Determination of pKa by absorbance and pH of buffer solutions
Make the plot $10^{\mathrm{pH}}(A_{\ce{L-}} - A)$ versus $A$, 
the adjusted equation will be of the form
$$
10^{\mathrm{pH}}(A_{\ce{L-}} - A) 
  = 10^{\mathrm{p}K_\mathrm{a}}A
  - 10^{\mathrm{p}K_\mathrm{a}}A_{\ce{HL}},
$$
the slope is $10^{\mathrm{p}K_\mathrm{a}}$, that is, $\mathrm{p}K_\mathrm{a} = \log_{10}(\text{slope})$.