Analytical Sciences

Polarographic Determination of Total Iron Content Using a Fe^2+/3+-Methylthymol Blue-NO₂^-System

Lizhu YANG,** Lirong WANG,* Li LIN,** Zhen PENG,*** and Guanghan LU*

*College of Chemistry, Central China Normal University, Wuhan 430079, People’s Republic of China
**School of Pharmacy, Wenzhou Medical College, Wenzhou 325027, People’s Republic of China
***Chaohu College, Chaohu 238000, People’s Republic of China

In the buffer solution of NH₃-NH₄Cl (pH = 8.5, 0.04 mo1 l^-1), iron-Methylthymol Blue (MTB) can produce a sensitive polarographic wave at -1.10 V (vs. SCE) in the NaNO₂. The peak current is linear with the concentration of the iron in the range of 3 × 10^-8 - 5 × 10^-6 mol l^-1, and the detection limit is 1 × 10^-8 mol l^-1. By studying the characteristics of the wave and the electrode reaction mechanism, we can prove that the catalytic wave is an adsorption wave and that the peak current comes from the reduction of Fe(II). The molar ratio of iron to ligand was found to be 1:1. Adsorption particles are neutral molecules, the saturated adsorption quantity of the complex on the mercury electrode is 1.92 × 10^-9 mol cm^-2, according with the Frumkin isothermal formula. In the experiments, the adsorption coefficient (β) is 4.05 × 10⁵; the adsorption factor (γ) is 0.70; the electron transfer number (n) is 2; the free energy (ΔG^o) is 31.99 kJ mol^-1; the transfer coefficient of the irreversible adsorption is 0.42 - 0.45; and the reaction velocity constant (K_s) is 1.35 s^-1. This method, whose result is satisfying, can be applied to the detection of trace total iron contents in medicinal products.

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