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We are doing a simple water electrolysis experiment but I have issues with the electrolyte solution.

Took 450ml of distilled water and slowly dissolved 50g of NaOH until heating ceased. If I'm not wrong that should be a 10% solution (in weight) and about 2.5M.

From this resource the electrical conductivity should be about 309 mS/cm, so 3.2 Ohm×cm; this seems consistent given that electrolysis uses a lot of current.

However with the highly non-scientific method of dipping the DVM probes in the solution I see more than 20 kOhm resistance, which is nowhere near what I'm expecting (sorry, no specialized equipment there). And, of course, the experiment fails (no significant current drawn)

Am I doing something wrong?

I fear that the sodium hydroxide is degraded (it's hardware store grade flakes, supposedly 98%), given that the bottle was not sealed (only a child proof cap, but not airtight). I know that it absorbs CO2 from the air but I've no idea of how much can influence it.

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2 Answers 2

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Note that DVM is Digital Voltmeter.

It may use too low DC voltage, what is OK for linear electronic components, but completely misleading for electrochemistry scenarios.

It may show high resistance after very short transient period, as after initial potential re-balancing, there is nothing to carry the current, if voltage is too low to perform electrolysis.

Better is to use AC several kHz with suitable amplitude.

Even with high enough DC, the I/U ratio is not equal to AC based conductance, because of electrode potential difference and possibly other boundary potential drops.

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  • $\begingroup$ Uhm OK so the issue is that a DC current is stuck if it doesn't reach the cell potential; I'll try with an AC measurement then, thanks $\endgroup$ Jun 17, 2022 at 9:49
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    $\begingroup$ Note that to get conductivity, you have to compare conductance, using identical geometry, with conductance of solution of known conductivity, typically KCl. $\endgroup$
    – Poutnik
    Jun 17, 2022 at 10:27
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For posterity: it was a bad/expired batch of NaOH.

Got a fresh supply from a reputable supplier and works correctly!

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  • $\begingroup$ Was it just the sodium hydroxide? Did you also adopt Poutnik's suggestions? $\endgroup$ Jun 20, 2022 at 11:25
  • $\begingroup$ NaOH converted by air exposure to Na2CO3 would have just decreased conductivity, what itself would not cause such low measured resistance. Electrode related potential drops must be involved. $\endgroup$
    – Poutnik
    Jun 20, 2022 at 13:13

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