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I want to electrolize salt ($\ce{NaCl}$) water into a solution of sodium hydroxide for soap making. The salt I have has yellow prussiate of soda (sodium ferrocyanide) added as the anticaking agent. What will happen to the ferrocyanide ($\ce{Fe(CN)_6}$) molecules during the electrolysis process?

I will be using an old battery charger to perform the electrolysis, $20\, \mathrm{V}$, $0.9\, \mathrm{A}$ or $1.8\, \mathrm{A}$ settings.

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    $\begingroup$ ""yellow prussiate of soda (sodium ferrocyanide) "" I doubt that. Look up "ferro" and "ferri" $\endgroup$ – Georg Dec 21 '14 at 22:38
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    $\begingroup$ @George Sodium ferrocyanide is frequntly used as an anticaking agent added to food grade salt. $\endgroup$ – Keith Reynolds Feb 2 '18 at 17:41
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If you want sodium hydroxide for soap making you should just buy some: it is a cheap and common chemical. This is because there are effective industrial processes to make it in vast quantities.

Trying to make it yourself is not easy and will cost you far more effort than just buying it. The least of your problems will be what happens to contaminants like the hexacyanoferrate (which is present in such low concentrations it isn't really significant). Have a look at the way industry makes bulk hydroxide and chlorine (e.g. here); it isn't as simple as just electrolysing brine. All the processes involve sophisticated ways to separate the products produced at the different electrodes. You don't get decent quantities of either product if you don't maintain this separation (one method uses flowing mercury; another expensive ion-exchange membranes). In addition to this, uncontrolled electrolysis is dangerous as it produces toxic and explosive gases. You really don't want to try it at home.

Since the most basic components for an effective reaction will cost you far more than the couple of dollars a large bottle of pure sodium hydroxide will cost, buy the chemical and ignore the electrolysis as a route to what you want.

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  • $\begingroup$ I think your answer is stating the obvious. OP was probably hoping to find out how to circumvent some or all of the obvious. I am also curious. $\endgroup$ – KalleMP Feb 2 '18 at 8:29
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Electrolize salt (NaCl) water without a proton exchange membrane is more likely to produce NaOCl. You need the membrane to keep chlorine and sodium hydroxide from combining to form bleach. The membrane may be made from a polymer which only allows positive sodium ions to pass through it.

Saftey note. The whole process also creates dangerous potentially deadly chlorine gas if not vented properly.

Its probably safer and cheaper to buy your sodium hydroxide from a commercial source as the producer is likely able to capture revenue from selling the sodium hydroxide as well as chlorine, and hydrogen that are made during the process, while buying their electricity at lower than residential rates.

Edit, I am not sure what will happens to the sodium ferrocyanide. The worse case is it is also broken down, and that at above room tempurature, cyanide gas will evolve at the same electrode as the chlorine gas, underscoring the need for proper ventilation.

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  • $\begingroup$ Same problem as the above, you are clarifying why it is hard, not how to make it happen. There are cases where buying is not an option. What if your underwear on Mars is dirty and you have to go to see the nurse. You have salt (oil too I hope) and electricity but no nearby shop to buy soap or sodium hydroxide but the Space-X people thoughtfully included a lot of salt as a raw chemical feedstock. (Pity about the anti caking agent though.) $\endgroup$ – KalleMP Feb 2 '18 at 8:34
  • $\begingroup$ KalleMP, The OP asked how to make NaOH from salt with electrolysis, not how to make soap to wash underware on Mars. He has all he needs except a proton exchange membrane. The first thing I did was tell him he needs one and why. Sure, I did not write a how to make a proton exchange membrane because that was not the question. I supose if one were to write a "how a dig a hole", one could start with cutting down a tree for the handle, and how to forge steel for the shovel blade, but where does it end, plant a tree and make an axe before he cuts down said tree for the shovel's handle? $\endgroup$ – Keith Reynolds Feb 2 '18 at 17:17
  • $\begingroup$ You are right. It looks like there are suitable films available from specialist suppliers. I was hoping that a salt bridge or some other lateral thought might be an option for someone trying to do this at home. It is good that you added something about the OPs question to your answer as well. - fuelcellsetc.com/2012/07/… $\endgroup$ – KalleMP Feb 3 '18 at 18:44
  • $\begingroup$ There are many options for a proton exchange membrane, including ones that can be fabricated at home with common materials. The breath of possibilities is largely why i thought it inappropriate to choose one specific type to use given the op's specific question. $\endgroup$ – Keith Reynolds Feb 3 '18 at 21:49

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