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.
What is the pKa of bromothymol blue from this measurement?
- A
0.50
- Bcheck_circle
7.10
- C
14.00
- D
3.55
Explanation
For an indicator HIn <-> H+ + In-, when [HIn] = [In-], pH = pKa. Midpoint pH 7.10 = pKa.