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How is the pH scale constructed , I mean the strength of an acid is determined by its pKa value and this one can be determined in different solvents if water doesn't suits it but how do you determine the pKa value of a substance in water after you have found the value of its pKa in another solvent ? Also I don't think I quite understand why the pH interval in water is running from 0 to 14 (I know that it has to do with the leveling effect but when you take this scale for other solvents the starting value can be different from 0 , if I am not mistaken for liquid NH3 the starting value is 5 ) can someone explain to me from where does this values come from ?

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  • $\begingroup$ First of all the scale in water doesn't go from 0 to 14, it can extend beyond those values (values of -2 to 16 are possible without too much effort in a lab). The scale in liquid ammonia is much larger, covering over 30 units. See here and here, as well as several other Chem.SE posts tagged with pH. $\endgroup$ – Nicolau Saker Neto Dec 21 '19 at 15:32
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The pH is determined experimentally with a glass electrode. A glass electrode is a small glass tube limited by a thin membrane and filled with an aqueous KCl solution with a silver electrode inside. There is also a similar silver electrode out of the tube, in a compartment in contact with the outer solution. When you dip a glass electrode into an aqueous solution, the experiment shows that the tension E read between the two electrodes is proportional to the logarithm of the H+ concentration of the external solution. It is : E = Constant + 0.059 pH. The Constant depends of the particular glass membrane. And the experiment shows that pH is -log[H+] at low acidic concentrations, and is measured as

  • 7 in pure water,
  • 3 in 0.001 M aqueous solutions
  • 2 in 0.01 M aqueous HCl solutions,
  • 11 in 0.001 M aqueous NaOH solutions
  • 12 in 0.01 M aqueous NaOH solutions.

These values are in agreement with the usual expression : pH = -log[H+], or : pH = 14 - log[OH-]. At higher concentration, the measured pH values are not exactly equal to the logarithm of [H+]. The measured pH is said to be the logarithm of the "activity" of H+.

The pH does not go only from zero to 14. It may be smaller than zero in concentrated solutions of HCl and of H2SO4.

In non-conducting solutions, the glass electrode does not give any stable values. The measured tension is fluctuating without any reason and can be ± 3, 5, 10 or 0, and this changes constantly.

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    $\begingroup$ Maurice's great experimental depiction clearly notes the measurements are created in DILUTE conditions. Otherwise, the 'activity coefficient' is important which is a function of ion charge, ion size and ionic strength. So, in the presence of high ionic concentrations of salts like FeCl3 or CaCl2 with a large ion charge, even dilute HCl can apparently 'act' as a much stronger acid due to the activity coefficient which is applied to [H+] to determine the pH (see csun.edu/~hcchm003/321/321100313.pdf page 4). $\endgroup$ – AJKOER Dec 21 '19 at 18:19
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The pH+ scale is a logarithmic scale, based on the number of $\ce{H+}$ ions available (with the caveat that $\ce{H+}$ is theoretical, it may be $\ce{H3O+}$ or other ionic species). I.E., in water at ~300 K, ~10−7 mol dm−3 of the positive and of the negative ions exist, a pH+ of 7 (the negative exponent).

For a strong acid, virtual all, 100%, of the molecules are broken into ions, e.g. $\ce{Cl-}$ and $\ce{H+}$, so the pH+ is 0 [10−0 = 1].

Note that at different temperatures, water may be more or less ionized, so a neutral pH+ is not necessarily 7. At its boiling point, "neutral" is about 6.14.

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