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Name: Thomas
Status: educator
Grade: 9-12
Location: VA
Country: USA
Date: April 6, 2011


Question:
I have read the similar articles in the archives but wanted to check on a specific. When trying to neutralize a sodium hydroxide solution in distilled water (0.3 N), would hydrochloric acid at a similar concentration be most appropriate? This would simply be for disposal means after an experiment. I will also be checking with the local authority before disposing any material.


Replies:
A fairly simple and safe method of neutralisation - Add enough Phenolphthalein to give a good colour - deep pink in basic solution. Only a few drops of solution or a tiny pinch of powder is required. Add a similar strength of Acid slowly, until the colour disappears. Dilute Hydrochloric is easy to manage, and creates no harmful salts. When the pink colour disappears indicates a neutral, or weakly acidic solution has been created. Add a couple of sprinkles of marble chips to remove the last of the acid. leave stand overnight to be sure. You can then dispose of the remnant fluid any way you see fit. It will be nothing more than salty water and there would be no requirement to notify the water board or other local authority. I would NOT recommend you tip it away to any live watercourse, as the salt (NaCl) could be harmful to frogs or fish.

Nigel Skelton
Barkly College
Tennant Creek AUSTRALIA


Thomas,

Personally, I would not use HCl for this, in theory this would work but actually you could spend ages getting the solution neutral, a little too much HCl will make it very acidic, then you have to add base, etc. etc. The easiest way is to use a solid weak acid, such as citric acid powder. You can add this, bit by bit, and test the pH until it is neutral then dispose of the sodium citrate solution down the drain. If you add too much it will not make the solution more acidic than lemon juice, so you should be fine.

Best wishes,

Tom Collins


Prior to performing your neutralization, please check with local and state regulators.

In Washington state, where I am from (and most familiar with the rules), what you are proposing is called 'elemental neutralization'. It does neutralize the hazards, but the amount initially generated still counts as waste towards your total waste generated.

In regards to the chemistry of your question, yes a 0.3N solution is easy stoichiometrically, but you can also use half as much 0.6N, etc. Just be aware that your solution will generate heat as it reacts.

Ian Farrell


That sounds about right to me. Chloride ion has the advantage being common in the ecosystem and nearly matching seawater.

Making it sodium bicarbonate by leaving it wide open to air for some days is another alternative, if it is practical for you. Bubbling air, or stirring and fanning, would accelerate this substantially. It has advantages of costing no purchased chemicals, and being an infinitesimal reduction of greenhouse gas if you spend little electric power doing it.

Phosphoric acid is a poor idea because it fertilizes algae, upsetting the downstream ecosystem. Sulfuric acid might be acceptable, I am not sure. I cannot think of any other alternatives offhand. Simple dumping into the drain would soon be neutralized by atmospheric CO2 as above, but I imagine the pH upset in the sewers might reduce bioactivity which breaks down sewage, that being the reason for the regulation.

Jim Swenson


Disposal regulations can be odd and arcane. Often "treatment" by the generator is prohibited. So please do not construe any of the advice below as pertaining to applicable regulations or laws. It is just chemistry.

An equal volume of an equal concentration of hydrochloric acid would neutralize your base. The result will just be dilute saltwater, which is not particularly problematic in small volumes. An equal molar amount of a slightly higher concentration of hydrochloric acid would also adequately neutralize the base and produce a smaller volume of waste, though you should not work with any higher acid concentration than is comfortable for you.

Other acids would also work, but probably the one giving you the least troublesome neutralization product is the one you suggest, hydrochloric acid.

Richard E. Barrans Jr., Ph.D., M.Ed.


Hi Thomas,

Your suggested approach will do the job. The important thing to do is make sure that the pH of the solution is between 6 and 8. Then it is safe to dump down the drain, assuming that the only thing in the solution is sodium hydroxide and hydrochloric acid. These will react to form sodium chloride and water. If you have a large volume of dilute sodium hydroxide for disposal, then you may want to use hydrochloric acid of a higher concentration to cut back on the volume of liquid. HCl is frequently sold in 5N solutions.

Matt Voss


The reason is not just a matter of disposal (which is not the issue to consider here). The issue is minimizing experimental error. Let the concentration of NaOH be (0.3 molar), and if every "drop" of HCl titrant contains (1.0 mols) then it only takes a few drops of titrant. At the other extreme, if every "drop" of HCl titrant contains (0.001 mols) of titrant, it takes a large amount of titrant to neutralize the NaOH solution. In the first case only a few drops of titrant will neutralize the NaOH solution and in the second case it takes a lot more titrant to neutralize the solution. If one does an error analysis, something like one-to-one sample and sample minimizes the error. That is the short answer because sometimes the relative amounts are not exactly one-to-one for practical reasons, like the sample size.

Another issue that bypasses the problem is to do a "weight" titration, rather than a volume titration. A "weight titration" eliminates is much more accurate than the usual volumetric titration, because you can weigh the sample and titrant much more accurately than their respective volumes.

Give the two methods a comparison. But also be careful about using NaOH. It quickly absorbs atmospheric carbon dioxide. This is the large error in the measurement.

Vince Calder





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