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Author: Subject: Electrolysis question
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[*] posted on 28-11-2004 at 16:14
Electrolysis question


I have a very basic question that I cannot for the life of me figure out. When a solution of NaCl is electrolysed what makes the difference between it going to perchlorate and chlorate or sodium hydroxide(?) and Chlorine gas?
But why HCl cannot be oxidised to perchloric acid. Does it have to do with he electrodes? Having a look at axe handle’s NaClO4 production system why doesn’t his just convert everything to NaOH and Cl2 gas?
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[*] posted on 28-11-2004 at 20:59
Electrolysis


It could very well end up as Cl2 and NaOH if the Cl2 is allowed to escape. As for your 2nd
question, someone with more experience will be needed to answer that 1.




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[*] posted on 28-11-2004 at 22:07


Just like MadHatter said for your first question. You get hydroxide and chlorine gas if the mixture is not agitated, and the anode does not go far into solution the Cl2 evolved at the anode will escape. But if the solution is agitated or otherwise setup in such a manner as to allow the chlorine produced at the anode to react with the hydroxide from the cathode then hypochlorites, chlorites, and chlorates are produced. The temperature determines which chlorine oxidation state you will achieve, but perchlorate requires further oxidation of chlorate and not just a higher temperature.

As for your second question, HCl can be oxidized to perchloric acid though electrolysis, it's just that electrolyzing a solution of acid really eats electrodes and once it becomes an oxidizing acid it eats them more. Temperature control is a problem and hot perchloric acid will oxidize the HCl and result in the reduction of the perchlorate anion.

The last part of your question revolves around the surface area of the anode in the electrolysis cell and other factors such as a cap which increases the probability that the Cl2 will become solvated and react with the hydroxide.




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[*] posted on 28-11-2004 at 22:15


So one could heighten the yield of chlorate using something like a magnetic stirrer? Also I should be able to make chlorate by bubbling Cl2 gas through Sodium hydroxide solution?
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[*] posted on 29-11-2004 at 00:33


That is how commercial sodium hypochlorite bleach solution is made. There is an equilibrium present of sodium hypochlorite with sodium hydroxide and dissolved chlorine, which renders the solution strongly alkaline. However, the bottles of it sold in supermarkets do not state how much chlorite, chlorate, and perchlorate are present as impurities.
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[*] posted on 7-1-2005 at 21:39


Haven't tried electrolisis on NaCl solutions in 15 years, but as I recall, Hydrogen and Oxygen gasses formed (relatively) freely. The water around the Hydrogen side began turning green around the electrode, and spreading. Best guess from memory is that the Clorine was dissolving in the solution as fast as it was being produced. This abundance of Clorine may help answer the original question. (Chlorine is produced, it just stays in the solution long enough to form something else)



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[*] posted on 8-1-2005 at 00:35


I am currently building chlorate cells at the moment. I am currently on designing my fifth one. Chlorate is formed when more than 10% of the solution is chloride ions. And the produced chlorine must dissolve back into solution. I am using fiberglass mesh in my next design to help that. Lower than that, perchlorate is formed. Graphite anodes can be used for chlorate production but not for perchlorate. Lead dioxide substrate, magnetite, or platinum anodes must be used for this. Stainless steel is a good choice for a cathode (I only just figured that out), and I'm going to use a spoon in my next one. Chlorine dissolves to form hypochlorus ions OCl and then reacts with sodium hydroxide to form (sodium chlorate, chlorite??). Actually I think there are about 8 different reactions taking place:o. Anyway sodium chlorate is what I am after.
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thumbdown.gif posted on 23-2-2005 at 04:40
Question?


Where the sodium chlorate came from( + or -) in NaCl elektrolysis?:(
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[*] posted on 14-3-2005 at 10:22


Neither actually. The hypochlour ions (ClO-) react with eachother to form ClO3- and Cl- ions. This happens most efficiently at higher temperatures.

The Cl2 bubbles that go up in the solution must pass through the OH- solution for the best effeciency. This can be achieved by placing the anode below the cathode in your cell. Also placing some sort of fine mesh helps to make the bubbles smaller which also increases the efficiency of the cell.




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