Oxidation of iron and galvanised iron in bleach

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Discussion Overview

The discussion revolves around the oxidation of iron and galvanized iron in bleach (sodium hypochlorite), focusing on the unexpected magenta color observed in a laboratory experiment, as well as the presence of gas bubbles in the solution. The scope includes experimental observations and potential chemical reactions involved in the process.

Discussion Character

  • Exploratory
  • Technical explanation
  • Experimental/applied

Main Points Raised

  • One participant reports observing a magenta-colored solution with ferrous hydroxide deposits after immersing plain carbon steel in bleach for a week, questioning the source of the color.
  • Another participant suggests that the magenta color may not be due to an iron compound and speculates about the potential involvement of manganese, although they express uncertainty about its oxidation state.
  • There is a discussion about the presence of gas bubbles at the surface of the solution, with one participant noting that the expected reduction reactions do not involve gas production, raising questions about the source of the gas.
  • One participant mentions that sodium hypochlorite can decompose into oxygen gas, particularly in the presence of metals, and questions why iron catalyzes this decomposition.
  • Another participant notes that there were more bubbles in the beaker with plain carbon steel compared to galvanized steel, questioning whether the reduction reactions should theoretically be the same.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the cause of the magenta color and the gas production, with no consensus reached on these points. Multiple competing views about the chemical processes involved remain present.

Contextual Notes

Participants mention the use of commercial bleach rather than laboratory-grade chemicals, which may influence the results. There is also a lack of clarity regarding the specific iron compounds involved and the conditions under which the observed reactions occur.

Hannichan
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In a laboratory experiment I have tested plain carbon steel in a beaker of bleach (sodium hypochlorite), immersed for a week.

The result is a magenta-coloured solution with a large amount of ferrous hydroxide deposits.

Q1) I haven't been able to find anywhere anything that might be causing this magenta colour. Fe(II) is meant to be green, so is Fe(II) Chloride, Fe(OH)2 is brown and insoluble... so why is is deep magenta?

Q2) There is also a large amount of trapped gas at the surface of the solution. Why is this? The most likely reduction reaction is the ClO- + H2O+2e- => Cl- +2OH-, followed by O2+H2O+4e- => 4OH-. In both of these cases, no gas is involved, or gas is actually used up.
I have found that sodium hypochlorite decomposes into O2 gas (2NaClO=>2NaCl+O2), which is catalysed by the presence of metals, but WHY is this the case? In this equation the oxygen is oxidised, but in my situation it should be the iron being oxidised and therefore I need a reduction reaction. So where is the gas coming from?

Thank you!
(2nd year engineering student)
 
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No idea about the magenta - it is definitely not an iron compound. Color makes me think about Mn, and it is not an uncommon element in steel - but even then I am not convinced it would be oxidized to permanganate.

Decomposition can occur as a side process, occurring completely separately from the iron oxidation.
 
Have you used laboratory grade pure chemical or commercial bleach ?
 
Borek said:
No idea about the magenta - it is definitely not an iron compound. Color makes me think about Mn, and it is not an uncommon element in steel - but even then I am not convinced it would be oxidized to permanganate.

Decomposition can occur as a side process, occurring completely separately from the iron oxidation.

Thanks Borek. Do you know why the presence of iron catalyses the decomposition?

Re: the magenta- my lecturer apparently mumbled something to someone else in my lab group about the fact that it was due to the presence of some kind of iron compound. No idea what...Maybe he was making it up?? But it was definitely extremely pink/magenta, not just a small hint due to trace elements of Mn. Strange
 
Nidum said:
Have you used laboratory grade pure chemical or commercial bleach ?
Commercial bleach
 
Borek said:
Decomposition can occur as a side process, occurring completely separately from the iron oxidation.

Also there were a lot more bubbles present in the beaker with plain carbon steel, compared to the beaker with galvanised steel, although theoretically the reduction reaction would be the same right?
 

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