I did this reaction on a very small scale, swirled it around to mix and react, and saw a yellow green gas come out, as Chlorine gas is one of the products. However, based on what I found online about the reaction I found it ambiguous whether or not it also produced Chlorine dioxide, as it has a similar appearance. Is it maybe dependent on the concentration of HCl, temperature, or pH? (I used 6 M HCl at around room temperature)

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    $\begingroup$ ClO2 is a red-brown gas [lambda max 36o nm] very different from chlorine. It can be produced by reduction of Chlorate by oxalate in acid and supposedly by disproportionation of chlorate in acid. The latter reaction is questionable as you seem to have found. It is definitely formed by chemical or electrolytic oxidation of chlorite or disproportionation of chlorite with acid especially HCl. Most of published equations involving ClO2 are at best only partially correct because the various chlorine oxidants interconvert, and yields are variable. Check Wikipedia for examples. $\endgroup$
    – jimchmst
    Jun 1, 2022 at 4:31
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  • $\begingroup$ I have done this experiment once. I have dropped about $10$ mL concentrated $\ce{HCl}$ in a $4$ liter cylindrical glass container, containing some potassium chlorate. This produced a heavy green gas which progressively filled the container. When half filled, I took some steel wool with a pair of pliers, put it in a Bunsen burner, and dipped it quickly into the green gas. It produced a small explosion. So the gas was not pure chlorine. I don't know what has happened, but I did not repeat this experiment $\endgroup$
    – Maurice
    Jun 1, 2022 at 7:35
  • $\begingroup$ Long ago in college lab a friend used concentrated sulfuric to clean some fused permangate + chlorates mess from a test tube . He dumped the result into a large stone sink - explosion. Lab instructor ran over and threw the tube into the sink- explosion. Next day the professor lectured about the dangers of chlorine dioxide. ( No, I don't know the chemistry . My friend got to stay in school). $\endgroup$ Jun 1, 2022 at 21:07

1 Answer 1


Sodium chlorate/hydrochloric acid mixture has been actively used as chlorinating agent and Wikipedia did mention that it generates $\ce{HOCl}$ or $\ce{Cl2}$ depending on pH which act as active agent in the chlorination reaction, the reaction does also generate chlorine dioxide. Again, it depends on the kinetics and pH of the reaction system. Following is the abstract from the paper[1]:

Based on the mechanism of $\ce{ClO3-/Cl-}$ reaction system, the kinetics for reaction of sodium chlorate and hydrochloric acid to generate $\ce{ClO2}$ was studied. The rate equation of this reaction system was deduced and simplified as a formula with mixed-order (combination of first-order and second-order) towards $\ce{ClO3-}$. This rate formula indicates that the initial rate of the reaction is the first-order with respect to $\ce{ClO3-}$, and the reaction rate is the second-order with respect to $\ce{ClO3-}$ when $\ce{[ClO3-]}$ becomes close to zero. The rate constants of the first-order were determined as $\pu{0.0168s^{-1}( 30 °C)}$, $\pu{0.0221s-1( 40°C)}$, and $\pu{0.0279s-1( 50°C)}$, respectively, and that of the second-order were obtained for $\pu{0.0019Lmol-1s-1( 30°C)}$, $\pu{0.0028Lmol-1s-1( 40°C)}$, and $\pu{0.0060Lmol-1s-1( 50°C)}$, respectively. The results of statistic test prove that the rate formula obtained in this work is credible.


  1. Li, X.J., Jiang, D.D., Zhang, Y.J., 2013. Kinetics of the Reaction for Generation of Chlorine Dioxide from Sodium Chlorate and Hydrochloric Acid. AMR 634–638, 546–550. DOI: 10.4028/www.scientific.net/amr.634-638.546

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