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I heard that hypochlorous acid, which can be used for sterilization, is generated by the electrolysis of salt water. So I actually tried this experiment. When salt (NaCl) water underwent the electrolysis using titanium electrode (for 60 min), sterilization was completed within 20 min. I sampled the solution in the electrolysis cell every 5 min during the experiment, and detected ions in the solution. Cl- was detected, and its concentration was decreased; However, ClO- was not detected. (The instrument I had can detect ClO-).
Why was ClO- not detected in my experiment, even though I heard that ClO- was detected in the experiment of others? Incidentally, pH was increased from 7.5 (before experiment) to 9.6 (after experiment).
KireiKirei
(Nov 2002)
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Answer:
Development Division, Permelec Electrode
LTD, kindly answered this question.
Cl2 is generated at the anode (positive electrode) by the electrolysis:
-
2Cl-
↔
Cl2 + 2e-
This chlorine changes form depending on pH;
Cl2 is the main component at low pH, hypochlorous acid (HClO) is the main component at neutral pH, and hypochlorous acid ion (ClO-) is the main component at high pH. A portion of
Cl2 is dissolved in solution, but the majority of Cl2 is eliminated from solution as a gas. These are expressed by the following equations:
-
Cl2 + H2O ↔
HClO + H+ + Cl-
-
HClO ↔
H+ + ClO-
Effective concentrations of chlorine, which is used as the indicator of bactericidal action, is total concentration of Cl2 (chlorine) and HClO (hypochlorous acid) and ClO- (hypochlorous acid ion) in the solution.
During actual electrolysis, oxygen (O2) is generated in a competitive reaction with reaction 1 (the generation of Cl2):
-
H2O ↔
1/2O2 + 2H+ + 2e-
The ratio of the generation of Cl2 and O2 varies according to pH or the type of electrode. HClO and ClO- are not generated, when only reaction 4 proceeds. My company’s DSE electrode is created to preferentially make reaction 1 rather than reaction 4 occur.
Although I am not sure of the exact conditions prevailing during this questioner’s experiment, I assume that HClO and ClO- were generated in his experiment, since these compounds are generated when titanium electrode are used, even though the amount would be less than the one generated using my company’s electrode. The facts that the concentration of Cl- decreased, and sterilization was completed also support my assumption. It would be better for this questioner to check whether there was any problem in his analysis. This questioner seemed to use the detector which can detect ClO-, but some commercial instruments which measure effective concentrations of chlorine can only detect HClO. If he used such a detector, he would not detect the concentrations of chlorine, because ClO- is the main component (about 98%) at pH 9.5.
It could be thought that ClO- is further oxidized and becomes chloric acid ion
(HClO3-) and perchloric acid ion (HClO4-).
However, the possibility that all the ClO- is oxidized would be low.
Although I mentioned above that Cl2 is also generated using titanium electrode, if the titanium electrode is used as an anode, the surface is gradually oxidized, which prevents the electricity from running through the surface. Reactions 1 and 4 do not occur when the electricity does not run.
Finally, I recommend that he performs the experiment in a fume hood, even though a large amount of Cl2 gas would not be generated in this experiment.
Acknowledgement
Permelec Electrode LTD
They are developing and selling electrodes which have many functions.
This article is translated by Chemistryquestion.com from the original article in Chemistryquestion.jp. Please let us know if you find any errors.
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