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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 p

by 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.

by toddm 4y ago

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.