Why You Should Never Mix Sulphuric Acid and Sodium Hydroxide

This post could also be called “Why you should never mix different drain cleaners?” and the answer is because it can have catastrophic results!

You can buy drain cleaners from Amazon. You’ll find a range of different types which are suitable for al sorts of applications – be sure not to mix them though as this can have potentially catastrophic results!

Drain cleaners come in essentially 3 forms:

1. Caustic

These contain lye (sodium hydroxide, NaOH) or caustic potash (potassium hydroxide, KOH). In this case, it’s the hydroxide ions that initiate the reactions to clear the blockage. These products work better to clean fat or soap-based substances

2. Acid

Typically, these contain high levels of sulphuric acid (H2SO4) or hydrochloric acid (HCl), which unclog a blocked drain by increasing the concentration of H+ ions. Attracted to the clog substance, the reaction releases heat, in turn melting the congealed grease.

3. Oxidising

The main ingredient in this type of drain cleaner is usually bleach sodium hypochlorite, NaClO, and other peroxides and nitrates), which oxidises the material clogging the drain.

Sulphuric acid and sodium hydroxide is a dangerous mix

When cleaning drains, it’s essential to use only one product and not to mix different chemicals. For example, the reaction between sodium hydroxide (NaOH) and sulphuric acid (H2SO4) generates salt (sodium sulphate, Na2SO4) and water (H2O).

2 NaOH + H2SO4 -> Na2SO4 + 2 H2O

This may not seem dangerous at all, but this reaction is highly exothermic, generating a considerable amount of heat. This means by mixing a caustic-based drain cleaner (with sodium hydroxide) and a more acidic product (with sulphuric acid), there’s the risk that hot water will gush out of the drain and potentially cause severe burns to the face and hands. Also, as it is unlikely that you managed to mix the two products in exact stoichiometric proportions to use all of the NaOH and H2SO4, the water gushing out will not only be hot, but in will contain left-over acid or base, which can worsen the burns.

Mixing sodium hypochlorite and hydrochloric acid is also risky

Sodium hydroxide and sulphuric acid is not the only dangerous reaction involving inadequate use of drain cleaners. Mixing a cleaner with hydrochloric acid (HCl) with a bleach-based product (containing sodium hypochlorite, NaClO) can also have risky consequences. The reaction generates water and salt (sodium chloride), which are harmless, but it also produces chlorine gas (Cl2). This gas can be extremely irritant to the throat and eyes, even in very low concentrations.

2 HCl + NaClO -> Cl2 + NaCl + H2O

If you ever wondered why bottles of drain cleaners state clearly and in big letters NOT to mix different products, now you know why.

Alex Reis
Alex Reis is a freelance science writer, with a particular expertise in the field of biological sciences. She has several years experience in scientific writing and research, with various scientific manuscripts published in high impact factor journals, including Nature Cell Biology, as well as articles promoted in more mainstream publications.
Alex Reis
Alex Reis
Alex Reis

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  1. Great delivery. Solid arguments. Keep up the amazing spirit.

  2. Mintundeer says:

    great im doing the wrinkle titration with these solutions

  3. I mixed Sodium Hydroxide with Sulfuric Acid on accident and it almost blew the lid off my toilet. Mom put towel paper down the toilet so I guess according to this webite I should have just used the sulfuric acid alone to eat the paper up. Use one or the other for your specific application as they mention above. My clog was gone but also almost my toilet seat (not really but sounds so)! Be careful and read the label!

  4. I did this too not realising any of the above… I used Castic soda first and that didn’t work, so I went to the local hardware store and he suggested I try spirit of salts acid. That didn’t work either and I managed to unblock it by jamming the cloth end is a shaggy mop in and out of the toilet pipe (suggested by my plumber). My problem now is that the salts have formed into a solid mass in the bottom of the visible toilet pipe – I’ve tried to shatter it but it won’t shift. It is also making the water cloudy and giving off a bad smell. Is this smell dangerous? I have 2 children and my husband is away!! And will I have to replace the toilet? Big oops.

    • Hi Laura,
      Oh no – it’s a shame you didn’t come across our post before this happened!
      The most important thing to do firstly is not add any further chemicals. Contact a plumber and have them come out and take a look. The exothermic reaction the two chemicals cause can be dangerous and you’ll need to have the pipe cleared by a professional. Make sure you let the plumber know exactly what you’ve put down the toilet.
      Sorry about the problems you’re experiencing,
      The Chemical Blog

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