Determination of the first pKa values of some sulfonphthalein dyes

An attempt was made to estimate the first pKa values of Bromocresol Green, Bromocresol Purple, Bromophenol Blue, Cresol Red and Phenol Red using a method similar to that of Reilley and Sawyer. A value of -0.75 for Bromocresol Purple; +1.05 for Cresol Red and +1.03 for Phenol Red was estimated. Trials to estimate the first pKa values of Bromocresol Purple and Bromophenol Blue were not successful due to the upper limitation on the strength of commercially available hydrochloric acid. A value of -0.85 or less for Bromocresol Green, and -0.95 or less for Bromophenol Blue has been predicted.