SWG Operation - Lessons Learned

[chem geek]

I understand that when I take H20, and NaCl and pass them across the electrically charged plates, I can see how H2 and Cl2 can be formed.

Since the H2 comes from water, the remaining oxygen will dissipate.
However the Cl2 come from NaCl. What happens to the sodium, where does it go?

At the cathode (the negatively charged plate), hydrogen is produced from either of the following reactions:

2H+ + 2e- --> H2(g)
2H2O + 2e- --> H2(g) + 2OH-

Note that oxygen gas is NOT produced and does not dissipate. Instead hydroxyl ion (OH-) is created. Either of the above can be considered to be occurring since there is an equibrium that always exists in water between its split components, though the vast majority remains as the water molecule at normal pool pH:

H2O --> H+ + OH-

At the anode (the positively charged plate), chloride ion gets converted to chlorine gas that then dissolves in water to from hypochlorous acid and hydrochloric acid as follows:

2Cl- --> Cl2(g) + 2e-
Cl2(g) + H2O --> HOCl + H+ + Cl-
----------------------------------------
2Cl- + H2O --> HOCl + H+ + Cl- + 2e-

As you can see, the sodium doesn't participate at all in these reactions. When you add salt to water, it dissolves into its two components:

NaCl(s) + H2O --> Na+ + Cl- + H2O

Nothing happens to the sodium ion in a salt cell. Only the chloride ion participates in any reaction. The sodium ion does contribute to the general conductivity of the water. The area around the cathode accumulates negatively charged ions while the area around the anode accumulates positively charged ions so the sodium ion and hydrogen ion (which are positively charged) will tend to migrate from the anode to the cathode while chloride ion and hydroxyl ion (which are negatively charged) will tend to migrate in the opposite direction.

Now it is possible for another reaction to occur at the anode and competes with the production of chlorine as follows:

2H2O --> O2(g) + 4H+ + 4e-

So it is possible for oxygen gas to be produced in addition to chlorine, but due to the differential liklihoods (activation energies) of these two reactions, the production of chlorine is the primary reaction.

There are some websites that talk about "splitting" the sodium chloride as part of the electrolysis in the salt cell, but basically they have no idea what they are talking about. It is simply not true. Sodium Chloride (salt) "splits" all by itself when dissolved in water (in fact, that is how it is able to dissolve in water) and sodium has nothing to do with the electrochemistry of making chlorine.

Richard