A student investigates the pKa of bromothymol blue (BTB), a weak-acid indicator that is yellow as HIn and blue as In-. Equal concentrations of BTB are placed in a series of buffered solutions and absorbance is recorded at 615 nm (the In- absorbance peak). The midpoint of the absorbance vs pH curve - where [HIn] = [In-] and the solution looks green - occurs at pH = 7.10.
BTB is most appropriate for which titration?
- A
a strong acid titrated with a weak base (equiv pH ~5)
- Bcheck_circle
a strong acid - strong base titration where the equivalence pH is near 7
- C
any titration regardless of equivalence pH
- D
a weak acid titrated with a strong base (equiv pH ~9)
Explanation
Indicator should change color near the equivalence pH; BTB (range ~6.0-7.6) matches strong-strong titrations.