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|Three solutions are combined in alphabetical order. |
Solutions A and B produce a red solution. Adding solution C after this turns the solution milky white, then blue after a time.
The hydrochloric acid in solution B turns the 2,4,2',4',4"-pentamethoxytriphenyl methanol (an indicator) red. The addition of solution C causes the pH to increase due to the formation of OH- ions in the IO4- oxidation of S2O32-.
2 S2O32- (aq) + IO4- (aq) + H2O (l) --> S4O62- (aq) + IO3- (aq) + 2 OH- (aq)
When the indicator turns colorless again, the milky-white suspension of Cd(IO4)2+ becomes apparent. The further increase in pH triggers another indicator (thymolphthalein), turning the solution blue.
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