If Methyl orange is used as only indicator, it's used in the scenario " strong acid to weak base"
Explanation:
A reaction between a weak acid and weak base, gives not strong enough changes in pH. The neutralization reaction (might) happen. But it would be difficult to get the titration curves. This means no indicator is suitable.
A strong acid to a weak base : The equivalence point for titrations between strong acids and weak bases occurs somewhere upward of pH=3-4, this depends on how strong the acid and the base.
⇒ A good indicator for this scenario is Methyl orange, because it changes colour between a pH of 3.1 - 4.4. We notice that the equivalence point is more or less in the middle of this range. This makes the indicator an useful one !
A weak acid to a strong base : The equivalence point for titrations between weak acids and strong bases occurs somewhere upward of pH=7, this depends on how strong the acid and the base.
⇒ A good indicator for this scenario is phenolphthalein, because it changes colour between a pH of roughly 8 - 10. We notice that the equivalence point is more or less in the middle of this range. This makes the indicator an useful one !
⇒Methyl orange has an equivalence point, more or less at pH=4, ( so will change colour there). This is quite away from where the equivalence point of a weak acid with a strong base.
HCl with sodium carbonate: Since this reaction has 2 end points (one at pH = 8.5 and one at pH =3.5). Methyl orange has range between pH = 3.1 and 4.4 ; phenolphthalein has a range between 8.3 and 10. Since it's colourless below pH 8, it does not interfer with the methyl orange.
For this scenario methyl orange is ONE of the indicators that can be used. As only indicator, it does not have a full spectrum of colour change. So the otherindicator that should be used is phenolphthalein.
If Methyl orange is used as only indicator, it's used in the scenario " strong acid to weak base"
