Chemists use methyl orange as an indicator in the titration of weak bases with strong acids. It changes from red (at pH 3.1) to orange-yellow (at pH 4.4):
pH-related color changes result from changes in the way electrons are confined in a molecule when hydrogen ions are attached or detached. Here is the structure of methyl orange in acidic solution:
In the basic form of methyl orange, a hydrogen ion is lost from the -NN- bridge between the rings, and the electrons formerly used to bind the hydrogen neutralize the positive charge on the terminal nitrogen, so that it is no longer able to pi-bond. Solutions of the methyl orange appear yellow in alkaline solution .
This reaction is called diazotization and it is extremely useful in organic synthesis. The nitrous acid provides NO+ which replaces a hydrogen on the -NH3+ group to produce -NH2NO+ and water; a second water is eliminated to produce the -N2+ group.
The diazonium salt is then coupled with dimethylaniline. The coupling takes place in the opposite ("para") position from the -N(CH3)2 group:
The sodium acetate makes the solution weakly basic; the acetate ion accepts the hydrogen displaced from the dimethylaniline ring.
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/print-methyl-orange.shtml