# How does pH change during the electrolysis of water? [closed]

How does the pH change during electrolysis of a water and magnesium sulphate solution? Also, will this pH change happen every time? Does the pH change differently when there is just water in the anode and cathode?

• If u electrolyse a solution. pH change is uniform no demarcation between anode and cathode pH as the electrolyte is the same. – Karmanya GB Mar 20 '18 at 14:13

The pH will increase or decrease or stay the same, depending on where and when and how you look.

At the cathode, $\ce{Mg^{2+}}$ ions will be reduced to magnesium metal. The magnesium will immediately react with water to evolve $\ce{H2}$ gas and form $\ce{Mg(OH)2}$, which is a strong base (though not very soluble). The pH increases near the cathode. The $\ce{H+}$ concentration due to the slight acidity of $\ce{MgSO4}$ is negligible but any $\ce{H+}$ which is removed by being turned into $\ce{H2}$ gas results in the same effect: the pH increases at the cathode.

At the anode, $\ce{SO4^{2-}}$ anions are oxidized, $\ce{O2}$ is evolved and the sulfate ion is converted into $\ce{SO3}$ and hydrates to $\ce{H2SO4}$. The pH at the anode decreases as sulfuric acid is formed.

If you stir the solution and then take a pH measurement, the pH will not have changed, because the $\ce{H2SO4}$ will react with the $\ce{Mg(OH)2}$ to produce $\ce{MgSO4}$ again, and the only change will have been that $\ce{H2O}$ was converted into $\ce{H2 and O2}$.

• Hi James! I see that you’ve been here for quite a while. Please use MathJax to format questions and answers. Thanks!. – MollyCooL Mar 21 '18 at 2:00

The pH will decrease, because magnesium sulphate is acidic.

As the electrolysis proceeds, the magnesium sulphate becomes more concentrated.

Your electrolysis will split water into it's compounds, you just have to look at redox potentials.

$\ce{2H+ + 2e− ⇌ H2}$ E° = 0 V

$\ce{Mg^2+ + 2e− ⇌ Mg}$ E° = −2,362 V

The redox potential of hydrogen is higher, thus it will be reducted before Magnesium. It is similar with the sulphate and oxygen, in this case the oxygen will be oxidated.

Edit:

DavePhD is right! The pH will drop a little bit until the solubility limit has been reached, then it should stay constant until there is no water left; no water no pH.

In addition to that Magnesium sulphate has a good solubility in water, the pH will decrease for a longer time until precipitation.