Why does METHYL ORANGE change to YELLOW in alkaline solution? Explain elaborately with structures.

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ncchemist | eNotes Employee

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I cannot draw the structures in this program but I can explain it to you because the color change is due to only a single difference in structure.  Methyl orange is an azo compound, meaning that it is composed of two benzene rings connected by a nitrogen-nitrogen double bond.  One of the benzene rings has a sulfonate sodium salt (SO3Na).  In a very acidic medium, the sodium salt remains intact and gives the chemical an orange color.  But when the solution is made more alkaline, the sodium salt is protonated to make the regular sulfonic acid (SO3H).  When this happens, the light absorbance properties of the chemical change and it becomes yellow in color.  This change occurs at about pH=4.4. 


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