Experimental correlation between the pKa value of sulfonphthaleins with the nature of the substituents groups

This work presents the results obtained from a spectrophotometry study performed on some indicators of the sulfonphtaleins like phenol red (PR), thymol blue (TB), bromothymol blue (BTB), xylenol orange (XO) and methylthymol blue (MTB). During the first stage the acidity constants of some of the indicators were determined using the data from spectrophotometry, potentiometry and with the use of the software SQUAD. These were as follows: for the equilibrium 2H + BTB ↔ H2BTB, log β2 = 15.069 ± 0.046 and for H + BTB ↔ HBTB, log β1 = 8.311 ± 0.044. For the XO and the MTB five values were calculated for each, namely, for MTB: log β5 = 42.035, log β4 = 38.567 ± 0.058, log β3 = 32.257 ± 0.057, log β2 = 23.785 ± 0.057, and log β1 = 12.974 ± 0.045 while for XO: log β5 = 40.120 ± 0.102, log β4 = 35.158 ± 0.062, log β3 = 29.102 ± 0.053, log β2 = 21.237 ± 0.044, and log β1 = 11.682 ± 0.044. During the second stage, a study was conducted on the effect of the substituents present in the indicators to determine the effect of different functional groups on the pKa value corresponding to the last indicator's dissociation.