Acid dissociation constants of some new 4-{[(5-bromo-2-hydroxyphenyl)methylene]amino}-5-alkyl(aryl)-2,4-dihydro-3H-1,2,4-triazol-3-one derivatives were determined experimentally with potentiometric titration method and theoretically with SPARC computer program. The effect of solvents composition on the acid dissociation constants is discussed and experimental and theoretical values are compared. In the DPPH, free radical scavenging assay was achieved. The compounds have shown hydrogen-donating ability in the reaction with DPPH (1,1-diphenyl-2-picrylhydrazyl) radical.