pH in SWG cell

mgtfp

Bronze Supporter
Mar 5, 2020
276
Melbourne, Australia
I found an interesting paper on calcium carbonate deposition on the electrodes of an electrolysis cell:


The paper is very recent, due for publication in the October 2020 issue of the Journal of Environmental Chemical Engineering. This issue is in progress, but contains articles that are final and fully citable. This article seems to be published under ScienceDirect's "open access" scheme and is freely available.

The authors' motivation was to use the cathode of an electrolysis cell as sacrificial material to protect other materials from scaling. They didn't investigate the electrolysis of NaCl, but the electrolysis of water. They added large amounts of CaCl2 (15000 ppm and 1500 ppm CaCl2, equivalent to 13600ppm and 1360 ppm CaCO3) and triggered the scaling process by injecting CO2 gas (in a pool, the CO2 is already dissolved according to the TA-level) . The results are quite interesting, and should also be applicable to SWG cells.
  • They measured the pH of the "bulk" solution exactly in the middle between cathode and anode, and the pHs right next to the cathode and the anode. The distance between cathode and anode was 7cm, the pH at cathode and anode was measured 4mm away from the respective electrodes. The pH at the cathode was higher compared to the bulk solution, and it was lower at the anode.
    This is probably not a revelation to us, but I haven't really seen this so far that nicely presented in a graph.

  • The surface of a reference rod (without voltage applied to it) was only slightly covered with white precipitate, whereas calcium carbonate deposition was accelerated at the cathode, and inhibited at the anode. This was explained with the local pH around each of the three rods.
    That really underlines that the scaling in an SWG cell is not an effect of an overall higher pH within the whole cell, but an effect due to localised increased pH around the cathode. The pH around the anode is more acidic and protects the anode from scaling. That's where reverse cycling of an SWG comes into play.

  • Higher voltage resulted in more scaling and larger crystallite size. Crystallites deposited from lower concentrations of CaCl2 were more homogeneously distributed over the surface and were smaller compared to those from higher CaCl2 concentrations.
    That seems to be in agreement with our experience with SWG cells:
    Also, scaling is far more likely in a cell that is run at 100% or near that output than a cell that is run at lower output but for a longer period of the day.
  • At the highest investigated voltage (10V), the deposits started to flake off. This was explained with erosive processes associated with intense gas release.
 
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JamesW

TFP Expert
Mar 2, 2011
19,760
They didn't investigate the electrolysis of NaCl, but the electrolysis of water. They added large amounts of CaCl2 (15000 ppm and 1500 ppm CaCl2, equivalent to 13600ppm and 1360 ppm CaCO3) and triggered the scaling process by injecting CO2 gas (in a pool, the CO2 is already dissolved according to the TA-level) .
If they added that much calcium chloride, the chloride concentration would be high enough that chlorine would be evolved preferentially over oxygen. So, it’s the electrolysis of brine and not water. The chloride does not need to come from sodium chloride.

Injecting CO2 would lower the pH and it would not trigger a scaling process.

Carbon dioxide dissolving in the ocean is acidifying the water and dissolving calcium carbonate or preventing its creation.
 

mgtfp

Bronze Supporter
Mar 5, 2020
276
Melbourne, Australia
If they added that much calcium chloride, the chloride concentration would be high enough that chlorine would be evolved preferentially over oxygen. So, it’s the electrolysis of brine and not water. The chloride does not need to come from sodium chloride.

Injecting CO2 would lower the pH and it would not trigger a scaling process.

Carbon dioxide dissolving in the ocean is acidifying the water and dissolving calcium carbonate or preventing its creation.
Not sure if I'd call it a brine already, I think it's usually called a brine from sea water concentrations onward. But certainly a very high concentration, at least the higher of the two investigated ones.

They don't really mention in their paper whether oxygen or chlorine gas is released at the anode. Redoxpotential for water oxidization is actually less negative than for chloride oxidization. If chlorine gas production is desired, usually an electrode material is chosen with a high overvoltage for water oxidization and the voltage is chosen so that water oxidization is more or less fully suppressed. I don't know which reaction is preferred for the chosen graphite electrodes and the chosen voltage range. But you are right, in most cases the overvoltage for water oxidization is sufficiently high so that Cl2 creation is favoured over O2 creation, unless the Cl- concentration is very low.

The experiment was started at pH=12. As the experiment proceeded, pH continuously dropped until below 7. It dropped much quicker at the anode (which protected the the anode from scaling), and the drop was delayed at the cathode.