DETERMINATION OF PK(A)S FOR THYMOL BLUE IN AQUEOUS-MEDIUM - EVIDENCE OF DIMER FORMATION

Formation constants for recrystallized thymol blue were determined in water, using the SQUAD and SUPERQUAD programs. The best model correlat ing spectrophotometric, potentiometric and conductimetric data was fit ted with the dissociation of HL- = L2- + H+ - log K = 8.918 +/- 0.070 and H3L2- = 2L(2-) + 3H(+) - log K = 29.806 +/- 0.133 with the SUPERQU AD program at variable low ionic strength (1.5 x 10(-4)-3.0 x 10(-4) M ); and HL = L2- + H+ - log K = 8.9 +/- 0.000, H3L2- = 2L(2-) + 3H(+) - log K = 30.730 +/- 0.032, H4L2 = 2L(2-) + 4H(+) - log K = 32.106 +/- 0.033 with SQUAD at 1.1 M ionic strength. (C) 1998 Elsevier Science B. V. All rights reserved.