0

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.

Mathew Mahindaratne
  • 39,943
  • 27
  • 55
  • 107
Quantonium
  • 341
  • 1
  • 6
  • Read this article. – Mathew Mahindaratne Mar 05 '21 at 07:02
  • 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. – Quantonium Mar 05 '21 at 07:44
  • 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. – Maurice Mar 05 '21 at 09:46
  • 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? – Quantonium Mar 05 '21 at 11:08
  • 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. – S R Maiti Mar 05 '21 at 16:03
  • What magnesium salt did you use? Magnesium hydroxide is sufficiently strong so that its salts with weak acids could be basic. – Oscar Lanzi Mar 05 '21 at 21:16
  • Mg(NO$_3$)$_2$. I guess I have gotten my answer from all the comments above. It doesn't make sense that Mg$^{2+}$ solution has higher pH than water. – Quantonium Mar 06 '21 at 05:22

0 Answers0