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

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Abstract
A spectrophotometric study of the complex formation between iron(III) and sulfosalicylic acid was carried out over the pH range up to 8; further, an attempt was made to test the applicability of sulfosalicylic acid as a reagent for the spectrophotometric determination of iron(III) in the presence of iron(II). The absorption maxima are found at 510 mμ independently of the pH between 1 to 2.5, while they shift in the direction of shorter wavelength as the pH rises. It was concluded that three complex species, Fe(Su), Fe(Su)23−, and Fe(Su)36−, are formed successively in acid solutions; their stability constants were determined to be: (Remark: Graphics omitted.) where Su3− denotes the sulfosalicylate anion, C6H3(O−)(COO−)(SO3−). If the pH of the solution is adjusted from 3 to 4.5 and if the absorbancy measurements are made at 480 to 490 mμ, sulfosalicylic 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 10 p. p. m.; iron(II) does not interfere, even at a concentration of 100 p. p. m.