I remember a bit of this in undergrad gen chem. If pH stands for -log([H+]), how can pure water (H2O) have a pH of 7? In theory, there shouldn't be any hydrogen ions in pure water. Turns out there are two simplifications being made:
i. H+ in aqueous solution is synonymous with hydronium ion (H3O+).
ii. Water is in an equilibrium autoionization reaction with itself and the hydronium and hydroxide ions:
2H2O <=> H3O+ + OH-
A pH of 7 means the negative log of the hydronium ion concentration is 7. Or about 10^-7 hydronium ions per liter. This reflects that the reaction above greatly favors water in the H2O state. We tend to make such simplifications (H+ for H3O+) in order to make teaching easier, but at some point it impedes a deeper understanding of what's really going on.
Fun fact: The exact meaning of the letter p in "pH" is disputed, as Sørensen did not explain why he used it. Sørensen describes a way of measuring pH using potential differences, and it represents the negative power of 10 in the concentration of hydrogen ions. The letter p could stand for the French puissance, German Potenz, or Danish potens, meaning "power", or it could mean "potential". All the words for these start with the letter p in French, German, and Danish—all languages Sørensen published in: Carlsberg Laboratory was French-speaking, German was the dominant language of scientific publishing, and Sørensen was Danish. He also used the letter q in much the same way elsewhere in the paper. He might also have labelled the test solution "p" and the reference solution "q" arbitrarily; these letters are often paired. Some literature sources state that the "pH" stands for the Latin term pondus hydrogenii (quantity of hydrogen) or potentia hydrogenii (power of hydrogen), although this is not supported by Sørensen's writings. Currently in chemistry, the p stands for "decimal logarithm of"
The stoichiometry which factors into the definition of pH is
H_{2}O <=> H^{+} + OH^{-}
which actually includes the activity of proton - that's H^{+} multiplied by a coefficient that reflects the ionic strength of the solution and in non-zero outside of really pure 18 mega-ohm RO water - but that's beyond the scope of this. I will note that your balanced equation is in fact valid, as it includes bulk aqueous water.
(1) H^{+}(aq) is not synonymous with H_{3}O^{+}; the former is aqueous solvated proton, the latter is hydronium ion;
(2) Moving on to the so-called "autoionization" reaction, what you mean to say is that water is self-ionizable or, more appropriately, amphoteric, i.e., is can act as an acid (in this sense, a proton donor) or a base (the hydroxide ion);
(3) The pH of 7 is an ideal and is dependent on temperature, ionic strength, and a few other things (including pressure) that do actually move us off the pH point of 7 for a so-called "neutral" solution; and finally
(4) The units are moles per liter; and hydronium vs. proton does make a difference, as proton is one hydrogen and hydronium is 3 hydrogens.
I don't think the GP is saying that H+ and H3O+ are literally synonymous, but rather that "H+" in the context of an aqueous solution is actually shorthand for a hydronium ion and not a solvated proton (with an H2O implied on the other side of the equation for balance). My recollection from high school and undergrad chemistry is that an unassociated proton is very highly energetically unfavorable in solution relative to a hydronium ion, so any "H+" in solution would exist almost exclusively as the latter. Is that accurate, or is it just a simplification they make for the sake of not overcomplicating things in intro chem classes?
I've always known protons to do things like no other particle.
I agree that H+ in solution is going to make temporary friends with whatever it finds in abundance until it comes in contact with something it can react less irreversibly with.
To build from fundamentals, what do we know?
Naked protons really are aggressive. Yikes. H+.
Water is just protonated OH-, it's neutral. pH 7 by definition.
The more the H+ outnumbers the OH-, the more aggressive the proton action.
If you water down HCl enough the solution strength will eventually fall into the narrow band known as "measurable pH", from pH 1 (quite acidic) to pH 7 (neutral).
pH numbers higher than 7 (all the way up to 14) are alkaline, not within reach for HCl in plain water.
It's good to have NIST-traceable pH measurements from precision electrochemical readings using pH-sensitive glass electrodes with temperature compensation. This is a type of glass that has a slight change in electrical properties according to the type and strength of ions it is exposed to, highly sensitive to aqueous H+ in particular. The range of readings is highly amplified by the instrument which is then calibrated logarithmically to the pH scale using known solutions. The known pH solutions correspond to the reference H+ concentrations according to the fundamental pH equations, without dependence on electrochemical measurement themselves.
Not quite so good using the proper pH-sensitive dyes which have been discovered useful or invented over the ages. These are strong enough pigments at visible wavelengths such that they can still be detected by eye (or spectrometer) at very low concentrations, while also possessing the property of noticeably changing color wavelength in response to degree of protonation of the dye molecules. These have been around since before electrochemistry.
In non-aqueous solutions, neither the electrodes nor the dyes behave exactly like they do with aqueous work, and I don't think the protons do either. The electrodes can be extremely useful non-aqueously, but it's not actually pH we're measuring any more. Plus different solvents are naturally going to have different water contents, sometimes whether intended or not.
And there are other things besides dyes which have visible indications of aggressiveness, like polyurethane and metals of many types. Some may take longer to notice than others, but eventually you conclude, yup, this stuff has been attacked by acid.
You will find the same number of molecules of HCl per liter of water will not be nearly as aggressive as the same number of molecules per liter in some non-aqueous solvents.
At other times it will be the water itself that is required for acid behavior to be fully as expected.
The best non-aqueous mixtures for any one purpose are probably not the ones commercially available. So you should be capable of coming up with your own.
Perhaps we've ended up full circle on this again as the current leading theory is H₂O actually forms the cation H⁺·(H₂O)₂·(H₂O)₄, where the H⁺ forms a hydrogen bond to two H₂Os which form two more hydrogen bonds to two H₂Os each: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2946644/
I think my mind was first blown when our physics teacher asked us if H2O conducts electricity. And naturally we all said yeah[1]. We were naturally wrong, but pure water is amazing in its own right.
edit: decided to add an edit just in case someone tries to test this claim
This video seems to confuse semiconductors with "materials with a negative thermal coefficient of resistivity". Saying "this gets more conductive as you heat it up" does not a semiconductor make, as far as I understand.
Any material that "gets more conductive as you heat it up" is a semiconductor, unless its conductivity remains negligible, in which case it is an insulator.
Not all semiconductors "get more conductive as you heat them up". A counterexample is provided by heavily doped semiconductors.
In any normal resistor, which is made from a conductive material, the resistance increases with the temperature, because the "friction" between the electricity carriers and the atomic lattice increases.
Only in resistors made of semiconductors, i.e. in NTC thermistors, the resistance decreases with the temperature.
The difference between a conductor and a semiconductor is that in the former the concentration of the carriers is constant, while in the latter the concentration of the carriers can be changed by various means, e.g. by temperature, by light, by impurities, by corpuscular radiation, by electrical fields, by contact with other materials, and by other means.
In most intrinsic semiconductors, the carrier concentration increases with the temperature, therefore the resistance decreases with the temperature.
This is a valid method to determine that a material is a semiconductor, even if false negatives are possible, when a material is a semiconductor though its resistance increases with the temperature.
The idea that pure water then frozen and kept sufficiently cold could be a functional transistor seems improbable, but because of that, someone will eventually get gain out of a water transistor.
Nice bit of insight! For anyone confused like me: I believe it should read "about 10^-7 mols of hydronium ions per liter", which is about 6*10^16 ions per liter.
Many people have never seen stainless steel rust. I have. At a slightly leaky Swagelok compression fitting on the HCl-containing gas mixture bottle for a XeCl excimer laser. The leaking HCl combines with the room humidity to make an acid that will rust stainless. And it’s brown oxide rust, induced by the stripping of protective oxides by the HCl.
Remember, it’s stain LESS not stain FEEE. It merely stains less than regular steel. Not that it is unable to stain at all.
Haha I remember when I first realised that.
Well, linguistically speaking, adding the suffix "-less" to a word implies "without", in English.
You can be penniless, having not one penny. You can be homeless- not just fewer homes but by golly none at all. You could be endlessly fearless and remain motionless when facing things that aren't harmless.
Since we are on a tangent anyway I've just realised that the current British PM just this week appeared to be using the non-standard definition when talking with a "homeless" man at a soup kitchen over Xmas. I think I'm this case the PM was corrected fairly quickly but the assumption that homeless means having fewer homes explains a lot of government policy
I have to agree with you on that point but I still think that the PM needs a lot more time at that soup kitchen before he starts understanding the problems it's actually his job to fix.
If English followed stricter rules "stainless" would mean "without stain", and to convey "unable to be stained" we would always use "unstainable" instead, a real but rarely used word.
> Well, linguistically speaking, adding the suffix "-less" to a word implies "without", in English.
It's also frequently an exaggeration and I doubt most people who heard "I'm penniless" or "It's endless" would think "they have zero money" or "this will continue until I die (and beyond)".
It seemed like you were making a joke until you talked about "realizing" something. So on the off chance you weren't, you should know that "___less" and "___ free" have identical meaning. The -less suffix means "present in the quantity zero", not "present in a quantity which is smaller than some unspecified other quantity".
> The -less suffix means "present in the quantity zero"
Prescriptively, sure, maybe, why not. Descriptively? Nah. Otherwise anyone using the world "endless" would be lying because nothing can be "endless". Most people using "penniless", the same, although some people may truly be "without a penny" (depending on your definition of "having a penny" - do material assets count?)
No, descriptively that is what it means. You're trying to argue that people who say "the best thing since sliced bread" actually mean something different by the word "since" than everyone else, or indeed than what they themselves mean in other contexts, but that is obvious nonsense. People make false hyperbolic claims all the time; hyperbole is not expected to be true.
It may have meant the absolute, because the term was likely invented by a marketer, but in reality "stainless" steel has never been rustproof. It really depends on the alloy, plus the manufacturing quality. Lower grades with less chromium rust pretty easily, just not nearly as much as regular carbon steel.
Another fun fact about "stainless steel": it's usually said to be non-magnetic: you can't stick a magnet to it. But again this isn't completely true: the lower grades are indeed magnetic, but the higher chromium-content ones are not.
Why bother with a low-chromium-content SS? Probably either cost, or performance reasons. The high-chromium alloys are more brittle.
What do the numbers mean?(slightly confused because your description of 316 makes it sound generally more badass than 304, but I can’t fit that in sequence with 430 being the worst).
Manufacturing quality matters, too. Cheap stainless steel (e.g. basically anything from a Lowes or Home Depot, or elsewhere standards aren't rigorously enforced, like so much exterior cladding, trim, and fasteners on modern construction) will often rust almost immediately in spots as the alloying isn't sufficiently homogenous or pure. See https://en.wikipedia.org/wiki/Nickel_pig_iron
I recommend to take a look at the research paper because it has a few drawing that show how the Cl- and H+ [1] are connected with the molecules of the solvent. I like Lowe's post very much, but this really need a few graphics.
[1] Note that the H+ is never alone. It's too difficult to remove that electron from the Hydrogen, so H+ is a shorthand for something more complicated.
> I've never thought anything related to chemistry was simple
I was an EE/CS major and getting A's in the two chemistry courses I took in college is one of my proudest achievements because I had to work so hard for them. Nothing about chemistry was intuitive or simple for me.
The first couple years of undergrad chemistry aren't too bad, it's stuff like advanced physical chemistry which finally beat my tiny brain into submission.
CS is much easier to grasp, the worst it really gets in undergrad is moderately difficult math.
No one has provided the actual answer yet. Here it is-
The gastroduodenal epithelium (stomach surface) is covered by a sticky mucus layer into which bicarbonate is secreted by surface epithelial cells. This bicarbonate-mucus layer creates a pH gradient with a near-neutral pH at the epithelial surfaces in stomach and duodenum, providing the first line of mucosal protection against luminal gastric acid.
We only learned this in the last 20 years so it’s relatively new science
Edit: either I misread the question or it was edited
Got to learn about tooth development, and the fact that our cells secrete apatite to then accrete into enamel. Quite mind blowing the diversity of stuff our cells can emit.
> ...and how does it not destroy the nutritional value of the food?
Acids destroying whatever they touch is mostly a "kiddie" version of reality. 99.9% of acids found outside of chemistry labs and industrial settings are no more dangerous or destructive (as acids) than soda pop, lime juice, or human vomit (all of which are acids).
The reason is that some far stronger and/or more dangerous acids occasionally show up in household settings. Example: "battery acid", splashed on the skin, can cause horrific & life-long damage in seconds. Teaching kids to stay far away from anything labeled "Acid" is kinda like teaching 'em not to play with fire.
> "battery acid", splashed on the skin, can cause horrific & life-long damage in seconds
In fact it's not true. For 98% suplhuric acid, you have at least 30 seconds to wash it away with just water and have no consequence at all (eyes is a different story, that's where it's actually seconds), and battery acid is only 35% or less.
And a really good stage magician can have a random audience member shoot him in the head with a loaded gun, and catch the bullet in his teeth, too. That I've heard, you don't see that trick performed too often. Because sometimes it goes horribly wrong.
> 99.9% of acids found outside of chemistry labs and industrial settings are no more dangerous or destructive (as acids) than soda pop, lime juice, or human vomit (all of which are acids).
> The reason is that some far stronger and/or more dangerous acids occasionally show up in household settings. Example: "battery acid", splashed on the skin, can cause horrific & life-long damage in seconds.
I had to look up "battery acid"; turns out it's sulfuric acid, which is well known to be very dangerous.
Of course, so is hydrochloric acid, which is the acid in human vomit. The destructiveness of the solution is not determined by the acid -- we use HCl because it is extremely destructive! -- it is determined by the acid's concentration.
Note that vomiting more than a normal amount (say, because you're bulimic) will cause noticeable acid damage to your teeth.
> "The destructiveness of the solution is not determined by the acid [...] it is determined by the acid's concentration."
I'd say you're at the "know enough to be dangerous" phase. The destructiveness of concentrated H2SO4 (sulfuric acid) to flesh is at least as much due to its extremely exothermic hydrophilic behavior as to its low pH. Hydrofluoric acid is typically weak (pH-wise) as an acid, but is a horrific delayed-action contact poison. Concentrated nitric acid is a dangerously powerful oxidizer, or worse. Yet concentrated phosphoric acid is arguably less dangerous than propane.
[DISCLAIMER - Neither HN nor I are in any way responsible if you pop any statement in this item your empty little head, and head off in search of your very own Darwin Award. If you have no professional safety training in handling dangerous and/or highly-concentrated acids, then you should completely avoid them.]
I'm struggling to see the relationship between my comment and your response. For example, you emphasize pH heavily, but I don't see that I mentioned it anywhere. Hydrofluoric acid will have all kinds of horrible effects on you, sure, but it can only have those effects if it's there. Whether it's there, and to what degree, is exactly what concentration measures. Aqua regia's ability to dissolve gold is unrelated to its pH. Does that mean it can't dissolve gold? Why bring up pH?
What point are you trying to make in response to the observation "99.9% of acids found outside a lab may be no more dangerous or destructive than human vomit, but that isn't particularly meaningful since the acid in human vomit is highly dangerous and destructive"?
Legit criticism. (And no, down in my harshly-worded disclaimer, you are not the "you".)
For a specified acid, both concentration and pH are fairly good, one-dimensional, metrics of how relatively dangerous/destructive various solutions are. (Or at least "to within household common sense". Anyone competent to cook spaghetti should understand that "10 gallons" or "boiling hot" would make the solution far worse than "3 drops" or "ice cold".)
What set me off was that your comment seemed far too close to "concentration is a good, one-dimensional, measure of how dangerous/destructive acids are." (Without the "specified acid" and "relatively" qualifiers.) That (or a similar statement with "pH" instead of "concentration") is the sort of "logical" simplification that kiddies and idiots can far too easily latch on to, when they're thinking about doing cool & dangerous things.
(So - NO, kiddies. The fact that 5% acetic acid is vinegar, which is safe to splash on your salad, does NOT make (say) 5% hydrofluoric acid safe in any way at all. Stay the hell away from the latter, unless you want to look like a cheap horror movie extra for the rest of your might-be-kinda-short life.)
> What point are you trying to make [...] 99.9% of acids found outside a lab [...] than human vomit [...]
Technically, vomit is (horribly impure) hydrochloric acid. But for household safety purposes? Any
good 11-year-old babysitter can safely clean up baby barf, using only routine baby-care supplies. But handling high-molar HCl, let alone dealing with spills of the stuff? That's a completely different world.
Low pH is needed for pepsin to work. Also to kill some pathogens. Doesn’t affect nutritional components of ingestion eg amino acids, vitamins and lipids.
The animals which are scavengers have more acidic stomachs than those which eat live prey, and the latter have more acidic stomachs than those which are herbivores.
This is consistent with the supposition that low pH is needed to kill some pathogens.
An interesting fact is that humans have a stomach pH in the scavenger range. This is consistent with the supposition that, before becoming the most successful hunters of this planet, for a long time humans had been mainly scavengers, who scared the other scavengers and predators by throwing sticks and stones, and then consumed completely the leftovers by breaking the long bones and the skulls with stones.
i. H+ in aqueous solution is synonymous with hydronium ion (H3O+).
ii. Water is in an equilibrium autoionization reaction with itself and the hydronium and hydroxide ions:
2H2O <=> H3O+ + OH-
A pH of 7 means the negative log of the hydronium ion concentration is 7. Or about 10^-7 hydronium ions per liter. This reflects that the reaction above greatly favors water in the H2O state. We tend to make such simplifications (H+ for H3O+) in order to make teaching easier, but at some point it impedes a deeper understanding of what's really going on.