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I observed a sharp decrease in the current when I carried out the electrolysis of water. Just to set the context, I used a voltage of $\pu{12V}$ and the concentration of my electrolytes were $\pu{0.15 mol dm^{-3}}$ for $\ce{Ca(NO3)2}$ and I used graphite rods which were flaking but I am not sure whether that might cause such a significant dip in the current. I did also notice some white material forming on the cathode. So I want to know why there is such a dip?

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  • $\begingroup$ Sounds interesting. Could you add a little more detail? How much current, peak and valley? Any bubbling that covers the electrodes? How large were the electrodes and how far apart? $\endgroup$ Commented Feb 25, 2021 at 13:48
  • $\begingroup$ Did you observe any white, solid material forming at your cathode? $\endgroup$
    – hBy2Py
    Commented Feb 25, 2021 at 14:03
  • $\begingroup$ The current was highest as soon as electrolysis started at 0.85A. Then I recorded the current value in 10s intervals and I got 0.5A, 0.37A, 0.3A, 0.26A and so on. This is for 0.15 mol dm-3 Ca(NO3)2 solution. The graphite rods with diameter 0.62cm and a height of 5.65cm with a separation of 2.56cm. It was a pretty small setup. $\endgroup$ Commented Feb 25, 2021 at 15:04
  • $\begingroup$ I did notice some whitening of the electrode. $\endgroup$ Commented Feb 25, 2021 at 15:06
  • $\begingroup$ Why did you choose calcium nitrate as electrolyte ? Why did you choose such a high voltage as $\pu{12 V}$. Have you ever observed a similar phenomena with a more common electrolyte ? On which electrode did you see a white deposit ? Have you observed an increase of temperature ? What sort of currant generator have you got ? Is it able to stabilize its tension ? Don't you think the trouble may be due to the feeder ? $\endgroup$
    – Maurice
    Commented Feb 25, 2021 at 17:40

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Given the whitening observed at the cathode, one potential explanation is that calcium salts were precipitating there, forming a layer blocking passage of part of the current.

I usually write the cathodic hydrogen-evolution reaction in neutral solution as:

$$ \ce{2 H2O + 2 e- -> H2 + 2OH-} $$

Thus, the pH in the vicinity of the cathode will be considerably greater than 7 during the electrolysis process, due to the $\ce{OH-}$ formed. Since you're using a calcium-based electrolyte, it seems likely that some combination of $\ce{CaO}$/$\ce{Ca(OH)2}$ is precipitating in that alkaline surface layer. Once formed, in my experience such solids will dissolve only slowly in neutral aqueous solution.

If possible, I'd recommend trying the experiment again but in a different electrolyte, such as $\ce{KNO3}$ or $\ce{NaNO3}$; those alkali cations shouldn't exhibit any precipitation.

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