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I was thinking of preparing a tiny oxy-hydrogen flame for soldering. I am going to use 6V DC and aluminum sheet as the electrode for this. Now, I have to add some electrolyte to water, that can be a salt, base or acid. But, if I add $\ce{NaCl}$ or $\ce{HCl}$, they will produce chlorine. If I add $\ce{NaOH}$, it will eat electrodes. Adding $\ce{H2SO4}$ may produce poisonous gases like $\ce{SO2}$ and $\ce{H2S}$. So, what can be a harmless/least harmful electrolyte for such purpose? Is $\ce{Na2CO3}$ a good choice?

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    $\begingroup$ More importantly, this is a very bad way to create a flame. It will require far more effort than just buying a butane or propane torch. $\endgroup$ – matt_black Mar 7 '17 at 16:42
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    $\begingroup$ @matt_black Obviously butane or propane torch are good ways, probably the best. But, I was just thinking of creating a flame myself or at least trying to do so. $\endgroup$ – Dwiparna Datta Mar 7 '17 at 17:32
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Electrolysis of water has become an increasingly common means of creating torches for soldering and welding, and there are commercial devices available as well as plenty of internet do-it-yourself sites. According to this site regarding the SafeFlame consortium:

"[...]the European Union-funded SafeFlame consortium has developed a torch system that generates a flame using nothing but H2O and electricity. SafeFlame utilizes an electrical current to electrolyze ordinary water, separating it into hydrogen and oxygen gas. Those gases are then mixed and ignited as they exit the torch's nozzle. By fine-tuning the proportions of the two gases, different types of flames can be produced for different applications."

A commonly used electrolyte / electrode combination is $\ce{KOH}$ or $\ce{Na2CO3}$ with stainless steel electrodes. I believe this would be pretty corrosive toward aluminum, but stainless steel for such a setup should be cheap and easy to come by.

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