The Experiment
A few days ago I saw a video of dissolving gold in the condition of $\ce{ICl3/HCl(aq)}$. In the video, $\ce{ICl3(s)}$ is added to a test tube, followed by adding conc. hydrochloric acid. The solid is dissolved in the solution and the solution turned yellow. No green color was shown.
A gold wire is then placed in the solution for a short period of time. Putting the wire on the starch indicator produces a color of blue.
As a comparison, a piece of titanium was placed in the solution and then indicated by starch. No blue was produced.
The Speculation
After adding $\ce{HCl(aq)}$ to $\ce{ICl3(g)}$, I can think of 3 types of reactions may occur.
1) Lewis acid-base reaction.$$\ce{ICl3(g) +Cl-(aq)<->ICl4-(aq)}$$ As I'm not sure whether $\ce{ICl3}$ is a strong Lewis acid and whether $\ce{HICl4}$ is a strong acid (I suggest it is), I don't know it may present as a molecule in the solution.
ii) Disproportionation. As no purple color was seen, which leads to no iodine was produced.
iii) Hydration. $$\ce{ICl4-(aq) +5H2O(l)<->HIO2(aq) +4Cl-(aq) +3H3O+(aq)}$$ This reaction is unlikely to occur as $\ce{HIO2(aq)}$ is unstable and will disproportionate but no disproportionation production could be observed.
The OP of the video claimed that the equation of the reaction of gold and the solution is $$\ce{2HICl4 +2Au->2HAuCl4 +I2}$$

a) Does the solution of iodine(iii) really contains much $\ce{ICl4-}$, and it is the cause of oxidation of the gold wire?
b) Does the equation claimed above really occur?
c) Is there any paper that investigates the electrode potential of $\ce{ICl4-/I2}$? Hence we can support the correctness of the equation thermodynamically.

  • $\begingroup$ That seems to be OK, at least approximately. $\endgroup$ – Mithoron Oct 24 '19 at 18:51

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