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The answer, of course, is very screwed; however, I want to elaborate on that more.

Background

Despite what common propaganda (D&D, Tolkien, etc...) might have taught you, dragons rarely ever use their breath weapon. When they do, it's primarily to handicap the enemy and make an opening for the killing blow, or an escape.

Dragons have a specialized bacteria in their stomach that can produce sulfuric acid with an acidity of at around 0.2 pH. The dragon's stomach lining (though the sulfuric acid isn't stored there), mucous layer and scales are adapted for high acid-stability, and should be able to take the acid on.

A dragon can usually store up to five liters of sulfuric acid, and with their flight muscles, propel it to distances of 6-10 meters.

So, assuming a human in plate armor got caught up in a fine mist of this acid (roughly 65% of the body is covered, head included), what injuries would they suffer and how fast would their incapacitation be?

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    $\begingroup$ Sulfuric acid can only reach such a low pH when it reaches a concentration of 1M/L. That dragon will use its breath weapon very often because it will be in a permanent state of acidic reflux. $\endgroup$
    – The Square-Cube Law
    Mar 16, 2020 at 19:58
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    $\begingroup$ Depends, if he breathes any in, pretty instant death... $\endgroup$
    – Plutian
    Mar 16, 2020 at 19:59
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    $\begingroup$ Likes the idea of using the very powerful flight muscles! $\endgroup$
    – MortenSickel
    Mar 17, 2020 at 9:30
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    $\begingroup$ Not an answer, because the question was not about this: Send a trusted and devoted sidekick first, and after the dragon spent his reservoir, finish him "the traditional way". And then, of course, bury the sidekick with full honours and feeling of gratitude. How much can a dragon spit in one go? If less than full 5 litres, employ more sidekicks. $\endgroup$
    – Radovan Garabík
    Mar 17, 2020 at 9:59
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    $\begingroup$ Not sure if any of the answers cover this, but plate armor, or armor with any metal in it, would be the knight's bane. Sulfuric acid oxidizes the metal, being itself reduced to sulfur dioxide, which when inhaled would irritate your lungs to the point of drowning themselves in mucous. Ironically, leather armor is the best bet against this one attack of the beast. $\endgroup$
    – Varad Mahashabde
    Mar 18, 2020 at 10:48

4 Answers 4

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So, assuming a human in plate armor got caught up in a fine mist of this acid (roughly 65% of the body is covered, head included), what injuries would they suffer and how fast would their incapacitation be?

I'm going to go for "instantaneous incapacitation, death within two minutes at the most".

The fine mist would inevitably be inhaled, leading to edema of the airways within seconds. An immediate tracheotomy is the only way I can think of that has any hope of salvation for the unlucky knight. Suffocation would probably do him in before shock has any real chance.

You can (reasonably) easily find images on the Internet of people who had about 10-20 cc of commercial sulfuric acid thrown at their faces, what is known as vitriolage. This quantity is often (80-90% of the cases) survivable, but even with immediate medical care brings loss of function and significant disfiguration. And that, to repeat, is the effect of a quantity two orders of magnitude less than your dragon can spew, usually thrown with much less proficiency and delivered in a significantly more inefficient form (inhalation of sulfuric acid vapours is almost invariably fatal; even the accidental inhalation of what vapours may effuse from a vitriolage attack can be fatal, and even when it is not, it leads to potentially life-threatening respiratory damage).

Defense against such an attack would require a way of sealing the armor so that, for a couple of minutes, the knight can breathe the air trapped inside. Not something to look forward to, given the average knight's reported level of personal hygiene, but survivable.

Then, something that can easily be sloughed off - maybe a mantle, or a large composite hoodie. Impregnated with water and alkali to neutralize the acid as much as possible, with a quick-release brooch. Maybe more than one mantle, in layers.

The best strategy if the dragon's attack cannot be thwarted or avoided entirely would be to draw the attack in such a way that it can be more easily defeated.

The knight would for example goad the dragon, assuming (as @JohnDvorak correctly points out) the dragon is not clever enough to see through the ruse, then quickly pivot and fall on his knees closing the visor of his sallet, leaving exposed the back and the (jettisonable) pauldrons. The back would be defended with a thick mantle of leather and maybe gold foil. Once the acid jet has been exhausted with certainty, the dragon can be battled with much better chances.

The comments by JohnDvorak and JohnMontgomery are just too good not to include:

Dragon: I know what you're thinking: "Did it ssspew 1.8 litersss, or just 1.7?" Well, to tell you the truth, in all thisss excitement, I've kinda lossst track myssself. But being thisss isss sssulfuric acid, the most powerful acid in the world, and would melt your face clean off, you've got to asssk yourssself one quessstion: 'Do I feel lucky?'. Well... do you, knight?

Dragon-killing knights would also very quickly employ a different kind of armor: some kind of transparent protection (quartz?) for the eyes. Probably a sealed or sealable helmet. Possibly a thick but still light leather suit instead of steel armor, to increase speed. This again assumes that the dragons don't also have, say, strong claws, in which case abandoning armor would not be advisable.

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    $\begingroup$ "Once the acid jet has been exhausted, the dragon can be battled with much better chances." - unless the dragon knows how little sulfuric acid suffices to kill a human, and is much more efficient than initially thought. $\endgroup$
    – John Dvorak
    Mar 17, 2020 at 12:18
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    $\begingroup$ @JohnDvorak Very right. It depends on the intelligence of the dragon. It could fake having exhausted all its sulfuric acid to lure the knight into attacking, then take him out. $\endgroup$
    – LSerni
    Mar 17, 2020 at 14:37
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    $\begingroup$ "Did it spew 1.8L of vitriol out of its mouth, or only 1.7L?" $\endgroup$
    – John Dvorak
    Mar 17, 2020 at 15:00
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    $\begingroup$ "But being this is sulfuric acid, the most powerful acid in the world, and would melt your face clean off, you've got to ask yourself one question: 'Do I feel lucky?' Well, do you, knight?" $\endgroup$
    – John Montgomery
    Mar 17, 2020 at 17:53
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The MSDS of sulfuric acid warns

Danger! Extremely corrosive. Causes severe burns and / or eye damage. Mist: Causes respiratory irritation. Harmful if inhaled. Harmful or fatal if swallowed. Reacts violently with water. Concentrated Sulfuric Acid will react with many organic materials and may cause fire due to the heat of the reaction. Not flammable, but reacts with most metals to form explosive/flammable hydrogen gas.

EYE CONTACT: Immediate pain, severe burns and corneal damage, which may result in permanent blindness.

SKIN CONTACT: Causes burns, and brownish or yellow stains. Concentrated solutions may cause second or third degree burns with severe necrosis. Prolonged and repeated exposure to dilute solutions may cause irritation, redness, pain and drying and cracking of the skin.

INHALATION: Causes respiratory irritation and at high concentrations may cause severe injury, burns, or death. Effects of exposure may be delayed.

From a more anecdotal point of view, during my PhD one of the lab technician, while dumping sulfuric acid in the discard bin, got a few droplets on his sleeves. Though immediate measures were taken, the few which went through the clothes was sufficient to cause a severe burn on the skin.

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    $\begingroup$ Note that 0.2 pH sulfuric acid is NOT a "concentrated acid", in chemical lingo. It corresponds to about 0.63M solution, which less than 10% by mass. $\endgroup$
    – Alexander
    Mar 16, 2020 at 20:24
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    $\begingroup$ @WOPR HF acid has WAY worse effects than sulfuric. $\endgroup$
    – fraxinus
    Mar 17, 2020 at 13:23
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    $\begingroup$ @OP please, pretty please, don't make dragons capable of HF excretion. ... it is one step before making them blowing sarin. $\endgroup$
    – fraxinus
    Mar 17, 2020 at 13:24
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    $\begingroup$ @fraxinus The Alfär air-superiority "dragons" (complete with tentacles) in the Laundry Files do indeed spit HF, which they produce by ingesting fluorspar. The ground crews are protected by inserting drainage tubes to prevent HF leakage until the dragon is needed for combat. $\endgroup$
    – Inductiveload
    Mar 17, 2020 at 18:07
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    $\begingroup$ @inductiveload: Actually, Laundry dragons spew out ClF3, which is far worse than either HF or hydrosulphic acid. blogs.sciencemag.org/pipeline/archives/2008/02/26/… Truly nasty stuff $\endgroup$
    – poncho
    Mar 17, 2020 at 20:36
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From a personal experience with a battery acid (25-30%, pH probably about 0.5):

Over the skin: not much of a problem. Some itching if not washed right away. Cotton clothes suffer much more damage.

Eyes: protect them, period. Low-tech survival without losing vision is an immediate access to running water and/or sodium bicarbonate solution. Any possibility of glass technology in your setup? Glass is pretty old, but needs infrastructure and qualified labour.

Other body openings: nasty, but manageable.

Breath can be protected w/ lime, limestone or sodium bicarbonate expendable mask (not much of a high tech). Mask must be tick and not made of cotton (wool is OK). Vapours are suffocating and invoke cough, mist will be fatal. Use mask.

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  • $\begingroup$ Sulfuric acid decomposes cotton pretty quick. Your mask will tear itself apart in a matter of minutes after the first droplets of sulfuric acid, long before you smell the acid. $\endgroup$
    – fraxinus
    Mar 17, 2020 at 11:34
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    $\begingroup$ If the knight can't kill the dragon in a matter of minutes after it spits the acid, he is in trouble anyway. $\endgroup$
    – Martin Grey
    Mar 17, 2020 at 12:19
  • $\begingroup$ Only if you have a single knight and a single dragon to kill. I imagined many to many situation with many battles one after another. $\endgroup$
    – fraxinus
    Mar 17, 2020 at 13:20
  • $\begingroup$ Glass itself is not a problem in medieval times, but its quality may be inadequate for clear see-through goggles. $\endgroup$
    – Alexander
    Mar 17, 2020 at 20:07
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    $\begingroup$ @fraxinus "... but only if the dragon is very small, and the knight is very very sharp." $\endgroup$
    – Will Crawford
    Mar 19, 2020 at 19:20
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I think they'd be okay because, of course, the acid is not going to catch them unprepared. In fact, they might not even bother with the plate mail, but might go with something like a coverall made of intestines or other material treated to withstand the acid, goggles and at the very least a 'gas mask' arrangement.

I'm sure after decades or centuries of dragon-slaying, protection would have evolved to far more advanced levels. They might even develop chemical means of using the acid against the dragon.

There are reasons why acid has never caught on as a weapon of warfare. If the dragon could spit a 5-kilo solid lump it might be a lot more dangerous, especially from a height!

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  • $\begingroup$ "If the dragon could spit a 5-kilo solid lump it might be a lot more dangerous, especially from a height!" Until the knights once again adapt to this danger. $\endgroup$
    – FreezePhoenix
    Mar 18, 2020 at 13:50
  • $\begingroup$ I don’t think there’s a solution to that one? $\endgroup$
    – David Hambling
    Mar 18, 2020 at 22:03

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