Electrolysis of sodium chloride solution

[lasha1]

In electrolysis of nacl where is sodium? Ok clorine is producing as gas but sodium?

 

[Bystander]

What cationic species are present in solution to be reduced, and which are more easily reduced?

 

[rootone]

The sodium binds with a hydroyxl (OH) from the water, the other hydrogen atoms from the water molecules are released as hydrogen gas.
So you end up with Sodium Hydroxide (NaOH) in the solution

 

[Windadct]

That's why on my damn saltwater pool I do not buy any chlorine but am now putting Muriatic Acid in every week!

 

[lasha1]

Therefor when the electrolysis whoud be done you chould have NaOH solution?

 

[rootone]

Yes if you leave it for long enough most of the Chlorine will be gone and the solution will mainly be NaOH.

 

[lasha1]

Thanks

 

[Radhakrishnam]

When we electrolyze aqueous NaCl solution using electrodes, such as graphite, Chlorine gas is evolved at the anode and Sodium metal is formed at the cathode - but Sodium being highly reactive metal - reacts with water producing Hydrogen gas and Sodium Hydroxide. Therefore, you don't find any Sodium as product of electrolysis from aqueous NaCl solution. However, if we use Hg (mercury) metal as the cathode, then, we get Sodium amalgam at the cathode, even in aqueous solutions. Mercury prevents hydrogen to form by having the property called high hydrogen over voltage for hydrogen evolution reaction ( this is just by way of info, you may not understand it, and you may ignore it without loss of understanding the process).

The other possibility of obtaining Na metal by electrolysis of NaCl is through the electrolysis of molten NaCl, where there is no water to react with the Na formed at the cathode in the electrolysis cell.

P. Radhakrishnamurty

 

[Borek]

When we electrolyze aqueous NaCl solution using electrodes, such as graphite, Chlorine gas is evolved at the anode and Sodium metal is formed at the cathode - but Sodium being highly reactive metal - reacts with water producing Hydrogen gas and Sodium Hydroxide.

While this is a good way of explaining why it is impossible to produce sodium from water solutions, I have never seen any reliable source confirming that's the way it works. As far as I know we don't get sodium - water reacts directly producing hydrogen. We observe exactly the same reaction in solutions that contain other cations, and the potential required for hydrogen evolution doesn't depend on the cation identity, but it depends on the solution pH, which strongly suggests it is water that reacts directly.

 

[epenguin]

What happens to sodium when you put it in water? If you had ever seen it you would remember.

 

[Radhakrishnam]

Dear Borek
What you wrote is not clear to me - 'water reacts directly producing hydrogen'. Water reacts with what?
'We observe exactly the same reaction in solutions that contain other cations'. What reaction are you referring to?

If we electrolyse water containing small quantities of any inorganic salt or acid, we get hydrogen gas at the cathode and oxygen evolution at the anode, at 1.23V, when the activity of H⁺ = 1 and P_O2 =1. For other values of these parameters the applied cell potential needs to be around this value of 1.23V. If the salt concentration is high, instead of oxygen evolution other reactions such as chlorine evolution from chloride containing solutions occur. In essence, the reactions that occur at the two electrodes depends on the concentrations of the ions present in the solution and the applied cell potential, besides the electrode materials.

Now, suppose our aim is to electrolyse a solution such as aqueous NaCl to produce sodium metal, then we go to much higher cell potentials when the potential of the cathode is forced below the equilibrium potential value for reduction of Na⁺ ions. Then Na deposition becomes possible. However, sodium is never found at the cathode in aqueous solution electrolysis. The reason is that Na reacts spontaneously with water to produce hydrogen gas and sodium hydroxide.

It is from this point of view, wherein the aim is to produce Na by the electrolysis of aqueous solutions of NaCl, I wrote the answer to Bystander's question, where he wanted to know where sodium was.

If, however, the aim is simple electrolysis of concentrated solutions of aqueous NaCl, we get Hydrogen at the cathode and Chlorine at the anode.

Overall, given an electrolyte solution, what products result on its electrolysis, depend on the cell potential and nature of the electrodes used.

P. Radhakrishnamurty

 

[Borek]

What you wrote is not clear to me - 'water reacts directly producing hydrogen'. Water reacts with what?

I am referring to the electrode reaction, that's what we are discussing here. Water reacts directly on the electrode, with electrons supplied (or taken from it).

'We observe exactly the same reaction in solutions that contain other cations'. What reaction are you referring to?

The same reaction involved above - electrode reaction.

Now, suppose our aim is to electrolyse a solution such as aqueous NaCl to produce sodium metal, then we go to much higher cell potentials when the potential of the cathode is forced below the equilibrium potential value for reduction of Na+ ions. Then Na deposition becomes possible. However, sodium is never found at the cathode in aqueous solution electrolysis. The reason is that Na reacts spontaneously with water to produce hydrogen gas and sodium hydroxide.

Nope. On typical electrodes, as long as there is water present, the reduction potential never goes that far down, we will be all the time in the range predicted by the Nernst equation for water. Even if you try to force lower reduction potential, it is still water that reacts first. Only once there is no water in the solution, we can start to observe Na⁺ reduction.

The case with mercury electrode is slightly different for kinetic reasons. Hydrogen is quite slow to react on the mercury surface and the reaction requires high overpotential. High enough, that the reduction of sodium becomes possible (I don't have tables here to check, but from memory the sodium reduction is also easier on the mercury - amalgam is not a standard state).

 

[Radhakrishnam]

'Even if you try to force lower reduction potential, it is still water that reacts first. Only once there is no water in the solution, we can start to observe Na+reduction'.

When we force the electrode potential to values lower than that corresponding to the equilibrium potential of sodium deposition (using a potentiostat, for instance), sodium deposits, but being active, reacts with water liberating Hydrogen gas. Hydrogen too evolves as a result of parallel electrode reaction but not as an exclusive reaction.
There is no first reaction (water, as you say) and second reaction and so on - there are only parallel or simultaneous reactions at the electrode!

If the OP understands and gets his answer, that serves the purpose and we are happy.

P. Radhakrishnamurty

 

[Borek]

When we force the electrode potential to values lower than that corresponding to the equilibrium potential of sodium deposition (using a potentiostat, for instance), sodium deposits,

Please provide a reliable reference (as defined in the forum rules) for the fact sodium can deposit from solutions containing water and was observed in the experiment.

 

[Radhakrishnam]

The following references may be consulted for electrolytic deposition of sodium from aqueous solutions of NaCl. It is a well known industrial process. Therefore, you may not find it difficult to get references for this process. I give below two, I gathered from the net.

1. See (b) The mercury cell, in the following reference.
http://www.essentialchemicalindustry.org/chemicals/chlorine.html

2. http://www.google.co.in/patents/US4290863.

P. Radhakrishnamurty

 

[Borek]

Nowhere it says about sodium production on electrodes other than mercury. I already told you it is a different situation. In the case of mercury electrodes sodium is never directly in the contact with water, and it is reduced preferentially due to the high overpotential for hydrogen reduction on the mercury.

Looks to me like you are mistaking both scenarios, or you incorrectly overgeneralize the reduction of sodium on mercury to other electrodes.

To reiterate: if you use mercury electrode, sodium gets reduced to amalgamate from water solutions. If you use any other electrode, H⁺ gets reduced and hydrogen is produced, there is NO* intermediate metallic sodium produced.

*added omitted "no"

 

[Radhakrishnam]

Thanks.

'If you use any other electrode, H+ gets reduced and hydrogen is produced, there is intermediate metallic sodium produced'. This is what I have been saying, too!

P. Radhakrishnamurty

 

[Borek]

Sigh, typo on my side. Intermediate metallic sodium is NOT produced. Corrected that.