For example, could one define a $\mathrm{pH}$ for pure acetic acid? It's a weak acid in water, but if someone handed you $1~\mathrm L$ of pure acetic acid, what would its $\mathrm{pH}$ be?

  • $\begingroup$ pH is usually for more or less diluted water solutions. But here is some insight chemistry.stackexchange.com/a/7000/9961 $\endgroup$
    – Mithoron
    Jun 11 '16 at 22:40
  • $\begingroup$ Is it? Strictly, it's just $-\log_{10}{\left[\ce{H+}\right]}$, right? Acetic acid may be a bad example, since I would think it wouldn't auto-ionize, but in general, if any solvated proton is present, I would think a $\mathrm{pH}$ could be defined, even if it wasn't particularly useful. $\endgroup$
    – hBy2Py
    Jun 11 '16 at 22:43
  • $\begingroup$ Edited, but it simply doesn't work the same in other situations. $\endgroup$
    – Mithoron
    Jun 11 '16 at 22:46
  • $\begingroup$ @Brian It does auto-ionize, but in pure CH3COOH will [CH3COOH2+] be more important then concentration of acid itself? In what context? $\endgroup$
    – Mithoron
    Jun 11 '16 at 22:53
  • $\begingroup$ H0 function is used is such cases. $\endgroup$
    – Mithoron
    Jun 11 '16 at 22:58

The IUPAC definition of pH is:

The quantity pH is defined in terms of the activity of hydrogen(1+) ions (hydrogen ions) in solution:

$pH = − lg [a(H^+)] = − lg [m(H^+) γ_m (H^+) / m^⦵]$

where a$(\ce{H+})$ is the activity of hydrogen ion (hydrogen 1+) in aqueous solution, $(\ce{H+})$(aq), $γ_m(\ce{H+})$ is the activity coefficient of $\ce{H+}$(aq) (molality basis) at molality m$(\ce{H+})$, and m$^⦵ = 1$ mol kg $^{−1}$ is the standard molality.

So since the definition specifically refers to "aqueous solution", pH is undefined unless an aqueous solution is being considered.

There would be a p[H+] in pure acetic acid based upon the self dissociation of acetic acid, where [H+] is the concentration of hydrogen ions in acetic acid solution, but this is not pH according to the IUPAC definition.

According to Acid-Base Equilibria in Glacial Acetic Acid. III. Acidity Scale. Potentiometric Determination of Dissociation Constants of Acids, Bases and Salts J. Am. Chem. Soc., 1956, vol. 78, pages 2974–2979:

the autoprotolysis constant of acetic acid is calculated to be $3.5 \times 10^{-15}$ (pK = 14.45)

Therefore p[H+] = 7.2 for pure acetic acid.

This value is not a measure of acidity, but simply the concentration of solvated hydrogen ions in pure acetic acid.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.