Question

The graph shows the effect of pH on quenching of luminescence of tris(2,2'-bipyridine) Ru(II) by 2,6-dimethylphenol. The ordinate, K_SV, is the collection of constants, k_q /(k_e + k_d), in the Stern-Volmer equation. The greater K_SV, the greater the quenching. Suggest an explanation for the shape of the graph and

estimate pK_a for 2,6-dimethylphenol.

Short answer

${\mathrm{pK}}_{\mathrm{a}}=10.8$

Step-by-step solution

Calculation of pKa value

The graph K_sv versus pH has two plateaus. First plateau is at low pH where the K_sv is approximately . The second plateau is at high pH where the K_svis approximately . 2,6-dimethylphenol is a weak acid, so at the beginning it is in acidic form HA, while after it is in basic form A^-. pK_ais the midpoint of the curve.