, not a technician or supervisor.

If you have stubborn stains on your laboratory glassware, you can dissolve them using chromic mixture - a solution of chromic anhydride (CrO3) in concentrated sulphuric acid. It is an extremely strong oxidant, and it can remove basically any contamination. It was the preferred one in the alcyon days before environmental regulations - nowadays, people tend to use the toxic Cr(VI) as little as possible. You can substitute it with potassium permanganate (which is dark violet, and stains clothes like nothing else - maybe pure iodine), but permanganate and sulphuric acid tend to form the explosive and shock-sensitive compound Mn2O7.

The exhaust pipes of two-stroke engines tend to become clogged with oily and tarry residues, and thus they need periodical cleaning. No solvent is good enough for that stuff (which is horribly dirty - only thinking of cleaning your bike's exhaust will cause black, oily, smelly patches to appear on you hands), so the best way to eliminate it is to heat the whole exhaust using a gas torch (the kind used for roofing with tar), or in a wooden fire if you don't have anything better. Another method is to fill the exhaust with gasoline, drain the excess and then set it on fire. But this method is kinda dangerous: the gasoline vapours can explode... It is rumored that a guy tried it, and the vapours exploded: he found his exhaust in a field fifty meters away. But thoroughly cleaned, they say.


You know this much chemistry but don’t know the Carbon Carbon bonding is the strongest around? BTW thanks for destroying my dreams of the space elevator. Oh well. I like HCL because of the way it bubbles as it eats away of anything it comes in contact with.
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