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I am a student and I take O.L. Chemistry, in my knowledge I know if I electrolysed concentrated aqueous NaCl solution I should get Hydrogen evolved at the cathode and chlorine at the anode.

I did an experiment, I add a lot of table slat to tap water to get the electrolyte ready (aq. NaCl) then I used graphite electrodes (just got it from a mechanical pencil) and passed electricity through the salty water. I used of course direct current using a 6 Volts battery I got.

Well my suggestions were half correct, Hydrogen really evolved and bubbled violently at the cathode, but at anode I didn't see any yellowish green gas indicating presence of chlorine, yet only colorless bubbles continue to evolve from the anode but was less violently than the cathode. I thought this is Oxygen evolving so I increased the concentration of solution by adding table salt but also only colorless bubbles evolves at the anode. Well I didn't really calculated how much precisely I should add salt but I added insanely a lot of table salt.

Only to discover after around 5 minutes of electrolyzing the salty water, it became darkish green in color, I didn't really knew what is this and this is my question, I used graphite electrodes so those doesn't react like if I used iron, the solution was just nice tap water and not really bad or muddy. My only suggestion is that this is actually chlorine dissolved in water ... So is this correct ?? I knew that chlorine is soulable in water but I didn't thought it will turn the whole water greenish in color....

Can anyone help me in that ? And please if you're going to answer me, answer me as if I am somebody who knows nothing in chimestry .....

Thanks in advance.

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  • $\begingroup$ @Mithoron unfortunately no, it doesn't and I already saw this answer, I am using graphite (carbon) electrodes there is no other ions except Na ions and Cl ions and H and OH there is no copper or any other metal. $\endgroup$
    – Shams M.Monem
    Commented Apr 26, 2020 at 18:49
  • $\begingroup$ Your "graphite" can be leaking who knows what, then. Maybe even copper ;) $\endgroup$
    – Mithoron
    Commented Apr 26, 2020 at 18:53
  • $\begingroup$ chemistry.stackexchange.com/questions/90529/… $\endgroup$
    – Mithoron
    Commented Apr 26, 2020 at 19:05
  • $\begingroup$ And your cables are still copper, aren't they? $\endgroup$
    – Mithoron
    Commented Apr 26, 2020 at 19:06
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    $\begingroup$ @M.Farooq I thank you very much for the advice .. I will try another pencil lead and see . And will try to attach image of my setup If I can. $\endgroup$
    – Shams M.Monem
    Commented Apr 26, 2020 at 20:05

1 Answer 1

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My speculation is based on the words specifying the use of 'tap water' during the electrolysis of aqueous sodium chloride.

In particular, the interaction of formed chlorine and existing ferrous bicarbonate (found in tap water) could form small amounts of aqueous FeCl2. [EDIT TO QUOTE SOURCE]

Bacterial Iron in Well Water – Iron bacteria is usually identified by slime in places such as toilet reservoirs or by the presence of a slimy mass fouling softeners or filters.

Ferric Iron in Well Water – Also known as red water iron, ferric iron in well water is essentially clear water iron that has been exposed to oxygen – usually from the air, thereby oxidizing. Carbon dioxide leaves the water and the oxygen combines with the iron to form ferric ions (Fe+++). This gives the water a red rust coloring.

Ferrous Iron in Well Water – Often called clear-water iron because it is clear when poured, this substance is found in water that contains no oxygen. Typically, it comes from deeper wells and groundwater sources. Carbon dioxide acts on iron in the ground to form soluble ferrous bicarbonate. In water this forms ferrous ions (Fe++)

Per Wikipedia on Ferrous chloride, to quote:

The compound is highly soluble in water, giving pale green solutions.

Repeat the electrolysis experiment with distilled water and see if you get the same coloration issue.

[EDIT][EDIT] Interestingly, low solubility Fe(OH)2 is clear and only acquires a greenish tint in the presence of oxygen (per Wikipedia, to quote: "Iron(II) hydroxide is a white solid, but even traces of oxygen impart a greenish tinge.). Further, well-water is associated with high levels of bicarbonate and low O2 levels (see this significant work). To claim that Fe(HCO3)2 is at all green is without basis, as I also could not find any source anywhere asserting a coloration, other than that it is, in effect, 'clear' as was claimed above per the comment: 'Often called clear-water iron because it is clear when poured'. Theoretical formation of the usual hydration sphere for Fe(OH)2 interacting with bicarbonate in low solubility conditions with high HCO3-, low O2, significant CO2 presence (and its associated partial pressure) suggest reasons why it may not be a valid argument. If I am wrong, please provide a single source citing color, else it should be likely assumed that my supposition is correct and Fe(HCO3)2 is clear. By the way, Atomistry on Ferrous bicarbonate, to quote:

Ferrous hydrogen carbonate or ferrous bicarbonate, FeH2(CO3)2, exists in solutions obtained by dissolving ferrous oxide or carbonate in water charged with excess of carbon dioxide. On exposure to air carbon dioxide is evolved, and a precipitate is obtained consisting essentially of ferric hydroxide.

which one could interpret as that clear-water iron, upon exiting the pipe, liberates CO2 and forms Fe(OH)2, which, with further oxygen exposure, may display a green tint (or, per the introduction of electrolysis formed chlorine).

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  • $\begingroup$ Cl2 + H2O = HCl + HOCl. Fe(HCO3)2+ 2HCl = FeCl2 + 2 H2O + 2 CO2. Further, HOCl will react in a fenton-type reaction with Fe(2+). Namely: Fe(2+) + HOCl = Fe(3+) + .OH + Cl- $\endgroup$
    – AJKOER
    Commented Apr 27, 2020 at 13:57
  • $\begingroup$ I have EDITED my thread to make very explicit the sources of iron common to well-water. However, city water, with iron pipes, is also an apparent source of Fe ions even if the water is free iron-free sources. The decomposition of iron pipes is likely sourced via galvanic based corrosion . $\endgroup$
    – AJKOER
    Commented Apr 27, 2020 at 14:39
  • $\begingroup$ I do thanks those who have doubts on the content of their water supply as I find the chemistry interesting and rich. I was personally shocked to find my water was sourced, per reading of my water bill, from 3 sources, reservoir, rivers, and well-water. The latter, I view, as more of a disclosure to explain the range of possible contaminants likely to be found in my tap water. $\endgroup$
    – AJKOER
    Commented Apr 27, 2020 at 14:48
  • $\begingroup$ Note, my source reference to 'clear-water iron' explicitly stated to be ferrous bicarbonate. $\endgroup$
    – AJKOER
    Commented Apr 27, 2020 at 17:01
  • $\begingroup$ Rewriting my comment to be more clear and concise, since it was apparently not understood: If FeCl2 is the source of the green color, the color should be apparent BEFORE the electrolysis has begun, since the concentration of neither ion should increase during electrolysis. If anything, they will decrease because Fe(2+) may be oxidized to Fe(3+) and Cl- to Cl2. $\endgroup$
    – Andrew
    Commented Apr 27, 2020 at 21:51

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