I am building a membrane electrolysis cell to make sodium hydroxide and chlorine gas. I then plan to combine the two to make sodium hypochlorite
(bleach) which I will feed into a pool.
I plan on bubbling the chlorine gas through a cylinder filled with aquarium filter balls (or something similar) and flow the sodium hydroxide and
water downward in a turbulent manner to combine the two products. (this method is often used by aquarists to dissolve co2 into planted aquariums).
I have read somewhere that the electrolysis cell may get heated and may need cooling, but I have heard that heat helps as well. I have also read that
the combination of chemicals to make bleach only happens below 110F (if my memory is correct). The recombining of chemicals to make bleach is my
biggest concern right now since I am reading conflicting information on the web. That and selecting which membrane to get. Any thoughts here? For
the membrane, I am considering Nafion or Membranes international.
My hope is to be able to control this whole thing in an automated process with an arduino.
What do you think?
I will post a sketch when I get a chance. tnphysics - 17-11-2011 at 10:08
It is not necessary to use a membrane-it is to PREVENT the formation of bleach, the desired product in this case.
It should not be difficult to make the process continuous. However, it must be VERY well cooled. If the temperature is too high chlorate will appear.
The temp must be kept below some value (I do not know what).dann2 - 17-11-2011 at 11:17
The definitely sell things that do exactly what you're saying, except the cell is not separated.Ramdough - 17-11-2011 at 17:22
@ tnphysics and 497
The reason for the membrane is to prevent the introduction of salt into the system. I read one source that said salt was produced when recombining
chlorine and sodium hydroxide so I am a bit concerned about that.
@dann2.
That reference is great. Thank you very much. It was very helpful.
I will try to Sketch my design when I get a chance.
497 - 17-11-2011 at 20:09
I don't understand. Whether the Cl2 and NaOH combine inside or outside the electrolytic cell, the reaction is still 2NaOH + Cl2 > NaCl + NaOCl +
H2O
And the NaOCl will end up as NaCl after oxidizing things anyway. So what's the problem with salt?Ramdough - 18-11-2011 at 05:26
@497.
When you buy Clorox, do you get one mol nacl for every mol Naocl? Or, is there a way to separate nacl from the products?
The paper dann2 posted did not mention nacl as a product. Does that process not produce nacl?
My goal is to chlorinate my pool without making a salt water pool. 497 - 18-11-2011 at 05:40
Yes chlorox is a mixture of NaOCl, NaCl and NaOH. Even if you could get pure hypochlorite it would turn into NaCl over time.
Why avoid the salt? If you absolutely must, you could use breaching powder which is Ca(OCl)2, or TCCA which will not introduce any inorganics at all.
Directly injecting Cl2 could probably work too, but is less practical.. it all depends on what the water is being used for.
You do realize the amounts of NaCl added would be inconsequential if the water is used for either swimming or drinking right?Ramdough - 18-11-2011 at 12:36
I am using the sodium hypochlorite as the primary sanitizing agent for a swimming pool.
I have considered making my pool a salt water pool, but I really did not want to deal with the negative aspects of a salt water pool.
No, i did not know that. I guess from your response that for the required amount of sodium hypochlorite to sanitize a recreational pool, the salt
added to the pool would be unnoticeable.
I appreciate all of your help. Ramdough - 24-11-2011 at 14:00
I have read that heat exchangers are used to keep the temperature of the sodium hypochlorite reaction below 95F or in one source 104F.
Is the reaction that combines chlorine and sodium hydroxide exothermic?
I have heard that the electrolysis cell produces heat as well, but does not need cooling since the products are separated by a membrane. is that the
case? AJKOER - 24-12-2011 at 18:43
Ramdough: On the reaction of Cl2 and NaOH, if your goal is NaClO, a temperature under 50 C is recommended. Higher temperatures (80 to 90) favor the
formation of chlorates. The key reaction for the formation of Chlorates is:
HOCl + NaClO --> NaClO2 + HCl
which is the slowest (determining) step reaction followed by the more rapid:
HOCl + NaClO2 --> NaClO3 + HCl
This is essentially equation (7) on page 23 in the prior cited reference ("Chlorine Oxides and Chlorine Oxygen Acids").
If your overheat (boil), some HOCl will distill off (predominantly Cl2O at first, followed by HOCl), and I suspect that Chlorate yield would decline.
With less water, the pH will rise and be above the optimal 6.9 impacting Chlorate yield as well.
An alternate good reference to the one provided above is:
