Which acid to choose when designing a world with acid seas?

Question

I'm currently writing a narrative and hoping to include a world which is home to a sea of some form of 'nasty' substance (doesn't have to be acid).

The varieties of inhabitants of this world have evolved mostly on the lands and eventually built their cities right up to the shorelines of this acidic sea. (In some cases I may even have cities that float and migrate across it).

I'm imagining some kind of scientific or industrial endeavor they perform whereby they've created ships that can withstand the chemicals (at least in short term) by lining their hulls with a type of ablative shielding.

Also desirable (although not essential) would be for the composition to not be immediately fatal. A carbon life form (or would they likely be silicate?) could fall in and maybe swim for a few minutes and leave with only some semi minor chemical burns. Yet prolonged exposure would most definitely be fatal.

I'm also wondering if the inhabitants could harness this chemical in some imaginative way such as for propulsion or agricultural/industrial means?

My question is, what kind of chemical/cocktail could I use to fit these kinds of parameters?

Disclaimer: also posted to chemistry.

Answer 1

Acids are very harsh on minerals so your world will be devoid of metals, bases, and basic rocks.

Additionally acids weaken with use, so you'd have to have them constantly replenished somehow, while also filtering out all the diluted substrate.

I suggest you maybe consider a water based ocean with heavy enzyme activity. The enzymes might be continually released by some microbial ecology while being quite adept at disolving non-native biology or machinery without natural defenses.

sort of a toned down, all natural grey goo.

there is precedent for this in earths history. When photosynthesis evolved and bacteria started dumping this toxic stuff called oxygen into the atmosphere and wrecked anaerobic lifestyles.

• proteases and peptidases split proteins into small peptides and amino acids.
• lipases split fat into three fatty acids and a glycerol molecule.
• amylases split carbohydrates such as starch and sugars into simple sugars such as glucose.
• nucleases split nucleic acids into nucleotides

feel free to mix and match, or invent something if you need to digest metals.

Answer 2

To maintain a toxic level of acidity in your oceans, you will need to have a large amount of acid-forming chemicals in your environment.

One possible example would be CO2 with a partial pressure of about 1 atm. The resulting carbonic acid would be about the strength you need. The acid will precipitate out as carbonates when available calcium ions, etc. are added to the ocean. Over eons, you likely must have a carbon cycle to justify the ongoing high CO2 level.

Of course, this level of atmospheric CO2 is toxic to virtually all animal life on earth, though a different animal biology would perhaps be compatible.

Other acids will also likely require their own equivalent to a carbon cycle. But they also share a common problem in that the corresponding atmospheric partial pressure will also often be toxic. For example sulfuric acid has a vapor pressure of 0.001 mmHg at 20 C. Not much, but enough to damage delicate lung tissues, etc., esp. considering constant exposure. Nitric acid has a much higher vapor pressure and hydrogen chloride is a gas at STP.

I don't know the vapor pressure of all the potential acids off the top of my head, but it is important to consider this for any suggested acid.

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If you don't need to ocean and atmosphere to be in balance because the ocean was recently polluted with large quantities of the acid that have not had the time to pollute the atmosphere significantly, you can ignore the condition needed for a long-term balance.

One realistic choice for recent or long term ocean acidification could be an ocean-based bacteria that produces abundant organic acids salts, perhaps the salt for oxalic acid. This could be a mutation of a common land-based bacteria that aggressively consumes the oxalic acid in the atmosphere -- this combination of bacteria could provide the required imbalance. It is not unrealistic for oxalic acid to be important to biology, in fact, you have oxalic acid in your blood and it is a bi-product of metabolic processes. Some plants are known to metabolise oxalic acid., Other organic acids have similar chances for production in the ocean and consumption on land that could maintain the imbalance between ocean and atmosphere.

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Also note evolutionary theory would suggest that ocean organisms would tend to develop resistance against oxalic acid though land-based organisms would be far-less likely to develop resistance. This may mean that ocean based food could be toxic (mildly or strongly) to land based creatures. It would depend upon whether the ocean adaption is passive -- developing tolerance to oxalic acid, or aggressive -- developing mechanisms that reduce oxalic acid levels within their bodies.

Why did I suggest oxalic acid?

• It is a strong acid, at 0.1 normality, pH is 1.3. For comparison sulfuric acid at 0.1N has pH 0.3
• It is known to be actively used in nature including consumption and waste output. Some bacteria are known to produce the oxalate. It is also a significant component of many common foods.
• It only consists of very common atoms, oxygen, hydrogen and carbon
• Accumulation within humans causes problems at low concentrations. It is also the chief component of kidney stones (calcium oxalate), which is why stones sufferers are advised to avoid sweet potatoes, asparagus, etc. that are rich in oxalates.
• It is colorless in solution, won't interfere with ocean photosynthesis, etc.
• It is cheap, about 600-700 USD per tonne at industrial grade (99.6% pure) from Alibaba today (2016-02-05)

Answer 3

There are acidic saline lakes on earth, down to pH 1 in ponds in Chile. That abstract mentions solutions of Cl-SO4-Na and Cl-SO4-Mg: I don't claim to understand the chemistry that determines their pH at a given concentration.

I'm not certain, but I think that's acidic enough to give you a bad day if you stay in there long enough. They do sustain carbon-based life, but extremophile cyanobacteria rather than fish.

As far as I know, these are generally formed by the concentration of whatever minerals are in them, over a geological period of time, by the evaporation of water that's run down from nearby mountains and whatnot and brought the minerals with it. That is to say, dead-end rivers.

Obviously your "sea" needs to be a lot larger than these ponds to be worthy of the name, but the general principle is that if you have a land-locked body of water, far enough away from the ocean that it hasn't been diluted and mixed any time in the last few million years, then it can concentrate pretty much anything water-soluble in the area, including acid salts. You don't need the whole planet or its atmosphere to be acidic, you need an enormous acid salt deposit plus just enough water to float a ship. The water might be seasonal or the result of the current climate in your world being somewhat wetter in this region than it had been during a previous epoc during which large acid salt plains formed.

For scale, Salar de Uyuni in Bolivia covers 4000 square miles and contains 10 billion tonnes of salt. It's not currently wet enough to constitute a sea, and it's not acidic, but combine the scale of this with the chemistry of acidic brines and you're there. You can argue what's "really" a sea and what's "really" a lake: this is a lot bigger than the Dead Sea, but I think the Dead Sea is normally considered a lake not a sea. I've seen the Sea of Marmara claimed as the "world's smallest sea" -- Salar de Uyuni is only a third the size.

One problem, though: I don't think building a ship to resist this level of acidity would be a huge big deal. It probably wouldn't require enormous ablative hull shields, but perhaps you're prepared to take dramatic license and/or increase the amount of sulphuric acid in there. Fundamentally, if you can use ceramics then I guess you can resist any acid not near-instantly fatal to humans.

It's true that acid gets "used up" as it reacts, but if you compare millions of years of deposit against thousands of years of humans operating boats, I don't see why something of this sort couldn't appear stable even though technically the salt deposits are being eroded as a result of currently having more water present than the long-term average, and a certain amount of stuff falling in and reacting.

Answer 4

There could be a lifeform (e.g. an algae or plankton) that filters salts out of the ocean that turns the salt products of an acid-base reaction back into the components. The lifeforms could "capture" the base, incorporating it into their body and protecting it from acid. This releases the acid back into the sea.

Apart from the salt itself, the organism would rely on two main things for this reaction: energy and hydrogen.

Since neutralisation reactions are exothermic (releases energy/heat), the inverse will be endothermic (requires energy). Thus you could expect the organisms to thrive particularly well around geothermal activity, and to live more slowly in smaller numbers on the surface (particularly in warm/sunny areas). Solar-adapted versions may have the ability to build a small "charge" from solar energy over time.

Hydrogen is needed to replace the base from the salt and turn it back into an acid. If they're in a water body, there's hydrogen in water (again, endothermic to split H20). This would release atomic or molecular oxygen into the water, which the organism or other organisms might use for respiration. This isn't a substitute for photosynthesis in the world, though; they're not splitting carbon dioxide.

So if we look at this, we have an organism that dwells on the surface and around geothermal activity, which captures bases from salts and keeps the ocean acidic. This will gradually erode the mineral landscape, but this may be a tectonically active planet. When the organisms die they may sink down and form protective layers (assuming however they hold the base safe from the acid is stable), preventing the acid from eating downwards.

So why are people living in places where these organisms live? Perhaps it's just because they want the same things - geothermal activity can provide uses for people, and often contributes to land fertility. Perhaps there are other dangerous organisms that the acidic seas keep at bay.

So, which chemicals??

Firstly, which salt? There'll be a few factors that make it more likely for this kind of organism to survive. The "lattice enthalpy" of a compound relates to the energy required to break the bonds, so different salts are more easily broken than others. I don't know if the organism would survive better with an easily ionised salt (because it takes less energy to break it) or a more strongly ionised one (because the bonds it forms by adding the molecules to its body are more stable). I don't know enough about chemistry to work out which salts have more or less lattice enthalpy!

Which acid? The stronger the acid, the harder it will be for the organism to store the base safely. However, a stronger acid will also react with other things faster (e.g. minerals in the landscape), forming food for the organisms. Because the organisms are limited in their "processing" by their intake energy, they might be more likely to have evolved with a medium-strength acid.

I'm no chemist or biologist, so I don't know which to recommend! Probably pick whichever's more suitable for the setting. Depending on the use of the setting you might not want to specify the acid or salt in question, and just describe the effect.

Answer 5

I would go with a weak dissolution of acid. Where most of the ocean is water, but has a 'high' concentration of acid. Take something like Hydrochloric acid say in "The Works" toilet bowl cleaner and water it down by 1/3 to 1/4. This might still allow some kind of water cycle to continue working on your planet.

It won't be immediately fatal, but you won't want to spend any more time in it than it takes to get back out, strip and rinse off.

Ships and docks would likely have stashes of ammonia or some other mild base to help quickly neutralize the acid, when people get pulled out.

The bigger problems would be how an ocean was turned to acid and even more what process is KEEPING it acid, since it tends to react with stuff and be neutralized. This can be handwaved away, but some thought might be put into that.

If you notice our oceans are full of salt, which happens when a base and acid are mixed.

Answer 6

This image might be helpful:

Hope it helps.

However, a list of some acids are: Sulfuric, Hydrochloric, Nitric. More here.