11

We recently bought a home and found about 2 litres of hydrochloric acid 20% (also known as muriatic acid) that was used to manage acidity in a swimming pool that no longer exists. I would like to dispose of it but our local recycling centre is unfortunately not accepting this substance.

Is there a safe way to dispose of it? I have done limited research and here are the 3 ways I could think of:

  1. Diluting the acid in water and dispose in a sewer when pH is above something like 5-6. I have testing bands so I can ensure that the water solution is not too acid.
  2. Neutralize with a basis, like baking soda or garden lime. Basically, it's similar to solution 1 except that it uses less water but produces dangerous gaseous chlorine as a side product.
  3. Use a solid metal (like iron or aluminum) to produce a precipitate and gaseous hydrogen. This might be safer (no toxic chlorine is produced as gas) but I have the problem of getting rid of the precipitate. I am not sure if that's simpler.

I am not a chemistry expert but I do remember some basic concepts. I can find online that the pH of HCl is 0 for a concentration of 3.647%. From there, I compute that the theoretical pH of my solution is -0.74. This means that, to reach a pH of 5, I believe that I need to dilute by solution by more than 500'000 times, assuming that the pH of the water in our pipes is around 7 (which is the case). That means that I would need to use 1 million litres of water, which is insane!

That leaves me with the last 2 solutions. Is it reasonable to consider using a metal to form a precipitate or is really the only option to use a basis? Also, what amount of garden lime or baking soda should I expect to need to bring my pH to around 5?

PC1
  • 221
  • 1
  • 3
  • 8
  • 7
    Concerning your second point: Why do you think you will get chlorine when neutralizing the acid? Baking soda and hydrochloric acid will give you a sodium chloride solution. – Snijderfrey Jan 23 '23 at 19:13
  • 1
    That's a fair question. The advice I got from my recycling centre was: "While wearing protective gear for your eyes and mouth, slowly add the acid to the bucket, keeping your face away while pouring. Caution that the reaction of lime and acid can release chlorine gas and/or hydrogen gas, so wearing a chlorine gas respirator is recommended (or at the least do this process in a well-ventilated area and keep the air moving away from you)." So I assumed that it could release some chlorine gas. – PC1 Jan 23 '23 at 19:31
  • That being said, if I only use sodium bicarbonate only, should I expect to have (almost) no chlorine gas production? The advice seems to be related to garden lime specifically. – PC1 Jan 23 '23 at 19:32
  • 9
    NO ! No chlorine ! Adding lime or chalk or baking soda will neutralize hydrochloric acid and produce some sodium chloride or calcium chloride. But it will not produce any chlorine gas. No ! – Maurice Jan 23 '23 at 19:59
  • OK this is reassuring then! I was worried when I read the advice to wear a "chlorine gas respirator". In any case, I plan to neutralize the substance outside (in open air) and measure the solution acidity (with test papers) before releasing it in a sewer. – PC1 Jan 23 '23 at 20:02
  • 1
    Pour 200 ml in your toilet and flush, 10 times. – Karl Jan 23 '23 at 21:47
  • 1
    Outdoors or well-ventilated area is a good idea just on general principle, and any other chemicals present might do something. Maybe wait for next spring if it's cold out, and you can't find anywhere to give it away. Protective gear is a good idea because the reaction could be energetic and fling drops of acid around. The chlorine gas respirator advice makes sense if the person giving it isn't sure about the chemistry and already had easy access to a respirator; better to wear a respirator you didn't need than to breathe poison gas! Chlorine likes to form compounds, hard to free it. – Peter Cordes Jan 24 '23 at 05:37
  • Oh, but as Robert points out, there can be HCL gas, some of which will escape instead of further reacting. So that might be a reason for a respirator if you're indoors or can't keep your distance upwind. – Peter Cordes Jan 24 '23 at 05:41
  • Yes I plan to neutralize it outdoor. I'll do small quantities at once so it doesn't get out of control! Will try to find a respirator too, just in case. – PC1 Jan 24 '23 at 05:57
  • 2
    Give it to someone who has a pool and can use it. – jimchmst Jan 24 '23 at 20:57
  • Just pour it in sewerage, don't neutralize with soda to avoid drops of HCl everywhere. – sa7 Jan 26 '23 at 22:47

4 Answers4

12

Thankfully, 2 liters is not alot. Muriatic acid is used to clean bricks and concrete. A local mason or contractor might happily take it for free.

But, if you love a good science experiment, start by being safe with googles and have a baking soda solution ready in case there is a spill. Use a large plastic tub for containment and you are ready to work. Make it outdoors or in a well ventilated area. Outdoors is far better.

Start by diluting a portion of the 20% muriatic 1 to 10 with water.
Add acid to water. Concerns about heat evolution are well founded. You won't get Cl2 gas, but you will get HCl gas during neutralization, which is only slightly less worse if inhaled. The chemistry is fairly simple:

$$\ce{NaHCO3 84g + HCl 36 g -> NaCl + H2O + CO2 fizz}$$

You can make a dilute baking soda solution as well and add slowly with stirring. Just keep adding until it no longer fizzles, then test with pH paper. Repeat.

Robert DiGiovanni
  • 1,116
  • 2
  • 10
  • 5
    Chemical equations contain atomic symbols and molecular or ionic formula. This is all. Please don't introduce masses in the equation ! – Maurice Jan 24 '23 at 15:56
  • 2
    I understand your intention but "fizz" is not a state of matter. You can denote with "^" to indicate gas. – Nilay Ghosh Jan 24 '23 at 16:10
7

$1$ liter $\ce{HCl}$ $20$% weighs $1.101$ kg, and contains $0.20 · 1101$ g/($36.45$ g/mol) = $6.041$ mol $\ce{HCl}$. $2$ liters $\ce{HCl}$ $20$%, contain $12.082$ mol $\ce{HCl}$. To neutralize this acid with baking soda ($\ce{NaHCO3}$), you will need $12.082$ mol baking soda, which weighs $12.082 · 84.02 g = 1015$ g $\ce{NaHCO3}$. To neutralise the same amount of acid with lime (impure $\ce{CaCO3}$), you will need $6.041$ mol $\ce{CaCO3}$, which weighs $604.1$ g lime. The reaction will be quickly finished. The obtained final solution may be thrown away with the waste waters. And I repeat, no toxic chlorine gas will be produced.

You may also replace baking soda or lime by a metal like iron or aluminium. But the reaction will be slow, especially at the end, when $90$% of the acid will have reacted. It may last more than one hour. Just to get an idea, you will need $6.041$ mol of metal, which weighs $338.3$ g with iron, and $163.1$ g with aluminium.

Maurice
  • 28,241
  • 3
  • 29
  • 61
  • 10
    I would just add a notice of caution: While the math is simple enough, hydrochloric acid at that concentration is hazardous. It will release some hydrogen chloride gas, which OP will notice in their nose instantaneously. And adding the carbonate or bicarbonate base too rapidly will result in a terrible mess due to the carbon dioxide produced. Be prepared. If you don't know what I mean don't do it. – Snijderfrey Jan 23 '23 at 21:08
  • 3
    Thank you for the answer and the comment. I will pour the acid very slowly, mix and monitor the situation for any smell. Just a last comment, I think that I should expect some heat coming from the reaction? I find +28.5kJ/mol for the reaction so I should expect about 340kJ of net energy (12 mols), which is around the energy to raise 1 litre of water from 20C to 100C. To be on the safe side, I will have at least 10 litres of water for the reaction and monitor the temperature. – PC1 Jan 23 '23 at 21:53
  • 2
    The reaction with aluminum may make a sludge that isn't really nice to dispose of in waste water. See the answers to my question https://chemistry.stackexchange.com/questions/112616/safe-and-responsible-disposal-of-aluminum-chloride – R.. GitHub STOP HELPING ICE Jan 24 '23 at 16:18
  • In the end, we neutralized with baking soda. I used slightly more than the theoretical value but that ensured us that we have a slightly basic solution, which is fine for our sewer. – PC1 Jan 29 '23 at 01:33
4

You are so lucky as to have found 2 liters of toilet bowl cleaner (concentrated). There used to be a variety in my grocery store that was 20% HCl, but now it is only 10%, so now I have to use twice as much. The commercial materials contain a little surfactant to make some foam, and usually some blue color.

However, you can make do with the clear stuff, and don't really need the surfactant; if you want to, you can add that to the toilet water separately.

As a first try, pour about 10-20 mL of the 20% acid into your toilet bowl and scrub around with the usual toilet brush. Then flush. Depending on how often your toilet needs cleaning, your hydrochloric acid will last about 2-3 years. Consider it a blessing. When it's gone, you may consider buying a gallon from the hardware store.

Now, if it really gets on your nerves to have a generic toilet bowl cleaner in your house, you could use twice as much, twice as often, and make it go away four times faster.

BTW, I have a gallon of 31% HCl that I use - but I dilute it to 15% for toilet bowl cleaning. I keep the jug (tightly stoppered) in a plastic bucket, in a place where there is plenty of ventilation (just in case of leakage). So, you do need to be careful, but aggressive chemicals can be useful if well contained.

James Gaidis
  • 14,011
  • 2
  • 26
  • 56
3

Some of the answers above go into a lot of good detail as to the chemistry behind the answer, but I want to get to the point and avoid extraneous detail.

Gradually pour chalk or baking soda into it, stir and repeat until it stops reacting.

Congratulations, you've neutralised your acid and now have a salty solution of water and sodium or calcium chloride (depending on whether you went with chalk or baking soda respectively). This can be poured down the drain no bother.

Just make sure you do it outside so the hydrogen doesn't build up and you do it slowly enough that the heat doesn't cause a problem.

ScottishTapWater
  • 131
  • 2