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I found a old bottle of methyl orange in my microscope kit, in solid form. I added water to it to form methyl orange solution. I tried putting 2 drops of methyl orange on a spot plate, then adding sodium bicarbonate. I got pink foam, which is not what i expected, as methyl orange is suppose to turn yellow on sight of a base. Does anyone know what this "methyl orange" might be instead of methyl orange? This did come from the manufacturer of the microscope kit, which has a label saying Methyl Orange, so I am confused on why it does not act as an indicator. It was originally in the kit to act as a dye for biological samples.

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  • $\begingroup$ mightve been oxidised due to exposure , since you mentioned its old. $\endgroup$ – Supernova Mar 29 '17 at 5:44
  • $\begingroup$ @Supernova In some dye collections, the samples of methyl orange, kept in glass jars, and hence maybe exposed to a little of air, are decades old and still work fairly good as pH indicator. $\endgroup$ – Buttonwood Mar 29 '17 at 9:50
  • $\begingroup$ "Formation of foam" upon addition of NaHCO3, a weak base, may be indicative for the liberation of CO2. Methyl orange is a sulfonate dye, i.e. an acid, and could ‒ if not delivered as a sodium salt ‒ in principle trigger the decomposition of NaHCO3. But its solubility in water is rather low and I assume your freshly prepared solution was a diluted, one, too. (Which colour spot?) That it is sometimes intentionally mixed with xylene cyanol (en.wikipedia.or/wiki/Methyl_orange) doesn't match with your likely addition of a lot of NaHCO3 (compared to the number of molecules of dye), too. $\endgroup$ – Buttonwood Mar 29 '17 at 9:54
  • $\begingroup$ so is this because it is old? $\endgroup$ – George Jones Mar 29 '17 at 23:15

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