Consider the switch from $$\operatorname{pH} = -\log_{10}\left(\left[\ce{H3O+}\right]\right)$$ to $$\operatorname{acidity} = +\log_{10}\left(\frac{\left[\ce{H3O+}\right]}{\left[\ce{OH-}\right]}\right)$$ in water.
I'm calling it "acidity" just so that I can refer to it below, don't worry about the name too much.

  • The neutral point is 0, when $\left[\ce{H3O+}\right] = \left[\ce{OH-}\right]$.

  • Acid solutions have a positive "acidity" instead of < 7.

  • Basic solutions have a negative "acidity" instead of > 7.

  • Both increase in strength as their value increases in absolute value.

  • The scale is open on both sides, which makes sense because there is no reason to restrict pH to [0, 14]. I know it's not restricted in reality, but for some reason I was taught that at some point.

  • In water, jumping from one scale to the other is as easy as $$\operatorname{acidity} = 2 \times (7 - \operatorname{pH}).$$

So why is that metric not used? I'm not a chemist so admittedly I have not seen enough chemistry works and maybe this scale exists, but in any case it's not mainstream while it seems like it would be easier to understand for the common folk.

  • 1
    $\begingroup$ pH as an acidity parameter is just its secondary usage. $\endgroup$
    – Poutnik
    Commented Sep 3, 2023 at 13:23
  • $\begingroup$ Btw, pH = - log ([H3O+]) is just approximate equality for low enough ionic power. More exactly, the thermodynamic activity should be used. $\endgroup$
    – Poutnik
    Commented Sep 3, 2023 at 14:24
  • 2
    $\begingroup$ This is not the worst thing we do because of traditions. $\endgroup$ Commented Sep 3, 2023 at 14:49
  • 1
    $\begingroup$ What is the use of this new scale ? $\endgroup$
    – Maurice
    Commented Sep 3, 2023 at 16:53
  • $\begingroup$ @Maurice Same as pH but with intuitive signs and neutral point. $\endgroup$
    – Guimoute
    Commented Sep 3, 2023 at 19:07

1 Answer 1


This was suggested on April 1st 2019 but did not gain traction. Sometimes, the historical definitions prevail even when there are (with hindsight) better definitions. For instance, learning chemistry might be easier if the electron had a positive charge and the proton a negative charge.


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