2-Phenyl-5-p-tolyl-1-salicylideneamino-1,3,4-triazole (PTST) was studied among 2,5-diaryl-1-salicylideneamino-1,3,4-triazole derivatives as a novel potential pH indicator. In terms of absorption intensity coming from the acid-base reactions, PTST showed strong yellow-green colour with high extinction coefficient in the pH range 7.0-9.5. In addition, the corresponding colour development at the transition point can be attributed to the resonance structures of PTST, which were caused by the hydrogen ion dissociation from the acidic form of the compound in the presence of alkali. The full geometric optimization and achievement of the electronic structure of the molecule were performed by an AMI semiempirical method. The triazole compound was compared with phenolphthalein (PT), which is widely used as an acid-base indicator in titrimetry, for accuracy test.