# Green deposition on the anode after electrolysis of sodium bicarbonate solution in tap water

I use $$\pu{12 V}$$ battery with copper wires and tap water with $$\ce{NaHCO3}$$ dissolved in it. I connected the battery and everything went as expected but after 5 minutes bubbling stopped. I took out the copper electrodes and saw green deposits on the anode that were blocking the current.

I think green stuff was malachite formed by the reaction

$$\ce{2 Cu + O2 + H2O + CO2 -> Cu2(OH)2CO3}$$

because there is oxygen in the anode water in solution copper in the wire but how did the $$\ce{CO2}$$ get there? I know bicarbonates hydrolyze to give carbonic acid which gives $$\ce{CO2}$$ but doesn't that only happen at the surface? Any ideas what that green thing might be?

• Tap water leaves room for imagination due to possible side reactions occurring with the dissolved salts and even organics. Ideally you want to use distilled water for any electrochemical experiment. Apr 1 '21 at 6:42
• If sodium bicarbonate is dissolved in water, it produces $\ce{HCO3-}$ ions which are in equilibrium with $\ce{CO2}$ according to the following equilibrium $$\ce{2HCO3- <=> CO3^{2-} + CO2 + H2O}$$ This is the origin of the $\ce{CO2}$ engaged in your equation. Apr 1 '21 at 9:57
• So are there $\ce{CO2}$ molecules everywhere in the solution? Apr 1 '21 at 10:00
• Yes. CO2 molecules are everywhere in a $\ce{NaHCO3}$ solution. Apr 1 '21 at 10:54
• Apr 1 '21 at 19:11