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I was checking the $\mathrm{pH}$ of $\pu{1 M}$ chromic acid solution, and I got its $\mathrm{pH}$ around -1.7. Yet, $\mathrm{pH}$ of $\pu{0.4 M}$ solution was around -0.6. Could these values be correct? The $\mathrm{pH}$ meter was calibrated before use.

I'm thinking not. I guess $\mathrm{pH}$ meters work in the interval 2-12 $\mathrm{pH}$.

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    $\begingroup$ I bet you were outside of the calibration curve and the extrapolation failed. Be also aware activity coefficients go wild for concentrated solutions and they can also affect the electrode functionality. Saying that, pH scale IS defined for negative values as well. $\endgroup$
    – Poutnik
    Apr 8, 2021 at 12:37
  • $\begingroup$ How about looking in your pH meter's technical documentation? $\endgroup$
    – fraxinus
    Apr 9, 2021 at 6:13
  • $\begingroup$ @Poutnik Is it even possible to meassure the pH in these extreme conditions? I am aware that negative pH exists theoretically. But can a glass pH electrode be even used to do that? I guess the electrode needs to be calibrated to work in the area around 0 pH $\endgroup$
    – marietiara
    Apr 9, 2021 at 9:12
  • $\begingroup$ Negative pH exists even practically, but measuring it is challenging and not necesserily involves a glass electrode. $\endgroup$
    – Poutnik
    Apr 9, 2021 at 10:06
  • $\begingroup$ I remember the old days when my teacher told me the good practical standard for pH 0.0 is 2 M H2SO4. Is has dissociated mosly just one hydrogen and activity coefficients just climbed back to 1 ( and will climb >1 for more concentrated acid ). But expect the glass electrode would be drifting and it may decrease its life or damage its responses. $\endgroup$
    – Poutnik
    Apr 9, 2021 at 10:41

1 Answer 1

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You are most likely getting an inaccurate value. As anticipated in Poutnik's comment above is difficult to reliably measure a $\mathrm{pH}$ of very acidic solution but the $\mathrm{pH}$ scale is indeed open, and negative $\mathrm{pH}$ are a real thing.

It is easy and more meaningful to state a very high concentration or activity rather than reporting inaccurate $\mathrm{pH}$. This, together with typical figures found in textbooks, has spread the misconception that $\mathrm{pH}$ is bound in the range 0 - 14, but it is not.

A simple example of a solution with negative $\mathrm{pH}$ is in every lab, namely conc. $\ce{HCl}$, 37% by mass, $\mathrm{pH} = -1.1$.

Edit: For sake of clarity, as for its definition, pH can exceed 14, too. As an example, a saturated NaOH aqueous solution has pH = 15.

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    $\begingroup$ Also, activity coeficients for H3O+ in 10-12 M HCl go "through the roof", above 10 or even 20. $\endgroup$
    – Poutnik
    Apr 8, 2021 at 12:17
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    $\begingroup$ @Poutnik indeed in my first lab they were scared of traces of acid or even water in the halogenated waste iron barrel. They claim a kind of superacid due to local extreme activity. Beside, super activity of H+ is at the base of some research in synthesis. $\endgroup$
    – Alchimista
    Apr 8, 2021 at 16:20
  • $\begingroup$ @Alchimista Reminds me of my first lab, they/we were (to a modern standard) extremely callous regarding what goes in the sink directly - but very careful about halogenated solutions and how they were not to go in the sink. $\endgroup$
    – Stian
    Apr 9, 2021 at 7:53
  • $\begingroup$ I'm writing a paper on the use of chromic acid on some inorganic material. The concen used were 1-0.01 M and my pH meter said the range is from -0.6 to 1.7, actually. In the literature the pH of the chromic acid solution is often related to the size of the polyanions that can be found. Which is important to us. I don't want to report something incorrect values, maybe I can calculate the pH. My boss asked me to report it. @Alchimista $\endgroup$
    – marietiara
    Apr 9, 2021 at 9:27
  • $\begingroup$ @marietiara Well that is another aspect. Again a mere calculation would imply to know exactly what is going on on the activity side. But surely it is something to be discussed. $\endgroup$
    – Alchimista
    Apr 9, 2021 at 10:11

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