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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

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.

A4ET8a8uTh0 4y ago
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 [1]https://www.usgs.gov/special-topics/water-science-school/science/conductivity-electrical-conductance-and-water https://www.usgs.gov/special-topics/water-science-school/sci...
- basementcat 4y ago
  Water ice is a semiconductor and can be doped, like silicon, to make transistors. https://m.youtube.com/watch?v=o61wtb7dSRA https://m.youtube.com/watch?v=o61wtb7dSRA
  - XMPPwocky 4y ago
    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.
    - brendank310 4y ago
      Why not? It's not practical for traditional uses of semiconductors, but it has a mechanism to control conductivity.
      - melony 4y ago
        Put it in a bell jar and pump the air out. Violà, vacuum tube.
      - ilyt 4y ago
        Because then you'd need to call resistors "semiconductors" coz they also change resistance with temperature and that's not useful to anyone.
        adrian_b 4y ago
        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.
    - kragen 4y ago
      it's true that conventional insulators also have a negative tc but as i understand it that's because they're just really cold semiconductors
    - adrian_b 4y ago
  - mikewarot 4y ago
    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.
dvh 4y ago
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"
- alwayslikethis 4y ago
  "negative decimal logarithm"
vore 4y ago
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/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2946644/
- aabajian 4y ago
  Ha, I wonder if that structure provides any more insights into pH.
- nyanpasu64 4y ago
  The water memory folks will be up in arms over this discovery!
mtlmtlmtlmtl 4y ago
Interestingly this was all in the curriculum for my high school chemistry class.
- hilbert42 4y ago
  Yes, it was. And it'd be nice if they left it alone (so I didn't have to relearn stuff a new every decade or so). ;-) Chemistry's a quixotic master.
dthul 4y ago
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.
acchow 4y ago
A key concept in biochemistry/chemistry is that almost all reactions are in some equilibrium between products and reactants, even H2O itself
toddm 4y ago
Ph.D. in chemistry here. 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.
- rcthompson 4y ago
  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?
  - fuzzfactor 4y ago
    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. Don't try this at home.

A4ET8a8uTh0 4y ago

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 [1]https://www.usgs.gov/special-topics/water-science-school/science/conductivity-electrical-conductance-and-water https://www.usgs.gov/special-topics/water-science-school/sci...

basementcat 4y ago
Water ice is a semiconductor and can be doped, like silicon, to make transistors. https://m.youtube.com/watch?v=o61wtb7dSRA https://m.youtube.com/watch?v=o61wtb7dSRA
- XMPPwocky 4y ago
  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.
  - brendank310 4y ago
    Why not? It's not practical for traditional uses of semiconductors, but it has a mechanism to control conductivity.
    - melony 4y ago
      Put it in a bell jar and pump the air out. Violà, vacuum tube.
    - ilyt 4y ago
      Because then you'd need to call resistors "semiconductors" coz they also change resistance with temperature and that's not useful to anyone.
      - adrian_b 4y ago
        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.
  - kragen 4y ago
    it's true that conventional insulators also have a negative tc but as i understand it that's because they're just really cold semiconductors
  - adrian_b 4y ago
- mikewarot 4y ago
  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.

dvh 4y ago

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"

alwayslikethis 4y ago
"negative decimal logarithm"

vore 4y ago

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/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2946644/

aabajian 4y ago
Ha, I wonder if that structure provides any more insights into pH.
nyanpasu64 4y ago
The water memory folks will be up in arms over this discovery!

mtlmtlmtlmtl 4y ago

Interestingly this was all in the curriculum for my high school chemistry class.

hilbert42 4y ago
Yes, it was. And it'd be nice if they left it alone (so I didn't have to relearn stuff a new every decade or so). ;-) Chemistry's a quixotic master.

dthul 4y ago

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.

acchow 4y ago

A key concept in biochemistry/chemistry is that almost all reactions are in some equilibrium between products and reactants, even H2O itself

toddm 4y ago

Ph.D. in chemistry here. 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.

rcthompson 4y ago
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?
- fuzzfactor 4y ago
  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. Don't try this at home.