Speciation of 2-hydroxybenzoic acid with calcium(II), magnesium(II), and nickel(II) cations in self-medium

Equilibria occurring between 2-hydroxybenzoic acid (salicylic acid, H2L) and calcium(II), magnesium(H), and nickel(II) have been studied by potentiometric titrations with glass electrodes at 298.15 K in self-medium constituted of 1.05 mol.kg(-1) NaHL. The self-medium composition can be considered constant since only a small amount of ligand HL- is transformed in the reaction products. The hydrogen ion concentration was varied from 10(-2) mol.kg(-1) to incipient precipitation of a basic salt of each metal, whose formation depends on the particular metal. The total metal concentration, C-M, ranged from 3.05.10(-3) mol.kg(-1) to 10.01.10(-3) mol.kg(-1). The higher C-M value was imposed in order to avoid changes in the ionic medium and in the activity coefficients. The experimental data have been explained by assuming the formation of the complexes: CaL; MgL, MgL22-, and MgL34-; NiL and NiL22-.