Sciencemadness Discussion Board

The most dangerous, expensive and inneficient way of making HCl!

Tacho - 13-6-2004 at 05:27

Electrolysis of a NaCl solution gives, initially, H2 and Cl2. The mixture of these two gasses is ignited by a spark, giving HCl gas that is absorbed by water giving a HCl solution.
You can get hurt or killed by chlorine gas (used in war as poison), by HCl gas (worse poison), by explosion of H2 or H2 Cl2 mix, or you can get electrocuted by the high voltage. At least, is not very efficient: I could only make 20ml of weak acid after 20 minutes fiddling with the setup to keep it working.

Making a short story long:
The electrode connected to the positive pole (anode?) has to be carbon (graphite). I used a solder’s carbon electrode. I had to strip away the copper layer by electrolysis first. For the negative electrode I used copper wire. The electrodes are kept in PVC pipes with one end closed. If you can’t understand why, you really are one little... aww, never mind.

Even with a strong brine and 24V (lots of current), not much gas is produced.

To produce the spark I used a Cockroft Walton voltage multiplier with 14 capacitors. I estimate in 3.000V the final output. Anyway, it was enough to give a spark every second or so with a half millimeter gap. The tubes that inserted the gases in the spark chamber are made of brass and one of them is connected to HV. A pencil lead graphite made the other electrode. THIS VOLTAGE MULTIPLIER MAY HAVE ENOUGH CURRENT CAPACITY TO KILL YOU! It certainly has voltage enough! Edit2: By the way, the capacitors hold the voltage even after you turn off the power.

Don’t mix the gases before the spark chamber. The biggest explosion I ever had in my attic was last week testing another configuration. H2 and Cl2 make one hell of an explosive! edit1: This mix can be triggered by light! Use only soft plastic pots for the spark chamber, just in case...

I put some water in the spark chamber to absorb the HCl. I figure that a drying agent (instead of water) would allow the production of reasonably dry HCl. Remember that, as the reaction goes on, NaOH is formed in the brine, producing oxygen, instead of chlorine and, therefore, water vapour instead of HCl gas.

The plastic tubes that conduct the Cl2 and H2 are silicone rubber tubes. I bought those along with the thin brass tube in a hobby shop. You know, the kind that sells models of aircraft and so. I guess these are used to conduct fuel for model engines. I presume silicone withstands chlorine better.

[Edited on 13-6-2004 by Tacho]

BromicAcid - 13-6-2004 at 05:53

Rather then spark gap you could just pass the resulting gasses though a tube packed with activated carbon or pumice at 300C then pass the resulting gasses though H2O. As it stands you say you made 20 ml of weak acid. Well, passing just Cl2 though water makes it weakly acidic, any quantitative tests for HCl (however they would not be any accurate seeing the equilibrium formed between Cl2 and H2O)? If you made your solution significantly concentrated then it would have to be HCl by default as only it gets to a significantly high concentration, especially in relation to Cl2.

One time my chemistry teacher was doing a demonstration and had a balloon full of H2/Cl2 to show the class the photosensitivity of the mix. But she had to run from the building where the chemicals were stored to where the lectures were given, halfway there the balloon achieved the activation energy it needed and detonated. Coincidentally it just so happned to be one of the days that we had a political figure on campus, a bad day for such an occurrence to happen.

BTW, I expecially like the madness to your idea, high voltages, explosive gas mixes, it doesn't get much better then that.

Saerynide - 13-6-2004 at 07:00

You must have a death wish :o:D

Tacho - 13-6-2004 at 11:22

Originally posted by Saerynide
You must have a death wish :o:D

Not really. I am very, very careful.

I make all those comments about the dangers involved to make sure anyone repeating the experiment knows what he/she should be careful about.

The "biggest explosion" was just an extra- loud "pop" that caused no damage but brine on my benchtop and some plastic bits in my lap.

darkflame89 - 14-6-2004 at 02:52

Do you need to use sparks to detonate the mixture? Why don't you just bring the mixture with water out to get some:D sunlight?

Tacho - 14-6-2004 at 03:07

Sunlight may or may not detonate the mix. It’s not a 100% sure catalyst. Besides, it may detonate only when too much mix is in the pot and that means an explosion.

The sparks are not a problem. The main problem with this procedure seems to be the chlorine. I think it may be dissolving in the brine instead of bubbling out. If I put the output tubes under water, hydrogen’s bubbles a lot, but chlorine’s doesn’t. Just a little bubble now and then.

Edit: I know the volume of hydrogen should be quite bigger but, even then, something doesn't seem right.

[Edited on 14-6-2004 by Tacho]

Organikum - 14-6-2004 at 03:32

The amounts of chlorine and hydrogen produced are to small for to make a noticeable reaction with the spark.

The glasstubes around the electrodes limit the amount of brine VERY badly. You will produce more oxygen than chlorine also I guess.....

this is some quick and dirty guessing of mine, no scientific anylysis.


Tacho - 14-6-2004 at 03:41

Those are not glass tubes, they are PVC pipes. They are 20 mm diameter and open in the bottom.

Under 24V there is a fair amount of hydrogen being produced, (edit) some chlorine that I can smell and there are small flames now and then in the spark chamber. Fumes are produced. The water becomes acidic.

It works. It’s just not efficient.

Edit: I'll try larger tubes.

[Edited on 14-6-2004 by Tacho]

[Edited on 14-6-2004 by Tacho]

Saerynide - 14-6-2004 at 06:08

when I used 9v to electrolyse a saturated salt solution, I got a ton of Cl2. It took only about 5 min to fill a large tube and show a noticable light green colour :o

Try not inserting the tubes to deeply. I inserted mine deep enough only to cover a bit more than the electrode's length.

rikkitikkitavi - 14-6-2004 at 09:15

actually, HCl has/is made industrial in this way.

Cl2 is burned in H2atmosphere (excess) in special constructed quarts nozzles and cooled , absorbed into H2O.

Tacho - 14-6-2004 at 10:35

WAIT!, why am I saying that I should get less chlorine? No! I should be getting exactly the same volume of chlorine! One electron to every H+, one from every Cl-! 22,4liters of gas for every mol of it!

Something wrong here! Maybe some copper was left in the carbon electrode and is depositing instead of chlorine!

Saerynide - 14-6-2004 at 10:45

Some water is oxidized, instead of chloride?

Tacho - 14-6-2004 at 11:13

Originally posted by Saerynide
Some water is oxidized, instead of chloride?

Please, could you elaborate a bit?

Saerynide - 14-6-2004 at 14:27

At the anode, water can be oxidized into 4H+ and O2, instead of 2Cl- being oxidized into Cl2.
However, at the cathode, theres nothing else to reduce, except water, so there will be more H2 than Cl2.

If you electrolyse plain water, there will be more H2 than O2 as well, because for each molecule of H2, you get only 1/2 an O2.

maxke - 15-6-2004 at 00:54

[Edited on 15-6-2004 by maxke]

maxke - 15-6-2004 at 01:02

Originally posted by maxke
2 H+ + 2e- --> H2 0.00

2 Cl- --> Cl2 + 2e- -1.36

Na --> Na+ + e- 2.71

2H2O(l) + 2e- --> H2(g) + 2 OH- -0.83

Very interesting effort.
But In the end you only have the four above things going on. I'have been a little out of service in my chemistry exercises ans such but you could calculate the exact voltage required to stimulate emission of H2/O2 or H2/CL2 given a reservoir of NaCL/H20.

But then there are some energetic losses so it would be best to give a little extra voltage on top. With your 24v I'm sure you are producing CL2. But ! If you want lots CL2 you need lots electrons. Needing a lot of electrons recquires high current, not a high voltage in my humble opinion.
Just make sure your voltage is high enough so that H2/O2 doesnt form.

Even with a strong brine and 24V (lots of current), not much gas is produced.

Have you measured the current ?

Preventing CL2 reaction of CL2 with H2O on the electrode position might be stopped introducing a pump, thus leaving the concentration of CL2 on water level at a minimum.
But then again, it might be better to find a valve or some other device which does the same while doing it explosion INSENSITIVE.
In the end you are still working with explosive gasses.

But my respect goes out to you Tacho.
You have crafted that which I've been thinking about doing it in this or another way.

By the way; where would one acquire electrodes as you have ?

Saerynide - 15-6-2004 at 03:13

Originally posted by maxke
If you want lots CL2 you need lots electrons. Needing a lot of electrons recquires high current, not a high voltage in my humble opinion.

More voltage = more current. Current is limitted by voltage, as i = v/r

Tacho - 15-6-2004 at 03:26


Thank you very much. Yes, water electrolysing might be my problem.
The copper thing I mentioned goes as follows: The graphite anode was covered with copper that I removed by electrolysis, but maybe some copper was left in the pores. As I begin to electrolyze the brine, the copper goes into the solution, reducing the output of H2. But now it’s being reduced at the cathode, instead of Cl. My solution doesn’t look blue though. I’ll clean the electrodes and change the brine.


I have not measured the current, but such a short path of strong brine must have little resistance and my power supply can provide more than 10A. Besides, the setup gets warm and the power supply hums noticeably when I connect the wires.

(snip) Preventing CL2 reaction of CL2 with H2O on the electrode (snip)

What reaction?

About the electrodes: they are used by electric arc welders, I presume as a cutting tool. They are graphite rods with copper eletroplated on the surface. I could not find them at the nearby (well supplied) hardware shop, I had to go to a shop specialized in soldering. They come in many diameters and are quite cheap, about U$ 0,33 (0,25 euros) a piece.

maxke - 15-6-2004 at 04:03

Originally posted by Saerynide
Originally posted by maxke
If you want lots CL2 you need lots electrons. Needing a lot of electrons recquires high current, not a high voltage in my humble opinion.

More voltage = more current. Current is limitted by voltage, as i = v/r

Ok, I agree , so basically you need a setup with low resistance because the powersource is one with a set voltage rating.

I had the concept of laser emission applied in this subject... so I might have some things mixed up.

Conclusion : there are 2 things you can do to increase current.

I= U/R
--> increase voltage
--> decrease resistance of the cell

Maybe overly saturating(NaCL) might give you better results ?? Distance between the two electrodes might also matter.

Tacho, I don't know if it is useful, but anyway : monitoring your current with a lowtech multimeter might also give an indication of the NaCl concentration in the brine. A limitation however : the more H2 CL2 produced , the more NaOH is in solution , which is also conductive.

What reaction?

Cl2 + 2 (Na+ + OH-) ––> (Na+ + ClO-) + (Na+ + Cl-) + H2O DrH°298 = - 103 kJ/mole.

I was reffering to the formation of NaOCL. This is reffered to in my native language as "javel", natriumhypochlorite. This is used for cleaning bathrooms and such. CL2 is basically absorbed in the watersalution containing the NaOH on the CL2 forming electrode.

Commercially "javel" is produced by bubbling CL2 through NaOH solution.

For finals : I'm sorry if I'm posting things that are obvious to you guys. I'm approaching this project as if I would try to do it myself. I want to recreate it anyway. Due to lack of time (examinations) starting projects such as these are on hold for an undetermined time.

[Edited on 15-6-2004 by maxke]

Tacho - 15-6-2004 at 06:21

Yes, NaOCl may be an issue here.

I'll try this new design:

I believe this will circumvent the concentration of NaOH and improve Cl2 yield.

Geomancer - 15-6-2004 at 11:01

Nice drawing. You're going for a divided cell type effect, no? I'm afraid the only other way to aviod build up of OH- would be to cancel it out, either with something simple like CO<sub>2</sub> (if this is acid enough), or possibly, if you feel like showing off, with anion exchange resin.

axehandle - 16-6-2004 at 03:58

This idea is so crazy that I just <i>have</i> to try it myself!

Very impressive work, Tacho.

I'm thinking (or rather speculating): Couldn't a small, thin Pt wire inside the combustion chamber be used instead of the spark generator? If memory serves me, it should catalyze the H2 + Cl2 --> 2HCl reaction....


Tacho - 16-6-2004 at 04:55

I was lucky and frightened last night! While testing the new design, there was another explosion. Nothing happened, just a loud bang. But it was inside the closed glass flask!
I could see the little fire running fast inside the silicone tube back to the generator and BANG! I believe it was the hydrogen/air mix.

I removed all copper from the anode using ferric chloride and used new brine. The new design produces lots of chlorine. The main problem now is SAFETY.


A catalyst would be wonderful. Something more available than Pt would be really, really wonderful!

Any ideas? Would nickel powder react with HCl?

axehandle - 16-6-2004 at 05:05


A catalyst would be wonderful. Something more available than Pt would be really, really wonderful!

Couldn't you remove the Pt film from a small, cheap type B, R or S thermocouple?

Note that I'm not entirely sure that Pt would initiate the reaction, so it could be an expensive failure...

How about this: I setup a small electrolysis, mix H2 and Cl2 in a test tube and then insert a Pt wire, then I'll tell you if it works or not? This seems like such a good idea if it works that I'm very excited to try it out....

Edit: PS, be CAREFUL! Glass shards would make a horrible addition to one's eyes... *shudder*

Edit2: The Merck says that nickel is "slowly attacked by diluted hydrochloric acid".

Edit3: Another idea just struck me: If the device is powered by solar cells, the HCl will be FREE!!!

[Edited on 2004-6-16 by axehandle]

Esplosivo - 16-6-2004 at 12:32

Note that Pt will ignite H2 in air, therefore if oxygen is present you could say bye-bye to your H2. I am not sure about the following but if I remeber correctly platinum does react with chlorine gas, although I think it must be in somewhat 'high' concentrations. Well anyway, be carefull about the catalytic ignition of H2.

Edit: Btw, sunlight was already mentioned, but following its disadvantage as a 'sure' catalyst why not use 'cheap' UV lamps. UV triggers the breaking of the Cl-Cl bond into free radicals therefore intiating the reactions.

[Edited on 16-6-2004 by Esplosivo]

sanity gone - 16-6-2004 at 14:01

I've been working on this with my own twist.
The electrodes are both graphite enclosed by PVC tubes (I did some percise drilling into my makeshift reaction chamber.) The PVC pipes the two gasses into a container with dH2O under UV lamp. I'm trying to think of a way to safely dispose excess HCL gas, but I'm thinking of running the excess gas under water to form some amount of HCL. I don't know if this will work.
I wrote a quick equation on my TI-83 and it says 262g of NaCl would make a concentrated acid in 250ml of H2O, although I could of easily not done the math right (I tend to make mistakes when doing speed stochiometry)

Esplosivo - 16-6-2004 at 21:55


but I'm thinking of running the excess gas under water to form some amount of HCL.

I know the amount of HCl is relatively small but be careful for suckback. Why not use the inverted funnel method. Excess gas will also bubble through but the method prevents suckback.

Tacho - 17-6-2004 at 04:29

UV light. I like that, thank you.

Does anybody know if borosilicate glass is somewhat transparent to short UV (germicide lamps)?

Esplosivo - 17-6-2004 at 05:01

Glass is somewhat 'impermeable' to UV but only to a small extent. UV has a high penetration power so 'thin' glass would not effect the UV to any large extent.

If you know the reaction between any alkane with Cl2 you know it requires sunlight/UV. Well if the mixture is placed in a closed borosilicate glass container proceeds normally.

axehandle - 17-6-2004 at 05:58

Slightly OT: Are there UV <b>diodes</b> on the market? A glass one could be INSIDE the chamber given adequate cooling.

t_Pyro - 17-6-2004 at 06:08

Isn't the Blu-ray disc read using a UV laser? Then there has to be UV diodes in the market, although their power output might not be sufficient for the current application...

See these pages:

[Edited on 17-6-2004 by t_Pyro]

axehandle - 17-6-2004 at 06:10

It only has to be a very narrow confined beam to initiate the reaction in a point of the gas mix, then the reaction will self-propagate, right?

t_Pyro - 17-6-2004 at 06:16

Yes, but depending on the rate of evolution of the Chlorine, the LED solution might, or might not be sufficient. It's still better than nothing, though.

An alternative solution using Mercury vapour lamps:

Tacho - 17-6-2004 at 06:33

UV germicide lamps are very common and easy to buy. I have a couple from my eprom erasers. One is very large, like 30 or 40 cm (many eproms at once). A glass tube along the lamp, wrapped with aluminum foil, will make, at least, pretty and tan molecules.

sanity gone - 17-6-2004 at 06:37

I don't know where to find the lamp, but the activation energy required in terms of light is 491.5nm.

Esplosivo - 17-6-2004 at 07:32

Are you sure the wavelength require to break the Cl-Cl is 491.5nm? If that is so the blue/violet visible light starts from 400nm. I see 491.5nm too low as an activation energy for the rxn.

sanity gone - 17-6-2004 at 07:37


The enthalpy for the overall reaction is -184.6 kJ per two moles of HCl. However, the enthalpy for the initial step is 243.36 kJ per mole of Cl2. This corresponds to the energy carried by photons with a wavelength of 491.5 nm This reaction is catalyzed by light toward the violet end of the visible spectrum.

Tacho - 17-6-2004 at 08:16

Originally posted by sanity gone
I don't know where to find the lamp, but the activation energy required in terms of light is 491.5nm.

UV range goes up to 400nm. So I guess the lamps I have are energetic enough (longer wavelenght, less energetic).

Edit: In fact, the reaction seems to be triggered by strong lights in general. Short wavelenghts are just to make it a reliable catalyst.

[Edited on 17-6-2004 by Tacho]

Esplosivo - 17-6-2004 at 09:45

Great! UV lamps will be therefore more than enough for the reaction to start. There are so many types of lamps which give UV light. There are many types of 'bulbs' emitting UV near the visible light spectrum - check out the phosphorescent tubes used in discos for example (also used in those thingies to attract and kill insects with an electrical shock through a grid, don't remeber what they are called)

Marvin - 17-6-2004 at 09:58

Last I looked blu-ray was just a spec and semiconductor blue lasers with a decent (more than a few hours) lifespan had yet to be produced.

UV LEDs are available but rather expensive last I looked, 370nm ish and rather weak compaired to mercury tubes.

Important note for anyone using the voltage multiplier - If using mains power directly make sure the live wire is the lower one in Tacho's circuit diagram, ie it is isolated by the first capacitor in the ladder.

Ideally the current to the cell would be enough that a tiny jet of gasses could be lit and the spark would just be backup. This would be in a small chamber isolated by inert metal gauze for example, so in the event ignition failed totally unburned gas leaving the tiny burning chamber could not be ignited later on (like a Davy lamp).

Yes, Explosivo, UV does the trick!

Tacho - 18-6-2004 at 03:42

I used a 15W, 43.5cm long, germicidal lamp. Part#G1578. It looks like a 20W fluorescent lamp but it’s not white, it’s transparent.

The reaction tube (about 50cm, soda-lime glass) and the lamp were wrapped in al foil, to improve light use and protect my eyes. I kept some distance between lamp and tube, in case the tube explodes.

I run 2.5A through the cell for 45 minutes, passed the gases through the tube and the output seems to be only HCl (a test for Cl2 would be easy but wasn’t done). The output seems to be irregular, in bursts. It passes over 20ml of water that becomes acidic.

Efficiency still seems to be lousy. It took more than 5ml (sorry, I stopped measuring at 5ml. I don’t have much time, you see. I have a life outside the attic) of the acidic water to neutralize an alkaline solution that was neutralized by 0,2ml of 7% HCl (approximate numbers, please, don’t quote me on that!).

Apparently it catalyses H2+O2 too, because there was condensation inside the tube, but it could be just mist from the cell.

Ooouuuww! You should see that setup working in the dark! Sci-fi scene! A picture of that will qualify me for a straightjacket! No less!

[Edited on 18-6-2004 by Tacho]

Esplosivo - 18-6-2004 at 04:23

Congratulations Tacho.


Apparently it catalyses H2+O2 too, because there was condensation inside the tube, but it could be just mist from the cell.

HCl sort of absorbs water from the air and dissolves in it. This condensation could be due to the HCl formed dissolving in water vapour in the air.

sanity gone - 18-6-2004 at 09:19

BTW how basic is the NaOH soln?

Tacho - 18-6-2004 at 10:27

Originally posted by sanity gone
BTW how basic is the NaOH soln?

I just dissolved some NaOH in water with a bit of phenolphtalein. Then I put equal volumes of this solution in two flasks and titrated with the two HCl solutions.

The test was comparative between the two HCl solutions, one with a known concentration. I was looking for a proportion, any NaOH concentration would work.

Tacho - 18-6-2004 at 15:56

That's the beast:

The aluminum foil is, of course, unwrapped. Notice the new H2 Cl2 generator, all made of soft plastic.

sanity gone - 22-6-2004 at 14:45

This is my idea for the generator:
Two towers connected by graphite salt bridge each contain a graphite electrode. Tubing is ran from the electrode from each tower into a reaction chamber under UV light. Then the HCl gas is pumped into water using the inverted funnel method.
Here is a MSPAINT sketch of my idea:

Esplosivo - 22-6-2004 at 20:52

The graphite rod connecting the two 'towers' will not act as a salt bridge. As shown in the setup you have drawn there will be effectively two cells, such that H2 and Cl2 are produced in both cells! Modifing the app by simply using a salt bridge instead of the graphite rod would do the trick.

sanity gone - 23-6-2004 at 00:51

oops... I didn't mean graphite salt bridge, I meant just salt bridge.

Esplosivo - 23-6-2004 at 02:03

The app is similar to Tacho's one but I think (at least it's my opinion) that there is a disadvantage using your apparatus. If I remeber correctly HCl gas is hygroscopic right? Therefore it will absorb moisture from the air and dissolve in it. This will cause Hydrochloric acid to collect in the container and not pass to the 'inverted-funnel - water' setup.

Btw, what are you intending to use as a power supply? A normal car-battery charger?

sanity gone - 23-6-2004 at 08:01

For my power supply I have a AC inverter that will give me 12V DC with positive/negative terminals.
And using Tacho's setup probally will work better, but I'm curious to see how an alternative will work so I designed this yesterday.

Chemtastic - 28-6-2004 at 19:33

I had an idea kinda like this, only the H2 and Cl2 were obtained seperately...

A deflated balloon would be stretched over the narrow mouth of a flask filled completely with vinegar (or other available acid) so that no air was in either the balloon or flask. Inside the balloon would be a limiting amount of magnesium ribbon. It would be dumped, and a balloonful of H2 would result...Mg + 2H+ --> Mg2+ + H2

Similarly, a deflated balloon would be stretched over another flask filled completely with vinegar, so that no air was present within. This balloon would hold either sodium hypochlorite crystals or a strong solution of the salts. When inverted, Cl2 would result...2OCl- + 4H+ --> 2H2O + Cl2.

In this manner, balloonfuls of H2 and Cl2 could be generated.

The apparatus would be something like this: A loosely sealed plastic container would have three additional openings on the side. The first would accomodate a spark plug or some other sparking device and would be caulked in somehow as to keep the container airtight. The second opening would have a narrow piece of plastic tubing leading from it, which would also be sealed in place, keeping the container airtight if the tube was blocked. The third hole would also be on the side of the container, a small hole just above the waterline (the container is filled with that water that will become acidic) used only in the pre-experiment setup.

Using the H2-filled balloons, air would be channelled down the pipe, sufficiently to fill the chamber and force all O2 and N2 out through the third small hole just above the waterline. Once complete, this whole is immediately plugged. The container should now be airtight and have an exclusively (or very closely so) hydrogen atmosphere within.

Next, Cl2-filled balloons are SLOWLY deflated down the tubing into the chamber. The sparker, set to spark at maybe 1-2 second intervals, will not ignite the hydrogen gas, as no oxygen is present, until the chlorine gas enters. In order to minimize reaction intensity, preventing explosion, the SLOW release of chlorine would be essential.

Anyway, this is my idea for an apparatus, and I am currently in the process of designing it. If anyone can spot any flaws, please let me know. Otherwise, I'll try it and post anything I can about, etc.

The attached picture is a JPEG of my proposed setup.

[Edited on 29-6-2004 by Chemtastic]

HCl.JPG - 39kB

The_Davster - 28-6-2004 at 19:41

Just a minor flaw, but sodium hypochlorite does not have a solid form. You could easily bypass minor problem this by using calcium hypochlorite(available as "unstabilized shock treatment";) instead. Other than that it seems viable.

darkflame89 - 29-6-2004 at 02:35

Actually, if Cl2 is collected in the balloon, why not just lead it through to dissolve in the water? A mixture of acids will result, with the hypochlorous acids decomposed in the sunlight.

Additionally, making hydrogen gas by reaction with vinegar, is a teensy bit too slow as the acid is weak. You could try instead mixtures of warmed NaOH and aluminium shreds. It would will give off H2 as usual, but with much more fervour.

Besides forcing out the oxygen out of the apparatus is hard unless, you have a certain chemical that removes oxygen from the air inside. Or instead of sparking, why not use a UV lamp or bring it to the sunlight?

Tacho - 29-6-2004 at 08:48

Nothing wrong with the general idea, but I would like to make a few comments and sugestions:

1) Using magnesium to make H2 seems like an awful waste of materials. I wish I had some magnesium. I would go for Al and lye, like darkflame89 suggested.

2) I think Cl2 will attack the latex of your balloon fast. A balloon full of chlorine exploding in your face is the stuff nightmares are made of. Why not use ziplock plastic bags? Take a look at this, I think you will love it:

I used this technique to make some gases and it’s simple and reliable. Think of using large syringes and ziplock bags in your project.

3) I doubt you will be able to drive away all oxygen from the plastic container, so be prepared for an explosion at your first spark. This may or may not be a great deal, just make sure the lid is loose. I agree with darkflame89 about removing oxygen, maybe an acid-wet steel wool hung on a wire.

Interesting what you said about chlorine in water. I wonder what would happen if I bubble hydrogen in bleach under UV light...

Chemtastic - 30-6-2004 at 17:57

Thanks, all. Just double checking one thing...UV light is enough for the activation energy of H2 and it enough for the activation energy of H2 and O2?

Cyrus - 30-6-2004 at 18:28

If there is an acid-wet piece of steel wool in the container, won't it detract from the quality and amount of HCl produced?

To chemtastic, I think the balloons would work until they were eaten through, but it would be much easier to have everything connected directly and not have to worry about balloons.

darkflame89 - 1-7-2004 at 02:27

NO, UV rays arn't strong enough to initiate the reaction between H2 and O2. UV photon breaks up Cl2 to Cl free radical which attacks molecules like H2 and O3 with full force..

Some positive observations

experimenter_ - 15-3-2016 at 10:04

This experiment was tried as a scientific curiosity mostly, to see if H2 and Cl2 gas can combine "peacefuly" or not. Two carbon rods are electrolysing a saturated salt solution at 2A and the two gases produced are collected together in a test tube through a bubbler. (You can see the setup in the first attached image). What was observed:

1) The two gases can react slowly with low intensity light. Leaving the tube by the window in a cloudy day will result in half of the gases to be consumed within a period of one hour. Leaving it in dark, would result in no gas consumption at all. In direct sunlight they would react within few seconds but no explosion. As the reaction was going on, I could see the water under the test tube rising. For some reason, not all of the gas in the tube could be reacted; probably because there was oxygen there too or because the Cl2/H2 ratio was not optimal (some Cl2 dissolved in the brine or the bubbler water maybe). This deviation from optimal ratio maybe the reason I couldn't observe an explosion out of it. This may be a key observation for designing a safe reactor in future.

2) As the electrolysis continues, NaOH is procuced. Within some minutes the solution is alkalic enough in order to be capable of consuming all of the Cl2 produced and making hypochlorite. Thus we get only H2 reaching the output. The effect takes place as soon as the first minute has passed. One can prove this by acidifying the electrolyte with HCl; Cl2 gas will be suddenly freed from the solution.

The title of the thread characterises the electrolysis way "ineffective" and "dangerous". There is a possibility by using carefully controlled UV light instead of sparks to make the reaction less dangerous. Also, if a membrane cell is used and the NaOH does not come in contact with the Cl2, this way could become much more effective.

Probably this device would never become practical enough for producing HCl, but here you have a proposed unit: (check second attached image)

If a separating membrane is not used, the solution must be kept acidic at all times by adding hydrochloric acid. In that way, it can only be used to purify impure hydrochloric acid and not to produce it from NaCl.

hcl2.png - 284kB hcl.png - 7kB

MeshPL - 19-3-2016 at 13:51

I had this idea for "safe" HCl generator for a while, it would probably need some cooling, but here is the rough idea. Not sure if it would work. Obviously, the goal for this one is production of conc. aquous HCl.

HCL maker2.jpg - 45kB

[Edited on 19-3-2016 by MeshPL]

It works

experimenter_ - 20-3-2016 at 05:46

Thanks for the input MeshPL.
In my opinion, one should avoid the usage of sparks for combining H2 and Cl2 since a simple UV lamp can do the same much more safer.

If you decide to use the sparks, you will have the benefit that H2/O2 gases will react too. I think it is unavoidable not to get O2 impurities through brine electrolysis. Using UV light in a closed system like the one you described is not practical (the inner chamber will fill with O2/H2), thus you must use the sparks in that setup.

Usage of UV light requires an open system but it has other advantages. There is no need to separate H2 from Cl2 prior to the reaction (which means a very simple and small electrolyser) and there is almost no explosion danger.

For the time being I don't fully understand how those two gases could combine explosively. My attempts to trigger the reaction with UV light are in vain. The reaction proceeds smoothly even near a black light tube. I think a much more intense UV source must be used in order to trigger it, but I don't care about that right now.

Bottom line is that the UV light method works. In one test, it was able to lower the pH of 50ml of deionised water around pH 2 only after 20 minutes of operation. I know it is aqueous HCl in there since the water also acquires a sour taste. There is barely any Cl2 smell in the output gases wich means that most of Cl2 reacts. Very promising...

I hope the next days I will come up with some serious measurements of efficiency and output capacity if you are interested. Until then, wish you guys a happy experimenting on whatever you do.

experimenter_ - 21-3-2016 at 23:13

Some news:

Finally, a violent reaction between H2 and Cl2 was observed when exposed to UV. In order for it to happen it requires purity; mix with the H2/C2 stream some air or other indifferent gas and you will not get it. However, the reaction to HCl proceeds smoothly even if the mixture is diluted with air and that's a fine thing to see! But keep the tubing thin and use a protective box around the device, just in case.

Still not any rigorous measurements on efficiency. Best thing I can say is that with 1.1A for 20mins I got 50ml of HCl solution that was capable of producing fizzing when mixed with soda. That is enough proof for me.

Now to the problems:

1) I noticed that the HCl solution has a smell like bleach. Obviously some Cl2 gets dissolved in there even if the pH is low :(
Well, this is bad news for me since I'm interested in the purity. Does anyone know if it is possible to remove the Cl2 from the HCl? I'm afraid it is difficult to do..

2) Still a solution on how to separate NaOH from mixing with Cl2 in the anode has to be conceived. Else, the device will not be able to work on salt only, it will require acidification of the electrolyte too.

So, for the time being it is useless; cannot be used to purify HCl acid neither to produce it from salt. Any practical ideas on how to solve at least one of those problems might kindle my interest again.

[Edited on 22-3-2016 by experimenter_]

pesco - 1-4-2016 at 19:06

Just few ad hoc suggestions:

a) use excess of H2
b) make sure your HCl collection vessel is under UV and bubble H2 though it on the way to H2+Cl2 reaction chamber
c) both above combined :D

2) divided cell


experimenter_ - 7-4-2016 at 08:44

Quote: Originally posted by pesco  
Just few ad hoc suggestions:

a) use excess of H2
b) make sure your HCl collection vessel is under UV and bubble H2 though it on the way to H2+Cl2 reaction chamber
c) both above combined :D

Thanks for the suggestions pesco. Both of them seem logical steps to take. I think (b) is the easier to do but also less possible to work.

My ideas on this was to use vitamin C or sodium ascorbate to react the chlorine towards HCl (according to this link: The sodium ascorbate will be positioned after the UV stage but before the water for the bubbling of HCl. In that way, chlorine will never reach the water to dissolve to; it will be reacted while it is still in the tube. Hopefully, HCl gas does not react with sodium ascorbate or else that way will not work. Anyway, I have three methods to check now, thanks to your input.

Quote: Originally posted by pesco  

2) divided cell

Good news for this one. I have already found that two half-cells connected with "salt bridge" made from ordinary filter paper works just fine. They produce separately H2 and Cl2 gases which are mixed in a later stage before the UV chamber. The rate of gas evolution is almost not affected by the "salt bridge"; it can carry 1A easily if elevated voltage is used to overcome the resistance. Added bonus is the production of NaOH also. I don't know yet what the maximum concentration of NaOH can be with a lengthy electrolysis.
I didn't want to use membranes that need to be bought from special shops. After all it is just some HCl that is produced. It must be made by the simpler possible way.

So, issue No2 is solved and issue No1 has a big probability to be solved easily too. When I get in the mood, I might dedicate a new thread to the device and take some pics; it is not very pretty but it is a piece of art in a way that only a chemist can appreciate :)

mysteriusbhoice - 11-9-2020 at 05:13

here to necro a thread back to life with my experiment on this.
its not explosive if the chlorine is wet! however dont pass too much at a time.
UV reactor setup

HCl test from setup

im wearing anti UV welding goggles so dont worry about my eyes.

Below is my chloro alkali PVA type sodium ion exchange membrane which produces an equimolar Cl2 and H2 mixture which if passed thru water safely wetts the chlorine so it dont explode!
I only have to buy salt and I can produce fairly concentrated HCl!!

test cell.jpg - 63kB

[Edited on 11-9-2020 by mysteriusbhoice]

[Edited on 11-9-2020 by mysteriusbhoice]

mysteriusbhoice - 19-9-2020 at 02:15

cost of production using my method of membrane cell + UVC is 82 cents per liter of 37% HCl so I would say its dangerous but not expensive at all nor inneficient.

khlor - 30-1-2021 at 23:39

Alright... I might just have dived into crazy territory here, but I've read on a big yellow chemistry book here that the H2+Cl2 reaction is facilitated at a temperature of over 250C, plus, on wikipedia and other places list Cl-Cl bond to require over 2.51eV to be broken, so I did some digging and in theory, you can use blue and violet leds to do it, since anything bellow the wavelengh of 450nm should provide enough energy in(eV) for that(I have some 3W UVA leds, not tested yet though) however, what I did was use an undervolted halogen lamp to reach the said 250C and turns out it works pretty fine, and for my surprise, no explosions(I was expecting such and PREPARED for such) nd I saw the pure HCl vapor comming out of the reactior as it formed a mist by absorbing the air humidity... kinda neat, and scary. then again, I turned the lamp on first, before turning on the gas, to make sure that all mixtured that got into the chamber reacted immediately.

Fyndium - 1-2-2021 at 05:25

Why not just use chinese quartz glass UVC bulb to catalyze the reaction? They cost few € a piece for 10-30W bulbs.

I actually bought one just for this specific chloro-alkali cell, but as I never got it working, I later dismantled the operation.

Would be much more controllable and economical than using spark plug. The reaction setup could be contructed such that the reaction chamber is buffered with something like mentioned carbon, and lined with material that is transparent to UVC when necessary, and otherwise opaque, to prevent it reacting the gases prior. The gases should also be led through different lines to the chamber. As it is energetic, the chamber should have water jacket. Perhaps a quartz insert tube into an ordinary multi-neck boro flask immersed in water bath with circulator pump to move water around? Scattered UVC would be enough to radicalize the reactants within.

IMO balloons and other static housings for gases are conceptual at most. Having a mass of 2g/L or likewise, one would either generate only grams of results minus losses. Imo a chloralkali cell is so laborous to construct that it would mandate the production of at least kg scale of reactants to be anyway excuseable. Good old bisulfate+salt is several orders of magnitude lower in lead-in costs and labor.

[Edited on 1-2-2021 by Fyndium]