which pH indicator should be used ?

Question

Suppose you are studying coordination compounds of Co(II) with the ligand pyridine (py, C5H5N, MW = 79.10). You isolate a crystalline compound and since the only available ions are chloride and nitrate, you hypothesize the empirical formula of the coordinaton compound must be Co(II)w(py)x(Cl)y(NO3)z. You analyze for pyridine (Kb is approximately 10-9) by dissolving 0.1000 g of complex in 10 mL of water and titrating with a 0.01 M HCl solution. Which of the following indicators sould be used to detect the endpoint? (Assume that the initial concentration of pyridine is approximately 0.01 M.)
1/methyl red (pH range of color change: 4.8 - 6.0)
2/thymolblue (pH range of color change: 8.0 - 9.6)
3/bromophenol blue (pH range of color change: 3.0 -4.6)
4/alizarin yellow (pH range of color change: 10.1-12.0)
5/bromothymol blue (pH range of color change: 6.0 - 7.6)

Answer 1

/bromothymol blue (pH range of color change: 6.0 - 7.6)