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Disclaimer: I'm asking this because I am curious. Not because I'm actually handling nitric acid.

In another answer I was reading, the poster mentioned that if you spill nitric acid, you shouldn't pour water on anything but a small spill, because it will generate heat (maybe enough to flash-boil the water?), but if you spill nitric acid on yourself, you should have an emergency shower ready.

Except, wouldn't the emergency shower be using water and cause the exact same problem? Is this just a case of deciding that possible burns from a hot liquid, while under a cold shower are much better than definite acid burns?

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    $\begingroup$ Anyway, with nitric acid (unlike sulfuric) heat is not much of a problem. $\endgroup$ – Ivan Neretin Jan 19 '17 at 21:13
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    $\begingroup$ Note that the standardized flow rate of safety showers for laboratories is 60 l/min if no requirements are stipulated by national legislation (e.g. 30 l/min in Germany). We could use such values to make a rough estimate of heat and temperature for a given amount of nitric acid. $\endgroup$ – Loong Jan 19 '17 at 21:23
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    $\begingroup$ Concentrated sulfuric acid has another danger. It is slippery. $\endgroup$ – MaxW Jan 19 '17 at 21:32
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If you spill nitric acid onto the table, you yourself are unharmed and you can use a cool head to decide what to do next. If the spill is small, pour water on it to both dilute it and dilute the heat — remember water has a high specific heat capacity. If the spill is larger, I would probably try adding sodium hydrogencarbonate to react away the acid before wiping it up. Adding larger amounts of water will work, too, but you need to get them from somewhere and then need to get rid of the resulting puddle.

If the spill is on you you want to get the acid off your clothes and body immediately. The quickest and most effective solution is to pour water over your head and lots of it. This is what the emergency showers are for. Lots, here, has two functions: it not only quickly dilutes the acid to a less harmful concentration but also (because it is washing down) serves well to actually remove the heat — remember again that water has a high specific heat capacity. Emergency showers have a high throughput for exactly this reason.

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    $\begingroup$ So more caveats about an emergency shower. (1) Using an emergency shower isn't the time for modesty. You strip bare ass naked under the flow of water. (2) Contaminated clothing should never be removed over your head to avoid eye exposure. Bandage scissors should be available at the shower. There isn't time to go looking for scissors if an accident occurs. $\endgroup$ – MaxW Jan 19 '17 at 21:16
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    $\begingroup$ True that; I decided to only concentrate on the effects of water + acid here ^^ $\endgroup$ – Jan Jan 19 '17 at 21:16
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    $\begingroup$ Adding water to acid may result in some of the acid going somewhere else in relatively uncontrolled fashion. If the acid is on the table, and the "somewhere else" is on you, that's bad. If the acid is on you, however, and the "somewhere else" is pretty much anywhere, however, that's good. $\endgroup$ – supercat Jan 20 '17 at 19:02
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This all comes down to a matter of relative amounts of acid and water. A large spill on the floor (say, 2L) would require water to be poured onto it, and the initial volume would be small compared to the acid volume. If you used a volume of water that would fill a typical garden bucket(9L or 30 seconds of flow from a tap), this would not mix spontaneously, and there would be parts of the spill flow where acid concentration is much higher, causing problems.

In the case where a spill occurs on a person, the volumes of acid resident on the individual are much smaller, and the relative amounts of water much greater. There may only be a matter of tens of mLs of acid remaining on clothing, and much less on the skin. The volume of water applied from a safety shower is much greater than this, and can dilute the acid before adverse reactions occur.

So, in the case of an acid spill in the lab:

  1. Remove the individual, remove clothing, immerse in safety shower
  2. Evacuate local vicinity as requried
  3. Contain spill, and use chemical absorbent to decontaminate hazard.
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