Making Hydrochloric Acid from Household Ingredients (reposted)

I used to do this when I was young.  I’m uncertain now:  could it not be considered a terroristic threat?  The times, they are a changing!  Anyway, into the science.

Hydrochloric (HCl) acid is simply a solution of the gas HCl (hydrogen chloride) in water.  The basic acid-forming reaction is:

HCl + H₂O ® Cl^- + H₃O⁺

H₃O⁺ ↔ H⁺ + H₂O

The main acidic species can be considered either H₃O⁺  or H⁺ , although the latter is usually used as it is clearer and more consistent.  In almost (but not all) all water based acids, this is the actual acidic species, whatever the starting acid (nitric, sulfuric, acetic, etc.) is.

There are a number of industrial and lab process to make HCl acid, usually from other strong mineral acids.  Another way, however, is to generate HCl gas directly and dissolve it in water (it is highly soluble, almost as much as ammonia).  The household method uses this approach.  Questions:  how do you make HCl gas, and how to you get it into the water?

Warning, Warning!  HCl gas is very irritating and corrosive, so you have to set up some kind of protection for your lungs and throat and eyes before generating it!

At a young age, I loved to tinker with chemicals (perhaps not a good idea when I look back on it, but I was usually reasonably careful), both those I found in the house and those I got in chemistry sets.  And I loved to read chemistry books and ponder what might happen if you mixed such and such with so and so and heated them or dissolved them in water.  Amazingly, I still have all my body parts and they all work well, which might be something of a wonder.

In this case I noticed something.  It seemed as though if you mixed ordinary table salt and baking soda and heated them strongly, you might get the following reaction:

NaCl + NaHCO₃ ® HCl­ + Na₂CO₃

In which the two reactants swapped the hydrogen and chlorine,  Further, since HCl was a gas, it would escape the reaction mixture (the upward arrow) and constantly drive the reaction to the right.

Of course I had to try it.  Now, if I’d had a balance, I’d weigh out 5.85 grams of salt and 8.8 grams of baking soda.  This is one tength of a mole of each product, thus an equal number of molecules of each, perfect for the 1:1 reaction.  It would have yielded 3.85 grams of gaseous HCl ( and 9.4 grams of Na₂CO₃) .  The two weight combinations on either side of the arrow equal, as they should.  I did not have a balance however, and so used a teaspoon or tablespoon of both reactions – good enough.

Now here comes the part where you shouldn’t have done what I did.  I would mix both reactants in an Kimex glass laboratory grade Ehrlenmeyer flask (the triangular shaped one), place the flask on one of our electric stoves, and (at least have the sense to) gradually heat the flask until the stove temperature was at or near high.  I know that gaseous HCl was irritating and corrosive, so I would carefully smell for any gasses coming through the top of the flask.  Sure enough, I found myself tearing and coughing pretty soon, and I knew my hypothesis was a triumph.  The question now was, how to deliver the gas into (preferably cold) water?

You’ve already noticed that household ingredients aren’t quite enough, you also need some laboratory equipment, mostly glassware.  I had such from my chemistry sets:  Ehrlenmeyer flasks, beakers, corks/rubber stoppers that fitted the flask and had a hole large enough for the glass tubing, the tubing, and an alcohol burner I could use to bend the tubing from the top of the Ehrlenmeyer over to the beaker (more than a ninety degree angle) – not as easy as it might sound for glass work requires some practice and experience.  (You can no doubt still get these things, though I don’t know if you’ll attract unwanted attention doing so).

Let’s assume you have a desktop balance for weighing chemicals, though don’t ask me how much they cost; anyway, you don’t need a highly priced one.  Now, if you weigh amount of reactants in the flask as described above, you should generate 0.1 mole (8.8 grams) of HCl gas when you heat it strongly.  After pouring the reactants into the flask, next, assemble the apparatus. The stopper should fit tightly inside the Ehrlenmeyer, the bent glass tube pass through the stopper (not too far, though, just enough to pick up any gasses and deliver them!), and the other end of the tube should reach the bottom of the beaker, which should hold about 100 milliliters (~ 1/10 of a quart, or half a pint or so – use a graduated cylinder if you can) of cold water.  Now strongly heat the mixture in the flask.  What you’ll observe is curious.  First, a stream of bubbles will emerge from the beaker end of the tube, rising and escaping into the air.  Don’t be alarmed; this is just the heated air being forced from the flask through the tube.  What happens next is the main show.  The bubbles stop, and the gas level in the beaker stays pretty much flush with the water.  What is happening here is that HCl gas is now being generated rapidly and, being highly soluble in water, immediately dissolves when it hits it, leaving no more bubbles.  Your HCl acid is starting to form!

You should keep this reaction/process going until you observe the following.  As the reactants are consumed, the HCl is produced in smaller and smaller quantities; and, again because it is so soluble in water, begins to suck liquid up the tube from the beaker.  At this point you should stop the reaction (turn off the heat and move the flask off the stove, remove the flask + stopper + tube from the beaker, etc.).  You DO NOT want water pouring back through the tube into the Ehrlenmeyer under strong heat – I never tried this, but I assume the water will flash into steam, at least cracking if not exploding the Ehrlenmeyer, thereby releasing a lot of acid and HCl gas into the atmosphere, any probably other nasties I haven’t thought about.  All in all, don’t let this happen!

Let it all cool down for a while, before disassembling everything and thoroughly washing out everything but the beaker and its contents (use lots of water, on your hands too).  Now, if the reaction has gone to completion (though remember, some HCl is lost), I figure the concentration to be 0.1 mole HCl gas dissolving into 0.1 liter water, giving around a 1.0 molar (M) solution.  This is a fairly potent concentration (if you get it on yourself, wash thoroughly with water).  It’s more than enough to dissolve aluminum and tin foil, magnesiumzinc, probably lead and iron and some other metals, giving off streams of bubbles of hydrogen gas as it does so (this is also potentially hazardous, and hydrogen gas is highly flammable).  Remember mixing vinegar (a dilute solution of  acetic acid, CH₃CH₂COOH) with baking soda and watching it fizz up?

CH₃CH₂COOH + NaHCO₃ ® Na⁺ + CH₃CH₂COO^- + H₂O + CO₂

The CO₂, or carbon dioxide, is the gas that fizzes up, just as from a can of beer or soda.  If you substitute the weak and highly diluted acid vinegar with fairly concentrated hydrochloric acid, the reaction ought to be considerably stronger:

HCl + NaHCO₃ ® Na⁺ + Cl^- + H₂O + CO₂

Not that I remember trying this.  Oh, one more thing; I’m pretty sure that you can make the acid highly concentrated (though I don’t recommend this, however, as it is VERY HAZARDOUS at very high concentrations), simply by upping the amount of reactants.  Multiply the reactants by five or ten (you may have to run the reaction several times, or find a large enough Ehrlenmeyer flask), and you should get five-ten molar acid.  Again, something you really shouldn’t play around with, unless you know how to do so safely).