The spectral behavior of some Schiff base compounds derived from 2-((2,4-dihydroxyphenyl) (hydroxy)methyl)-2-phenylhydrazono)propanal,2-(2-(hydroxy(2-hydroxy-4-methoxyphenyl)methyl)-2-phenylhydrazono)propanal,3-(2-(hydroxy (2-hydroxy-4-methoxyphenyl)methyl)-2-phenylhydrazono)butan-2-one and 2-(-(2,4-dihydroxybenzylidene) hydrazono)-1,2-diphenylethanone to investigate the acid ionization constants (pKH) of these ligands using Half-height method and Modified limiting absorbance method. A series of new metal chelates of Co(II), Cu(II), Gd(III), Sm(III) and Tb(III) with Schiff base ligands derived from condensation of glyoxal, biacetyl or benzyl bishydrazone with 2,4 – dihydroxybenzaldhyde and 4-methoxysaliycaldhyde in ethanol and characterized via potentiometric titrations. Stability constants of these metal complexes were determined using a potentiometric method by calculating the values of n ̅A, n ̅ and pL and found in the order Co(II)