Correspondence

Rio de Janeiro, June 27, 1998

Dear sir:

The paper "Spectrophotometric Determination of Iodate in Talbe Salt", wrote by Rosa Lina G.N.P. Silva, A. Fernando de Oliveira, and Eduardo Almeida Neves that appeared in J. Braz. Chem. Soc. 1998, 9(2), 171 has called my attention because of the molar absortivities found by these authors for the ion I₃^- at the wavelengths 288 and 352 nm. As can be seen from the tables presented below, these values are much higher than those obtained by other authors.

As the iodate content in the table salt is measured by the UV-Vis spectroscopy at these wavelengths it would be convenient for these authors to justify why their molar absortivity values are so different.

Respectfully,

Roberto de Barros Faria

Departamento de Química Inorgânica

Universidade Federal do Rio de Janeiro.

References:

1. Awtrey, A.D.; Connick, R.E. "The Absorption Spectra of I₂, I₃^-, I^-, IO₃^-, S₄O_6-² and S₂O₃^-2. Heat of the Reaction I₃^- = I₂ + I^-", J. Am. Chem. Soc 1951, 73, 1842-1843.

2. Allen, T.L.; Keefer, R.M. "The Formation of Hypoiodous Acid and Hydrated Iodine Cation by the Hydrolysis of Iodine", J. Am. Chem. Soc 1955, 77, 2957-2960.

3. Palmer, D.A.; Ramette, R.W.; Mesmer, R.E. "Triiodide Ion Formation Equilibrium and Activity Coefficients in Aqueous Solution", J. Solution Chem 1984, 13(9), 673-683.

4. Troy, R.C.; Kelley, M.D.; Nagy, J.C.; Margerum, D.W. "Non-Metal Redox Kinetics: Iodine Monobromide Reaction with Iodide Ion and the Hydrolysis of IBr", Inorg. Chem. 1991, 30(25), 4838-4845.

5. Thomas, T.R.; Pence, D.T.; Hasty, R.A. "The Disproportionation of Hypoiodous Acid", J. Inorg. Nucl. Chem. 1980, 42, 183-186 (note that the correct e value for the 353 nm band is equal to 2,58 x 104 and not 2,58; it can be obtained from Eqs. (1) and (2)).

Author’s Replay

Our molar absortivity data, e, are correct and are referred not exactly for the I₃^- anion but indirectly for IO₃^- as they are the slopes of analytical curves for iodate in salt NaCl. The reported data are based on the following redox reaction that takes place between iodate with iodide in acidic medium:

IO₃^- + 5 I^- + 6 H⁺® 3 I₂ + 3 H₂O

3 I₂ + 3 I^-® 3 I₃-

–––––––––––––––––––––––––––

IO₃^- + 8 I^- + 6 H⁺® 3 I₃^- + 3 H₂O

On this basis it is clear that 1 IO₃^- ion generates 3 I₃^-. Thus our indirect iodide e data should be divided by 3 when referred to I₃^-:

At 352 nm, e = (73 200 / 3) = 24 400 L mol^-1 cm^-1

At 288 nm, e = (110 300 / 3) = 36 767 L mol^-1 cm^-1

These values are colse to those from the literature, 40 000 or 38 790 at 288 nm and 26 400 or 25 800 at 352 nm. Small differences are acceptable due to different experimental conditions, viz. ionic. strength and the presence of chloride ion in our working soutions which can compete to some extent with iodide to form a complex like I₂Cl^-.

Eduardo Almeida Neves

Co-author

DQ-UFSCar

São Carlos - SP

Publication Dates

• Publication in this collection
  06 Nov 2002
• Date of issue
  Sept 1998