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Topic: Sodium hypochlorite vs Hypochlorous acid  (Read 1138 times)

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Offline Inorganic

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Sodium hypochlorite vs Hypochlorous acid
« on: February 05, 2019, 11:48:23 PM »
Hello everyone.

I have a question and hoped someone would be kind enough to give me an explanation in layman's terms as chemistry is not my strength.  I would like to better understand the difference between sodium hypochlorite and hypochlorous acid.
 
As for the context of my question, I am a hobbyist farmer who likes growing a variety of vegetables such as lettuce, swiss chard, and tomatoes. I have used a few hypochlorous acid products over the years always with good results, but the products I have used are a bit expensive. The HClO is mixed into the resevoir and helps prevent bacteria from taking over the solution.

I recently had read that bleach could be used as a cheap alternative and that "Sodium hypochlorite becomes hypocholorous acid when it comes into contact with water", but I have no idea if there is any truth to that. The poster also gave the following "NaClO + H20 -> HOCl + NaOH
Which I assume is saying bleach + water = hypochlorous acid + lye. Is it possible to produce hypochlorous acid without producing lye or isolate it? I don't see lye listed on the MSDS sheet of the hypochlorous acid product I have been using and would think it wouldn't be great for plants.

Thanks for reading through my unintelligent post, and any help is appreciated.



Offline Borek

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Re: Sodium hypochlorite vs Hypochlorous acid
« Reply #1 on: February 06, 2019, 02:52:55 AM »
I recently had read that bleach could be used as a cheap alternative and that "Sodium hypochlorite becomes hypocholorous acid when it comes into contact with water", but I have no idea if there is any truth to that.

It is not entirely correct, but it is also not completely wrong. It is one of these situations where oversimplification hides the reality.

Hypochlorous acid is - not surprisingly - an acid. When it reacts with lye it produces (in what is called a neutralization reaction) salt, sodium hypochlorite.

Trick is, hypochlorous acid is a very weak acid. Between other things it means in solutions of its salts hypochlorite anion (ClO-) will hydrolyze:

ClO- + H2O :lequil: HClO + OH-

So even if you start with the salt some minute amounts of the acid will be always present.

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Is it possible to produce hypochlorous acid without producing lye or isolate it?

Not easy. If anything, you could start with the bleach and add some hydrochloric acid

NaOCl(aq) + HCl(aq) :rarrow: HOCl(aq) + NaCl(aq)

(aq) means "dissolved in water" - and there is no practical way of isolating the acid and removing the salt.

Is this all about hydroponics? You have never mentioned it, but just growing things in soil doesn't call for any disinfectant. If so, IMHO the only problem with using bleach is pH control (IOW you should be very careful to add just enough acid to make the solution about neutral, or just a tiny bit acidic). Not easy without pH meter.
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Offline Inorganic

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Re: Sodium hypochlorite vs Hypochlorous acid
« Reply #2 on: February 06, 2019, 10:42:53 AM »
Borek, thank you sincerely for your response! :)

Yes, this is for a variety of hydroponic applications where there is no soil/soilless medium being utilized. If you are familiar with hydroponics I grow leafy vegetables in a raft type of setup, and larger plants I tend to do in an aeroponic system, if you aren't familiar and would like to know or see a little more of what I am doing, I would love to show you if interested. I am just a small hobbyist, so nothing too exciting. It's really great being able to run a high-intensity discharge light indoors, use that ambient heat to warm your house in the winter, and get fresh vegetables in the process.  ;D

I do have a ph meter as it is fairly essential when doing hydroponics to prevent a lockout of nutrients due to ph level.

Again, I must apologize for my ignorance, I am going to have to look more into this :lequil:,  is that the left equilibrium symbol? I think I need to pick up a book on chemistry, obviously, the Idiot's Guide to Chemistry sounds like it would be suited for my knowledge level. Any recommendations?

Thanks for your time!

Offline Borek

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Re: Sodium hypochlorite vs Hypochlorous acid
« Reply #3 on: February 06, 2019, 11:57:40 AM »
I am going to have to look more into this :lequil:,  is that the left equilibrium symbol?

Yes. What it means is that the equilibrium is shifted to the left - most of the ClO- put in the water when dissolving sodium hypochlorite will still be in ClO- form (beware: I was quite precise when selecting that wording, it is quite easy to misinterpret it here).

Sorry, no idea about good introductory books in English. In general any GenChem101 textbook will do, but they contain much more than you need here.

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It's really great being able to run a high-intensity discharge light indoors, use that ambient heat to warm your house in the winter, and get

If not for the fact you mentioned vegetables I would think it is about completely different plant ;)
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Offline Inorganic

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Re: Sodium hypochlorite vs Hypochlorous acid
« Reply #4 on: February 06, 2019, 02:28:23 PM »
I made this quick little video for you Borek, I am sure answering elementary level chemistry questions is one thing, answering them for someone trying to make drugs is probably at the bottom of your priorities. It's just a minute 1:55 long and shows some of the seeds I am getting ready for this spring.

https://www.youtube.com/watch?v=t58wjSWuW2c

Sorry I don't know how to embed a video.

Offline Borek

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Re: Sodium hypochlorite vs Hypochlorous acid
« Reply #5 on: February 06, 2019, 03:39:36 PM »
Wow, thanks, unexpected :)

Never tried hydroponics, could be one of my next projects. I had some limited success with hot peppers in pots in the past, the only picture I have at hand is this one (that was more for a joke, although they are perfectly edible):
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Offline Enthalpy

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Re: Sodium hypochlorite vs Hypochlorous acid
« Reply #6 on: February 06, 2019, 03:59:39 PM »
[...] you could start with the bleach and add some hydrochloric acid [...]
How dangerous is that? I expect a release of chlorine.

Offline Borek

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Re: Sodium hypochlorite vs Hypochlorous acid
« Reply #7 on: February 06, 2019, 06:44:53 PM »
[...] you could start with the bleach and add some hydrochloric acid [...]
How dangerous is that? I expect a release of chlorine.

Yes, if done fast and with concentrated solutions. Depends on the concentration and final pH though (which, as it was already said, has to stay in the the safe zone).
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