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Need help understanding the production of Hypochlorous Acid and byproducts

The goal is to aid in the fight against COVID-19 by producing stable Hypochlorous Acid at 200 ppm FAC or more to sanitize hospitals and high touch places through the use of a fogger. HOCl is far more efficient than bleach and much safer. 

I am researching the creation of HOCl through the electrolysis of pure water with 40g of pure table salt NaCl per liter, with and without a Bipolar Membrane.

This is the best explanation of what happens that I have. But I am not sure it is correct. I feel there is more to it and more chemical compounds created.

2 NaCl + 6 H2O + Electricity = 2 HOCl + O2 + 4 H2 + 2 NaOH


2 NaCl + 6 H2O + Electricity = 2 HOCl + 2 H2O + 2 H2 + 2 NaOH

or simplified to

2 NaCl + 4 H2O + Electricity = 2 HOCl + 2 H2 + 2 NaOH

I especially need to know what gases are released at each end (Anode & Cathode). As they can be a safety hazard. I thought they were Oxygen O2 and Hydrogen H2. However several articles have referenced Chlorine being released. I thought it had to stick around to create the HOCl and if that is not true there is something very flawed with my equations above.

This diagram shows chlorine gas released at the Anode.

Chemistry of Hypochlorous Acid 

This site gives chemical equations I do not know how to read.

Electro-Chemical Activation 

This diagram shows Oxygen gas released at the Anode.

Fig. +. Schematic of electrolyzed water generator and resulting compounds.  | Download Scientific Di... 

This diagram also shows Oxygen gas released at the Anode.

What is HOCl? – eco-i-on 

This is a question-answer that clearly says Oxygen is not released. 

6 Replies
New Contributor

Re: Need help understanding the production of Hypochlorous Acid and byproducts

First off, hypochlorous acid is inherently unstable. Even sodium hypochlorite isn't particularly stable but hypochlorous acid is even more unstable.  Metals catalyze the decomposition of hypochlorous acid so any traces of metals will cause its decomposition. So if you're planning on making hypochlorous acid, you'll need to be using it soon after you've prepared it.

You will be better off adding some base to the sodium chloride. This will convert any chlorine gas produced at the anode to sodium hypochlorite. It will also stabilize any hypochlorous acid formed by converting it to sodium hypochlorite. You can then convert that to hypochlorous acid by the addition of HCl. But, you need to be careful that the pH doesn't drop to the point where it reverts to chlorine gas. Any pH below 5 will be problematic.

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New Contributor III

Re: Need help understanding the production of Hypochlorous Acid and byproducts

Alex, I  second Christopher's good advice about keeping the solution to pH greater than 5 (neutral or basic). If you were to go acidic, to pH <5, you would generate Cl2. Here are some thoughts about sanitizing high-touch places and about the chemistry involved.

First, about sanitizing in hospitals.The aqueous solution that you are spraying is 200 ppm HOCl (~4 x 10-3 M) and 40g/L (~0.67 M) NaCl. You will be leaving a solid residue, the salt. Is that OK? Hospital settings including emergency vehicles have a lot of electrical equipment, valves, and fine moving parts. Is it OK from a design viewpoint to use a salt fog and leave behind a solid residue? HOCl may well be OK from a personal safety viewpoint, but it's going to release Cl2 unless it's already reacted with the biologicals as intended, or with some of the equipment. Will that be OK at the concentrations you intend and with the ventilation available? The design and engineering are going to be important. You may want to look at what De Nora ( is offering as disinfecting equipment; their chlor-alkali technology is quite strong and it's an informative website.

As for "can I really make HOCl this way?" the answer is yes. For the work you are doing you may need to look at the chlor-alkali chapters in the Kirk-Othmer Encyclopedia of Chemical Technology or Riegel's Handbook of Industrial Chemistry. The references that perplexed you are true under some conditions but maybe not under the conditions where you will operate! 

Here is a short version why it's a real research project to get the conditions you want and the HOCl concentration you want for spraying. If you were electrolyzing pure water, the simple equations are:

         4H (aq)+ + 4 e- = 2 H2(g)                      at the anode

         4 OH-(aq)         = 2 H2O + O2 (g) + 4e- at the cathode

That's not the full story, though. At the anode you first get  

           OH-(aq)          =  OH. (on the cathode) + e-    You will want that OH radical to make HOCl.  Then

         2OH. (cathode) =  H2O2 that might desorb before it can make water.

If you electrolyze a concentrated salt solution, 350 g/L instead of the 40 g/L you want, you make Cl2 at the cathode:

         2 Cl-(aq)           = Cl2 (g)           + 2e-      That's two steps too:

            Cl-(aq)           = Cl. (cathode) +  e-         

If you electrolyze a 40 g/L salt solution as you intend, you produce Cl2 and also O2 at the cathode. Because you are making OH. and Cl. at the cathode, they can combine to make HOCl and go off in solution so you can spray it. As Christopher said, best to keep the pH at 5 or greater. If you go  more acid, you make Cl2 instead. If you go strongly basic (high pH) you get OCl-. It's quite a project!

Stay safe and God bless all here,

Mike Dowell

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New Contributor

Re: Need help understanding the production of Hypochlorous Acid and byproducts

The swimming pool industry has been doing this for years but for much lower concentrations of free available chlorine (FAC). Now some of the manufactures are adapting their products to make 200 - 1000 ppm FAC solutions.


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New Contributor II

Re: Need help understanding the production of Hypochlorous Acid and byproducts

I think one has to answer if the proposed cure is worse than the illness here?

Inhaling 200 ppm FAC mist during fogging is corrosive to lung tissue, eyes, skin, etc. plus damages equipment and finishes.   Swimming pool water is biocidal on the order of a few 1-3 ppm.  

Leave it to the experts, please.   The questions raised do not indicate a fundamental understanding of what is proposed to even carry out the experiments safely.   They are plenty of other common agents for this. 

Also chemists have responsibly here to respond responsibly as well.

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New Contributor

Re: Need help understanding the production of Hypochlorous Acid and byproducts

I have a question.
What is the difference between creating hypochlorous acid and sodium hypochlorite.
I bought 2 different machines one produces hypochlorous acid and the other sodium hypochlorite.
But in both I use salt and water and both (I think) use the same generation system (Single Cell Electrolysis)
Maybe Im wrong thats why I am not understanding. Im searching in the web but none explain that.
Why one product or the other?
I hope you can help me.


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New Contributor

Re: Need help understanding the production of Hypochlorous Acid and byproducts

Id be really interested to know the answers too...

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