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Methyl Orange is used an indicator because it changes colour at ph 3.2-4.4 due to a transition of one chromophore to another. Use resonance structures to demonstrate the changes in the chromophore during this change in ph. Would Para Red make a good acid-base indicator? How would you tell experimentally?
Accepted Answer
Methyl Orange's color change is due to the protonation/deprotonation of its azo group (-N=N-), which acts as the chromophore. In its acidic form (pH 4.4), the azo group is deprotonated, the conjugated system becomes smaller, and a yellow color results. The deprotonation disrupts the extended conjugation, leading to a higher energy transition and the absorption of blue light. This transition between the two forms creates the noticeable color change used as an indicator.
Para Red, with a similar azo group, could also act as an indicator. To determine if it would be suitable, you could test its color change in solutions of known pH values, similar to how you would verify the color change range of Methyl Orange. Comparing the color changes at various pH levels would help establish if Para Red exhibits a sufficiently distinct and sharp transition at a useful pH range, making it a suitable acid-base indicator.
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