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Author: Subject: Making Sodium Hydroxide (lye) ??
ktw_100
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[*] posted on 21-4-2006 at 10:34
Making Sodium Hydroxide (lye) ??


Anybody know how to make sodium hydroxide? I am a biodioesel maker, and would like to make it vs buying it! I have heard of it being done with DC voltage and electrodes in salt water, but that's about it.... Or making from wood ash.

Thanks
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[*] posted on 21-4-2006 at 11:10


Best way would be from slaked lime (calcium hydroxide) and sodium carbonate solution.

Ca(OH)2 + Na2CO3 -----> 2 NaOH + CaCO3

An excess of Ca(OH)2 is agitated for several hours with a solution of sodium carbonate. It is filtered or decanted from the resulting chalk. There you have your lye solution.
Boiling it down is messy and dangerous, and hot NaOH solution attacks glass containers.




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rot
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[*] posted on 21-4-2006 at 13:31


Yes, you can make it from salt water:
First, you make a saturated salt solution. (35.9g/100mL).
Then you electrolyse this solution with graphite electrodes.
At the anode, chlorine gas will evolve, At the cathode Sodium Hydroxide and Hydrogen gas will form. you can just keep running electricity through it until no more chlorine gas evolves, then boil the solution down. Care must be taken to prevent the chlorine gas from mixing with the dissolve lye, as this will react to make sodium hypochlorite (bleach).
This can be done by a salt bridge.
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[*] posted on 21-4-2006 at 13:49


In Industry, there are suppose to make NaOH electrolytically by passing an electric current through a Sodium Chloride Solution.
Through the use of a special membrane, NaOH solution is allowed to exit the cell and be collected while the other products remain behind.
In a diagram I have in one of my chemistry books, it shows two graphite anodes in an NaCl solution surrounded by a wire screen cathode with an asbestos diaphram in between. NaOH is produced at the cathode and drips out of the cell while the Cl2 gas bubbles out without reacting with the water. This is called a "diaphragm cell".

Another process, called a "Mercury Cell", Hg serves as the cathode, which reduced Na+ ions to Na metal which then dissolves in the Hg. The Hg-Na amalgam is then exposed to pure water (not NaCl solution) and the Na will react to produce NaOH and H2 gas. This procedure is suppose to produce much purer NaOH.


"rot", How could you set up a salt bridge in the electrolytic cell in order to produce NaOH? I never quite understood the idea of a salt bridge any further than it is used to keep two solutions seperate during an electrochemical reaction.
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[*] posted on 21-4-2006 at 20:37


Well, the point of the salt bridge is just to keep the chlorine gas evolving in the electrolysis seperate from the sodium hydroxide being produced, which would form sodium hypochlorite.

I've been interested in thermochemically decomposing sodium carbonate using a large fresnel lens to produce sodium oxide, which would of course react with water to form sodium hydroxide. At the maximum focal length, a fresnel lens should be able to reach temperatures of 1500 degrees celcius on a sunny day. I'm not sure how feasible this approach would be, especially because white sodium carbonate would barely absorb the rays of the sun. The best way I can think of using this approach would be to heat a black substrate (graphite? metal oxide?) with the sodium carbonate over it. I'll probably be testing this soon enough when I can get enough money to buy the lens from this webstore: http://www.goldmine-elec-products.com/prodinfo.asp?number=G1...

$50 doesn't seem bad does it? For 40" x 31"?




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[*] posted on 21-4-2006 at 20:44


Don't count on it- sodium compounds are volatile, you'll mostly boil it off. Electricity, by resistance, arc, or else flame heat, is a lot easier and a lot less weather-dependent than solar heat.

Tim




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rot
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[*] posted on 21-4-2006 at 23:07


Quote:
Originally posted by mrjeffy321
"rot", How could you set up a salt bridge in the electrolytic cell in order to produce NaOH? I never quite understood the idea of a salt bridge any further than it is used to keep two solutions seperate during an electrochemical reaction.


I've never understood it completely either, but as far as I know it's just a tube filled with electrolyte (in this case NaCl solution) connecting the two beakers containing NaCl Solution, to prevent the chlorine gas from mixing with the lye.
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[*] posted on 21-4-2006 at 23:18


Yeah, thats all they are. I've used them a couple times for electrolysis, huge cell resistance, makes your reaction take much longer compared to an undifided cell.



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[*] posted on 22-4-2006 at 00:28


Quote:
I've never understood it completely either, but as far as I know it's just a tube filled with electrolyte (in this case NaCl solution) connecting the two beakers containing NaCl Solution, to prevent the chlorine gas from mixing with the lye.

At least it is not just me. Seriously, my only "experience" with them is in diagrams inside a text book, in all my practical experience with electrochemical cells, I have never seen one, much less used one. If the "salt bridge" is made of salt, why wouldnt it dissolve?...we assume it is made of the soluble salt which is being used as the electrolyte. Anyway, but that isnt important.

Decomposing Sodium Carbonate into Sodium Oxide using a giant frensel lens would be pretty energy intensive considering how hot you can get the focal point...especially with such a huger lens. You might just vaporize anything in the path of the light.
But if you could indeed get it to work successfully, that would be an excellent method to use since the Na2O would form NaOH in solution, but you might end up decomposing the Sodium Oxide with that intense heat while making it.
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[*] posted on 22-4-2006 at 05:46


salt bridge will not dissolve because the solutions in each cell is saturated.



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[*] posted on 22-4-2006 at 07:45


Then the bridge will dissolve, because as electrolysis continues the Sodium Chloride concentration drops, so the solution is not saturated anymore, so the bridge will dissolve.
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[*] posted on 22-4-2006 at 16:55


Hmmm, I'd like to see if I could at least calculate the direct energy output of the area of the lens. One could always change the range of the focal length to reduce the energy output of the focal point, then calculate the amount of energy absorbed by the heating medium underneath, and find the temperature range of the medium to prevent sodium oxide decomposition/boiling, no? As long as I can stay in the 800 - 1000 degree range, I hope I'll be fine.

I'll probably do some testing next month.




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[*] posted on 22-4-2006 at 21:43


A salt bridge is a tube filled with a salt solution, not a piece of solid salt. Each end is plugged with a permeable membrane that prevents the two solution from mixing but allows the ions to transfer charge.
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[*] posted on 24-4-2006 at 06:20
Poor Mans Salt Bridge


For a temporary salt bridge, use a filter paper (Chromotography) strip soaked in 1M Potassium Nitrate.
Put each reagent and electrode into separate beakers and insert the bridge between them;)
When the bridge starts to dry out, just add more 1M Potassium Nitrate from a dropper to wet it.
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[*] posted on 24-4-2006 at 11:29
Making Sodium Hydroxide (lye) ??


Thanks for the input....
I would assume that the chlorine gas is toxic, even at low levels? Just how much is generated when making the NaOH? Also, the electrodes I assume could be made from 1.5V battery cell posts - I think they are the right material.

What would be the best DC voltage to run on the cathode and plate, and how long a process is it? Does it need to be really 'flat' DC, or very well filtered?

Anything else I should be aware of?
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[*] posted on 25-4-2006 at 00:29


you end up electrolysing all the chloride into chlorine, so it's quite a heap. Electrolysis is slow, howver, so if done outside it should be OK. Alternatively, you can lead the chlorine through 'something plentiful and OTC' that reacts with it in a train of 'washing bottles' and end up with a lot less smell.

In electrolysis, I'd say the ideal voltage is the voltage at which you see a nice amount of bubbles. Just be sure that your current is not so big that you fry your power source.
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[*] posted on 30-4-2006 at 11:46


The danger of chlorine is not that high, chlorine is detectable by the humon nose in quantities far under the lethal limit, plus the fact that chlorine evolution is very slow. Do it outside and you'll be fine. Don't do it inside though, I had once ran a cell next to my computer (used as power supply ;)) within about 30 minutes I got a headache even though I couldn't smell any chlorine. I'm sure it was because of the chlorine though. I turned off the cell and opened a window and headache was gone :P
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[*] posted on 1-5-2006 at 08:11
Making Sodium Hydroxide - again


I THINK I have some Sodium Hydroxide (?)

I saturated some distilled water with rock salt. (35.9g/100ml). Then put equal amounts of this into two separate glass containers. Containers joined with a homemade salt bridge (cloth stuffed inside some good plastic tubing, saturated with salt solution). Applied 12VDC to carbon electrodes placed in the cells.

After a couple of days, I have white crystalline stuff in both containers, as electrolysis taking place. At the cathode is Sodium Hydroxide. What is the crystalline stuff at the anode - Sodium Chloride?

I did measure the pH of the Sodium Hydroxide - seems to be around 9 or so at the moment.

(PS: The salt bridge didn't dry out, or need any resoaking, due to capillary action keeping the cloth inside the tube wet.)

Also, when it comes to de-watering the Sodium Hydroxide, as mentioned, it can be done by distilling the water off. However, Sodium Hydroxide is hygroscopic, is it not? Therefore, I will never get all the water out of it.

I am thinking of making a saturated solution of NaOH/H2O, and using this as part of the process of producing biodiesel, versus using crystalline form of dry NaOH that I have been using.

From what I understand, if NaOH has been exposed to air, the best thing when making biodiesel with it is to increase the amount necessary for the process by 25% (maybe off topic here, but I don't know if I can get these questions answered on the biodiesel newsgroups - perhaps there is someone here making biodiesel?)

Appreciate any info already supplied! This is fun stuff!
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[*] posted on 1-5-2006 at 09:16


pH 9 isn't much at all, you need pH up around 12-13 before you have any percentage of NaOH, IIRC.

The crust is most definetly a salt crust. As you electrolyze water, you remove it, and the salt loses solubility. Also, even if it is still soluble in the solution, brine likes to crust up around its container anyway.

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[*] posted on 1-5-2006 at 10:11


I agree with 12AX7, with a pH of only 9, you really have not made very much Sodium Hydroxide.
Even small concentrations of NaOH will yeild very high pH values (on the order to 12, 13, ...14). If you evaporate the solution now, you will get a little bit of NaOH but most of the NaCl still remains.

Just for illustrative purposes, say you had a 500 mL solution with a pH of 9.0, this would mean that your [OH-] concentration (the same as your NaOH concentration) would be roughly 1 E-5 Molar, or about 5 E-6 moles of NaOH.
NaOH has a molar mass of 40.00 g/mol, so you would get (in theory) .0002 grams of NaOH by evaporating off the water.
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[*] posted on 1-5-2006 at 13:22


Remember that small amounts of NaOH in contact with the air will react rapidly with atmospheric CO2 to form (bi)carbonates, thus neutralizing the hydroxide.

There was a thread on biodiesel around here somewhere. Search around a little.




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[*] posted on 1-5-2006 at 13:38
Making Sodium Hydroxide (lye)??


Thanks for the info ..... I will let the process finish, and try to scrape off the brine from around the edges, and see what I have. I guessed at ph 9 using a wide range indicator. That was yesterday morning, so 24 hours have passed. My liquid solution is about half of what I put in there to begin with. I thought it was evaporation!

so if I get the NaOH out of it as soon as it is done, and then 'dry' it as best I can, I assume I will get a bit of NaOH anyway. If I had put 35.9 grams of salt in the solution, would I get 35.9 g of NaOH (in a perfect world)?

And still wondering about the non-sodium hydroxide side of the process. Is the precipitate also NaOH, or what?

Thanks for the help ...
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[*] posted on 2-5-2006 at 10:39


If you are using a salt bridge, then at the positive side, chloride is converted to chlorine, and chloride ions from the salt bridge are moving out of it, towards the positive electrode.

At the same time, sodium ions move to the other side. The net result is that the salt bridge becomes depleted of ions. Of course, the mechanic capillary effect and diffusion keep it filled with ions, so the liquid remains conductive.

Net result will be that at the anode part, the liquid remains salty and only chlorine gas escapes. At the cathode side, certainly some hydroxide will be formed, but it will be very hard to obtain it in a pure state. You always have NaCl over there as well, at high concentration.




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[*] posted on 16-1-2013 at 10:29


What about a drying tube, plugged or covered at both ends with filter paper, and filled with a conductive solution of your choice?



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[*] posted on 18-1-2013 at 10:08


Here is questionable path to NaOH (or Ca(OH)2) that some may find interesting and others may find insane (appropriate for Sciencemadness perhaps) on many levels. However, if you are short on ingredients, here is not very cost effective method that requires Hypochlorous acid. Note, one path to HOCl would be to add a weak acid (acetic from vinegar, ascorbic from Vitamin C, critic...) to Chlorine Bleach (essentially NaOCl and NaCl and a touch of NaOH) and distill (stop when you have obtained 1/2 of the starting solution which contains most of the HOCl and Cl2O gas). There are other paths as well (search on Sciencemadness and the internet).

Now, to quote from "A comprehensive treatise on inorganic and theoretical chemistry", by Joseph William Mellor, page 256:

"According to F. von Tiesenholt, there is a reversible reaction: NaCl + HOCl = NaOH + Cl2, and with 2 or 3 grams. of anhydrous calcium chloride dissolved in the smallest possible quantity of hypochlorous acid, there is an energetic development of chlorine, and a formation of calcium hydroxide; some chlorate is formed at the same time. According to J. L. Gay Lussac, hypochlorous acid at about 100° attacks metal chlorides with the evolution of chlorine and oxygen, and the formation of chlorates; according to A. W. Williamson and J. Kolb, the metal chlorides are not attacked in the cold, and on warming the chlorates are formed. Hypochlorous acid precipitates the higher oxides from the chlorides of manganese, tin, lead, iron, cobalt, and nickel; and copper oxychloride from cupric chloride. Silver chloride decomposes the acid catalytically."

Link: http://books.google.com/books?ei=hWP3UP78GoLi0gHd14GYCw&dq=f...

{EDIT} Interestingly, the following equation is given in a recent reference, confirming Mellor:

Cl2 + -OH <-----> Cl- + HOCl

See "Handbook of Detergents: Production", by Uri Zoller abd Paul Sosis, page 443. Link: http://books.google.com/books?id=dXn3aB1DKk4C&pg=PA443&lpg=P...

I would, nevertheless, not be surprised, if one does better making Ca(OH)2 from CaCl2 (as moving the reaction in the direction of expelling chlorine may be easier), and if this is the case, react the Ca(OH)2 with NaCl as detailed above to form NaOH. In either case, one must avoid Iron and heavy metals presence as these are suggested catalysts for the production of chlorates.

With my suspicions noted, one questionable embodiment of the process would be to spray a small amount of HOCl evenly over dry NaCl (maintaining an excess of NaCl so assume a high estimate of the Hypochlorous acid strength) and lightly heat in the open air (best in a fumehood or outside) to vent any toxic and irritating Chlorine fumes.

NaCl + HOCl --> NaOH + Cl2 (g)

However, I questioned whether the reaction can be really stopped at this point. For one, local concentrations may still result in some undesirable side reaction including:

NaOH + HOCl --> NaOCl + H2O

3 NaOCl --> 2 NaCl + NaClO3

NaOCl + HOCl --> NaClO2 + HCl

NaClO2 + HOCl --> NaClO3 + HCl

As such, this probably not a practical or particular easy to execute route, however, the basic starting chemicals are easy to obtain (salt and/or CaCl2, bleach and vinegar). Note, my side reactions listed are not unique to my suggested synthesis, and actually, may be of even greater concern in an electrolysis approach where control over concentration, pH and temperature is perhaps more difficult to reduce disproportionation into chlorate.


[Edited on 19-1-2013 by AJKOER]
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