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In a recent inorganic chemistry lab session, we tested the pH of various ions in water. The water turned out to be on the acidic side (5.5 with $\mathrm{pH}$-paper, 5.59 with $\mathrm{pH}$-meter). A solution of $\ce{Mg^{2+}}$ gave some strange results. The $\mathrm{pH}$ kept rising when using a $\mathrm{pH}$-meter, going from around 4.2 to 5.83 (higher than that of the pure water). We stopped at this point, thinking something must be wrong. A classmate experienced the exact same thing, and stopped at 6.5 or something.

I expected $\ce{Mg^{2+}}$ to act as a very weak cation acid, making the solution at least somewhat more acidic than the water. In my limited knowledge, I would think that the only thing $\ce{Mg^{2+}}$ can do in this scenario is to slightly polarize the water ligands, thus making them deprotonate.

Was this a fault in the equipment, or was there some other reaction going on?

I'm having a hard time finding expected $\mathrm{pH}$ of $\ce{Mg^{2+}}$ in aqueous solution, so any data on that would also be welcome.

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  • $\begingroup$ Read this article. $\endgroup$ Mar 5, 2021 at 7:02
  • $\begingroup$ Thanks, but this doesn't explain my results. I know what cation acids are, and Mg$^{2+}$ should be a very weak one, but still lower the pH of the water. $\endgroup$
    – Quantonium
    Mar 5, 2021 at 7:44
  • $\begingroup$ The pH of pure water in contact with air is never $7$. It is about $5.5$, due to the action of $\ce{CO2}$ in air, which is slightly soluble in water and produces some ions $\ce{H3O+}$ . As the amount of $\ce{CO2}$ in air is not constant and changes from time to time, the pH of pure water is also changing in time. $\endgroup$
    – Maurice
    Mar 5, 2021 at 9:46
  • $\begingroup$ That's fine, but the water pH was tested straight before the water+Mg${^2+}$ mixture was tested, and the latter still showed a higher pH. That shouldn't make any sense, am I right? $\endgroup$
    – Quantonium
    Mar 5, 2021 at 11:08
  • $\begingroup$ pH meters are not very accurate, and don't give consistent results. I have used one in my lab, and the pH value kept changing even though it was dipped in a buffer solution, which is supposed to have a constant pH, regardless of CO2 from air etc. I had to recalibrate it every 5 minutes or so, therefore, you shouldn't place too much importance on the pH meter reading changing from 4.2 to 5.83, that may well be machine error. $\endgroup$
    – S R Maiti
    Mar 5, 2021 at 16:03

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