Complexes of the Picolinium Ion and Picolinic Acid with Cresol Red

A spectrophotometric analysis of acidified aqueous solutions of picolinic acid and cresol red indicator suggests the existence of two complexes. Both the picolinium ion, a moderately strong acid, and picolinic acid, a weak acid, form complexes with the negative basic cresol red indicator species. The equilibrium constants for the formation of these complexes at 25 °C are 5.3 ± 0.6 and 1.6 ± 0.3, respectively. The existence of complexes of moderately strong acids with indicators has been known for some years. However, complexes between a weak acid and an indicator appear not to have been previously reported.