Determination of pKa Bromocresol green

[vine_shun]

i am having problem in calculating pKa
in my experiment,a formula of pKa=pH + log10 [HB]/
my data obtain only gotthe pH of HB,that is 5.17
how can i calculate the pKa??

[HB]=concentraion of undissociated form indicator
=concentration of dissociated indicator

 

[chemisttree]

Describe your experiment for us.