Could neutralisation of safe chemicals produce dangerous ones?

I'm basically thinking whether I could potentionally poison myself while neutralising strong acid/base by some weak base/acid when I accidentally spill it, touch it or when I'm cleaning reaction product.

Combinations of chemicals that are non-toxic in my opinion:

• $\ce{HCl + NaHCO3}$
• $\ce{H2SO4 + NaHCO3}$
• $\ce{NaOH + CH3COOH}$

None of these are poisonous. I just thought these up, for example the first one produces table salt, $\ce{NaCl}$.

But could there be a combination of non-toxic acid and base that would produce some nasty salt when neutralised? Something worse than what I would be getting rid of?
Is there a combination where I should better just wash myself with water?

For "everyday chemistry", I cannot think of an example besides that with bleach above.

Incidentally, mixing hydrogen peroxide and sulfuric acid to make piranha solution is another unsafe mixture. Aqua regia, mixing nitric and $HCl$ would be another problem. Both reagents should be avoided. That's not what you're asking.

Consider that typical "strong" acids and bases are highly ionic compounds. That is, the definition of a strong acid or base is one that dissociates completely in water.

So typical neutralization with weak acids and weak bases will still produce soluble salts.

There are caveats.

1. Neutralization often is exothermic, and with common acids ($HCl$, $H_2SO_4$ and $HNO_3$ the heating effect can drive release of the gases, which are corrosive. So if you have concentrated acids, you should dilute significantly by pouring the acid into water before neutralization with weak base.
2. Nitric acid. The various nitrous oxides are dangerous, and care must be used to prevent them from being released. Worse, nitric acid is a strong oxidizer and shouldn't be used with organic reagents, especially organic bases.

So the usual rules apply. Dilute, then neutralize carefully.

While not an acid-base reaction, an example of mixing two relatively "safe" chemicals to form dangerous ones is bleach (sodium hypochlorite) plus ammonia - both common household chemicals. This reaction yields chloramine, dichloramine, and nitrogen trichloride, all more toxic and hazardous than bleach or ammonia.

• That's a very good example! – Geoff Hutchison Sep 15 '14 at 15:39
• Thank you, but I'm directly interested in acid-base neutralisation. Though a list of dangerous combinations would be very useful thing for everybody. – Tomáš Zato - Reinstate Monica Sep 15 '14 at 15:48
• I'm asking about acid-base because I neutralise acid on my hands or equipment or solid product very often. – Tomáš Zato - Reinstate Monica Sep 15 '14 at 15:48
• If we consider just acid-base reactions, then neutralizing a metal phosphide or arsenide would be an example - this would produce gases phosphine and arsine, which are quite toxic by inhalation. The metal phosphide or arsenide salts are toxic too, but do not pose an inhalation hazard. Same considerations apply to metal sulfides/selenides/tellurides. – iad22agp Sep 15 '14 at 21:06

I can't think of any acid base examples where you will make something more toxic than what you are trying to neutralize.

That said I think you should always be washing with copious water. There's no reliable way to know how much material is on your skin, which means you won't know how much neutralizing agent to use. You are better off trying to dilute the acid or base dramatically to avoid its adverse effects. Furthermore, this gives you an immediate procedure to follow if a spill should occur. Instead of going through the process of considering what the nature of the contamination is, finding the appropriate counter-reagent, and approximating the amount needed for neutralization, just immediately go to the sink. The time saved is critical for minimizing injury.

• Indeed. I think one caveat here is $HF$. But if you're working with $HF$ you should research the safety protocols carefully. – Geoff Hutchison Sep 16 '14 at 3:32
• Great point. I'm hoping that HF isn't considered a household chemical. – jerepierre Sep 16 '14 at 16:07