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I'm developing a system for educational purposes where electrolysis of water takes place. The aim is to practically show how electrical energy can be converted to mechanical, by electrolyzing water, igniting the mixture of $\ce{H2}$ and $\ce{O2}$ produced, and accelerating a mass upwards.

Currently I'm using citric acid in distilled water ($4\%$ in weight, $18~\mathrm L$ water), platinum-coated mesh electrodes, $500~\mathrm W$ max power supply ($0\!-\!65~\mathrm{V_{DC}}$, $0\!-\!10~\mathrm A$). I'm wondering if there's any better option that meets the following requirements:

  • Not harmful to maintainers (that have to prepare and replace the liquid) and bystanders
  • Stable (not producing precipitates, and in general that can minimize the need for replacing the liquid)
  • Low/no interaction with the materials in the tank (Plexiglas, brass, platinum, silicone tubes, ABS)
  • Maximizes conductivity (the current setup at $20~^\circ \mathrm C$ has a conductance of $250\!-\!300~\mathrm{mS}$)
  • Not suitable for microorganism development (in the current setup algae develops in about two weeks)

If the last point can't be improved with just the electrolyte, I'd kindly ask for a suggestion for an additive that is not going to spoil the points above.

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  • $\begingroup$ Sodium carbonate, maybe? $\endgroup$ – Ivan Neretin Jan 14 '16 at 12:03
  • $\begingroup$ @IvanNeretin Signigificantly lowered conductance $\endgroup$ – permeakra Jan 14 '16 at 12:46
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    $\begingroup$ Welcome to Chem.SE - great question! I think something's wrong with your conductivity measurement, though. Per here, citric acid at 4% w/w should have a conductivity down around $4~\mathrm{mS \over cm}$. $\endgroup$ – hBy2Py Jan 14 '16 at 12:47
  • $\begingroup$ Diluted sulfuric acid. High conductance (because hydrogen cations), definitely no electrolized itself (except persulfuric acid formation at low temperatures), nontoxic. Unfortunately, dangerous in high concentration, but that's workable. Phosphoric acid may work as well. $\endgroup$ – permeakra Jan 14 '16 at 12:51
  • $\begingroup$ @permeakra Phosphoric acid has terrible conductivity at anything under 10% w/w. Probably not safe enough. $\endgroup$ – hBy2Py Jan 14 '16 at 12:53
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I suggest $1\%$ w/w $\ce{NaOH}$. Point-by-point:

  • $1\%$ w/w is $\sim\!10~\mathrm{g\over L}$, which for $\ce{NaOH}$ at $40~\mathrm{g\over mol}$ is $\sim\!0.25~\mathrm{M}$, corresponding to a $\mathrm{pH}$ of $13.4$. Standard personal protective equipment (coats, gloves, goggles) should suffice for observers and maintainers.

  • $\ce{NaOH}$ is extraordinarily soluble, so you will have no difficulties with precipitation. You will want to keep the system covered when not in use, if you're not already, to minimize evaporation. You may get some riming on the interior, but rinsing with DI water should take care of it.

  • According to Schweitzer$^\dagger$, the materials in your system have the following resistances to $10\%$ (not $1\%$) $\ce{NaOH}$:

    • Plexiglas (acrylic): Resistant, to $80~^\circ\mathrm F$
    • Brass: Good (less than 0.02" penetration per year), to $200~^\circ\mathrm F$
    • Silicone elastomer: Resistant, to $80~^\circ\mathrm F$
    • ABS: Resistant, to $140~^\circ\mathrm F$
    • Platinum: ..... darn well better hold up! (Though, if there are any gaps in the coating, the substrate might be exposed to attack.)
  • Per the first chart here (PDF link), the conductivity of $1\%~\ce{NaOH}$ should be about $50~\mathrm{mS\over cm}$, which is over ten times the $\sim\! 4.5~\mathrm{mS\over cm}$ of your current $4\%$ citric acid recipe.

  • I salute any common microorganisms able to grow in a $\mathrm{pH}~13+$ solution. (See, e.g., Table 3-5 here, where the maximum survivable $\mathrm{pH}$ for any species listed is $11$.)

Obviously, if you should switch, monitor the health of the system materials to be sure the above literature chemical resistance data actually holds in reality. There's always the chance some small component of the system is of a material you haven't listed, too.

(To note, I re-thought the vinegar-plus-sodium-bisulfate approach after discovering that both brass and acrylic/Plexiglas are poorly resistant to vinegar, and that the bisulfate is not likely to provide as much of a conductivity boost as I was thinking.)

$^\dagger$ P.A. Schweitzer. "Corrosion Resistance Tables, Part C, P-Z." 4th Ed. New York: Marcel Dekker, 1995, pp. 2685-2688. (Amazon link)

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  • $\begingroup$ Awesome suggestion, @Brian! I've just ordered a kg of pellets and I'll be able to perform a test early next week and feed the results back here. $\endgroup$ – oxullo Jan 14 '16 at 17:49
  • $\begingroup$ Under test since yesterday, the solution stays clear, conductance is high (350mS at 10A, 20∘C) even though not 5-fold, electrodes look healthy (no fancy deposits), materials compatibility looks good (even though I can't really predict what happens on long term). I'll give it a try on one of our prototypes, crossing my fingers for a decent blast :) Thanks! $\endgroup$ – oxullo Jan 19 '16 at 10:29
  • $\begingroup$ @oxullo Great! Hope it works out well for you. If you end up wanting more conductivity and the system appears to hold up well, you could probably bump the $\ce{NaOH}$ concentration up to $2$ or $3\%$ without much worry to safety ($\mathrm{pH}$ still below $14$) or to attack on the materials. Caveat emptor, of course.... $\endgroup$ – hBy2Py Jan 19 '16 at 11:44

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