methyl orange

Meaning: Methyl orange is a weak-acid pH indicator whose acid form (HIn) and conjugate base form (In⁻) differ in color. It is red below about pH 3.1 and yellow above about pH 4.4, with orange between, so it changes color in the acidic range, near its pKa of about 3.5.

Equation: pH = pKa + log([In⁻]/[HIn]). pH and pKa are unitless, and the brackets are concentrations in mol/L. At pH = pKa the two forms are present in equal amounts, and the color shifts across roughly pKa ± 1.

On the MCAT: Titration passages ask which indicator fits. Pick one whose transition range lies on the steep part of the curve, around the equivalence point. Methyl orange suits a strong acid–weak base titration (HCl with NH₃), where the equivalence point is below 7. For a weak acid–strong base titration the equivalence point is above 7, so phenolphthalein (about pH 8.2–10) fits instead.

Common confusion: Indicators do not change color at pH 7; each changes near its own pKa. The color change marks the endpoint, which is close to, but not the same as, the equivalence point.