A Spectrophotometric Study of the Complex Formation between Iron(III) and Salicylic Acid

Abstract A spectrophotometric study of the complex formation between iron(III) and salicylic acid was carried out over the pH range up to 6; further, an attempt was made to test the applicability of salicylic acid as a reagent for the spectrophotometric determination of iron (III) in the presence of iron(II). The absorption maxima of the mixture of iron(III) and salicylic acid are found at 530 mμ independently of the pH between 0.7 to 2.5, while they shift to shorter wavelength when the pH rises above 3. The isosbestic point occurs at 550 mμ between pH values from 3 to 6. It was concluded that three complex species, Fe(Sa)+, Fe(Sa)2−, and Fe(Sa)3−3, are formed successively in acid solutions; their stability constants were determined to be: (Remark: Graphics omitted.) where Sa2− denotes the salicylate anion, C6H4(O−)(COO−). If the pH of the solution is adjusted from 2 to 3 and if absorbancy measurements are made at 515 to 500 mμ, salicylic acid can be used as a reagent for determining iron(III) in the presence of iron(II). The optimum concentration of iron(III) is 2 to 12 p. p. m.; iron(II) does not interfere, even at a concentration of 100 p. p. m.