[Agalychnis]

I’m experimenting with running a small current between two electrodes in an aqueous solution of NaCl with small amounts of calcium and magnesium and sufficient exposure to air for gaseous exchange.  The goal is to accrete CaCO3 onto the cathode in a reaction caused by the production of hydrogen gas (Ca++ + H2CO3 + 2e- => H2 + CaCO3).  However, Mg(OH)2 also forms and mixes with the CaCO3, and this is undesirable.  I’ve done a little testing and it seems that some materials accrete more CaCO3 than others, and that’s what I’m wondering about.

As I understand from the reading I‘ve done, in electrolysis different anode materials can require different current densities to produce whatever chemical is being produced.  The materials have different "reaction overpotential" for the concerned chemicals.  The example given was different anodes in brine, and that most metals will produce chlorine gas before oxygen gas given a current density starting from zero and increasing; however, magnetite supposedly has a lower "production overvoltage" for oxygen than chlorine, and thus it would produce oxygen gas before chlorine gas in the same situation.

My question is whether the same can be said of the cathode, if there is a material that would produces the optimal (highest) ratio of CaCO3 to Mg(OH)2 and how I would find out what it is.  

It also seems that some materials may separate the two precipitates out better than others (copper being coated in almost pure CaCO3 with a pile of Mg(OH)2 below it, and titanium being coated with what seems to be a mixture of the two that easily flakes off), and I’m wondering if that makes sense and if so how to know how materials will act in this regard so that I can choose the best one for maximum CaCO3 purity.

Thanks!

[kevins]

Have you use the titanium electrodes?

[Agalychnis]

Yes, I've also tested graphite and copper.  Graphite was my plan A, but it deteriorated (not at all what I expected from the cathode).  Plan B was titanium, which accreted a mixture of Mg(OH)2 and CaCO3, with pieces flecking off and pilling up below along with some loose Mg(OH)2. This was notably different from the copper electrodes I'd been using while testing anode materials.  The two precipitates had separated out much more with the copper, leaving a pile of Mg(OH)2 below and a coat of almost pure CaCO3 on the cathode.  I'd like to find out what caused the difference, and how I can use it to my advantage.

[kevins]

Graphite is not a good one in this case becasue the produced gas will damage the its structure. The metallic electrode is much better. I perfer to use Platinum and Titanium electrode, not both Ti because the oxygen gas may attach the Ti structure. Slightly stir the solution.
Please try and post any good news, because this is a good subject for me.