Safe disposal of hydrochloric acid

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?




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](https://en.m.wikipedia.org/wiki/Sodium_bicarbonate) solution as well and add slowly with stirring. Just keep adding until it no longer fizzles, then test with pH paper. Repeat.


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$1$ 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.


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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.


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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.




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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.


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