this post was submitted on 02 Sep 2023
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Water is the most common substance that can be either an acid or a base (on earth), but lots of other compounds are also amphoteric.
In fact, on other planets where ammonia fills the same role as water, ammonia would be the most common amphoteric substance, so most solutions would be in a liquid ammonia solvent. This means neutral pH on those planets would substantially higher!
K_w is the auto dissociation constant for water, and at room temp, K_w is about 10^(-14). Taking the negative log of the square root of K_w gives the pH of pure water of about 7. The auto dissociation constant of ammonia, however, is about 10^(-30), so the pH of pure liquid ammonia is about 15! Basically, as soon as we start using solvents other than water, pH gets really funky
Edit: and before anyone jumps in to say "ack-shully, pH is based on the concentration of hydronium ions in solution, so you can't use pH for systems based on solvents other than water," pH can also be considered to be based on the protonated form of whatever the solvent is. So in an ammonia-based solution, you would find the pH by taking the negative log of ammonium instead of hydronium. Instead of defining pH as
pH = -log [H_3 O^(+)]
A more universal definition would be
pH = -log [H_2 A^(+)]
Where the auto dissociation reaction of any amphoteric solvent can be written as
HA + HA -> H_2 A^(+) + A^(-)
This is more detail than most people care about, but there's always lurking pedants on the Internet, so I thought I would leave a more detailed explanation
I knew that other planets had other chemicals as the most abondant substance on them but it being replacing water is something i never knew. Perhaps, aliens are sipping ammonia based cola as we speak !
Care to explain the neutral PH thing ? I don't really understand it. Does it mean ammonia based liquids wan to stabilize to 15 PH or something like that.
Sorry if it sounds dumb, English isn't my native language and i wasn't really a science guy at school when kid. Now, everything fascinates me. I never was good with math but i saw its beauty in programming ( if we taught kids math by making games with it we'd have a whole generation of math lovers )
Not dumb at all! In order to not write an even bigger wall of text, I assumed some things, like everyone already knowing that in water, a pH of 7 is considered neutral. This is because that solution would have an equal amount of acidic ions and basic ions, each with a concentration of approximately 10^(-7) moles per liter. But with a different solvent like ammonia, the change in auto dissociation constant means that to get an equal number of acidic and basic ions, you would only need a concentration of 10^(-15) moles per liter.
So, it would change a lot of the standard practices in a lab, like making buffers, neutralizing solutions, etc. Since it's Saturday and I'm doing this all off the top of my head, I don't know what other implications there might be, but basically a lot of things that chemists and biochemists take for granted would need to recalculated. Acids would be more acidic, bases more basic, etc. In ammonia, even water would be a fairly strong acid!
The chemistry doesn't really change, but a lot of the standard practices would need to be done differently (including the way we make buffers, measure pH, and the range of pH that a solution could be).
Thanks a lot for you explanation !
I knew water have a pH of 7 and is neutral and after reading your response i very very vaguely remember our teacher telling explaining what pH meant in middle school but a reminder was definitely needed. The rest is extremely interesting so Again, a huge thank you !!!
I'll likely go read about chemistry for dummies because i feel a bit ashamed of my limited knowledge with basic chemistry.
No need to get ashamed! Lots of people had bad experiences in chemistry classes at a young age and don't remember much beyond "it was hard, it didn't make sense, and I was really bad at it." So, you're in good company!
This is at least partly because chemistry was traditionally a "weed out" class, meaning it was used to determine whether people "had what it takes" to succeed in the sciences. As a result it was usually taught in a way that made it harder than it needs to be and a lot of people decided not to pursue STEM careers/education because chemistry felt too hard. But lots of times , it felt too hard just because it was taught poorly (on purpose).
Basically, don't be afraid to get back into chemistry! Even though I'm in chemistry education, I don't really have any great book recs for someone starting from scratch, as I'd want to recommend a textbook that's not necessarily easy to work through in your own. However, The Poisoner's Handbook by Deborah Blum and Uncle Tungsten by Oliver Sacks are both really fun to read and relatively accessible. To get more of a well rounded, academic understanding I would highly recommend taking a class at a local college (community college if you're in the US, to keep the cost down, but there are probably similar options in other countries). It would be more work and deadlines, but trying to educate yourself about this stuff can be really hard and intimidating, and if you take a class, you'll be much more likely to stick with it and get something out of it.
Thanks for the books recommendation. Where i live, I don't think there's any way to learn chemistry at a school other than going to middle school again, which I doubt I'd be even allowed.
There's probably a book or an app that can teach the basics of chemistry. Most people i know are so illiterate about chemistry that they mix household products and create toxic gases.