Clarifying SWCG chemical products

[Aqua Lab Rat]

I've read several postings on here including the Pool Water Chemistry one by Chem Geek. That has gotten me pretty far along in understanding that Na+ and Cl- are separate ions when dissolved, which by itself dispels lots of mythology. And I'm on-board with aeration being the primary cause of pH rise. But I could use some clarification to firm up a couple of things in my mind.

1. Does hydroxide at the negative plate NOT form sodium hydroxide, or is it just completely irrelevant whether or not there are sodium ions involved, since you still have hydroxide that gets at least partially negated by the products at the other electrode (see #2)?

2. Chem Geek's post shows two of the products of the positive plate as H+ and Cl- instead of showing HCl. Is that because the hydrogen ion and chloride ion are the same as what you'd get by adding HCl to water?

I just want to have all this straight in my brain to make sure that I don't pass along myths. I give mini pool chemistry presentations on things like chloramines, CYA, etc. and you can almost see people's brains twisting because they have bought into myths for 20 years.

 

[JoyfulNoise]

It really doesn't matter what the counter ion is to hydroxide as OH- is a strong base and remains completely dissociated in water from the other positive ions.

Chloride (Cl-) is NOT formed at the other electrode. Chloride is oxidized back to chlorine in a +1 state (Cl+). This effectively forms chlorine gas which dissolves rapidly and dissociates into hypochlorous acid, hypochlorite and chlorinated cyanurates.

The hydrogen ion (H+) at the electrode forms hydrogen gas (H2) which has very low solubility in water and bubbles out of the returns.

 

[Aqua Lab Rat]

I know chloride isn't formed at the anode, but in the Pool Water Chemistry sticky post, the "which nets out to..." section shows that when the Chlorine gas is added to water, you get "HOCL + H+ + Cl-". Wouldn't the chlorine gas added to water form HOCL + HCL, with the HCL dissociating into H+ and Cl- ? It's not an explicitly shown step but I assume that's what happens, or is that wrong? The cathode generates hydrogen gas and I get that.

Keep in mind that I'm not a chemist, just a marginally self-educated lab rat (my actual education is electronics). It's ok to hit me with a science book until I get the cheese.

 

[JoyfulNoise]

So this is the relevant section from the Pool Water Chemistry post (I added embellishments) -

At the anode (positive plate):
EQ. 1 2Cl- --> Cl2(g) + 2e-

At the cathode (negative plate):
EQ. 2 2H2O + 2e- --> H2(g) + 2OH-

which nets out to the following where the chlorine gas dissolves in water:

EQ. 3 2H2O + 2Cl- --> Cl2(g) + H2(g) + 2OH-
EQ. 4 Cl2(g) + H2O --> HOCl + H+ + Cl-
EQ. 5 H+ + OH- --> H2O
----------------------------------------------
EQ. 6 2H2O + Cl- --> HOCl + OH- + H2(g)

So, you add EQ. 1 and EQ. 2 together and that produces EQ. 3. The dissolution of chlorine gas (Cl2) in EQ. 3 gives you EQ. 4. Hydronium (H+) and Hydroxide (OH-) add together in EQ. 5 to give water (the basic water dissociation equation that gives us pH). When you add EQ. 3, EQ. 4, and EQ. 5 together, you get the overall equation which is EQ. 6.

If you looked at it explicitly it would look like this in expanded form -

2H2O + 2Cl + Cl2(g) + H2O + H+ + OH- ---> Cl2(g) + H2(g) + 2OH- + HOCl + H+ + Cl- + H2O

When atoms and molecules exist on both sides of the reaction, you can simply cross them out. Chemical reactions happen, for the most part, very quickly and so the intermediate reactions can be ignored to a certain extent while the overall reaction, EQ. 6 is what really exists. The details of the intermediate reactions are just not separable on an observable timescale unless you were to somehow physically compartmentalize the anode and cathode reactions (which happens when one uses a diaphragm cell in the color-alkali process).

 

[Aqua Lab Rat]

OK, thanks. I think where I'd gotten hung up is that I keep comparing it to adding chlorine gas to water, which I've seen represented as producing HOCL + HCL, instead of looking at the whole pie. Overall it's more like adding liquid bleach to water, which has been talked about on other threads.

 

[JoyfulNoise]

OK, thanks. I think where I'd gotten hung up is that I keep comparing it to adding chlorine gas to water, which I've seen represented as producing HOCL + HCL, instead of looking at the whole pie. Overall it's more like adding liquid bleach to water, which has been talked about on other threads.

Correct. The net effect of running an SWG or adding bleach are equivalent. The pH shift seen with bleach comes from the small amount of lye added with each dose and, for an SWG, it's CO2 outgassing from aeration.

Note that if the SWG were attached to a really short length of pipe before entering the pool, chlorine gas could possibly escape and that would cause a very large increase in pH.