o-Phenanthroline is a colorimetric reagent for iron detection. It is also used as a reagent for the solvent extraction of anions. This reagent is slightly soluble in water (3.3 g/l at room temperature) and benzene (14 g/l at room temperature), and fairly soluble in alcohol (540 g/l), acetone, and diluted mineral acids. Its proton dissociation constants are reported to be pKa1=0.70 and pKa2=4.98 (μ=0.1, 25ºC). o-Phenanthroline forms a red chelate with Fe(II) [logβ3=21.1(μ=0.1, 20ºC), λmax=510 nm, ε=1.1×105]. This complex is not easily extracted by organic solvents. The redox potential of FeL33+ -FeL32+ (L: o-phenanthroline) in 1 M H2SO4 is reported to be 1.06 V, and the color of the solution changes from red to pale blue. This reagent also forms colored complexes with Cu, Ni, and Co, and colorless complexes with Cd and Zn. Therefore, o-Phenanthroline can be used as a colorimetric reagent for Cu, Ni, and Co, and as a masking reagent of Cd and Zn. FeL32+ is also utilized for precipitation detection and ion-pair extraction.
Reduction and oxidation indicator: Ce
Colorimetry: Ag, Fe(II) :510 nm, detection limit 0.8 ppm
Fluorometry: Ru(II), Eu
Extraction colorimetry: halogen, ClO4-, PtCl62-, ReO4-, HCrO4-, SCN-, AuCl4, Ag(CN)2-, Sn(C2O4)32-, phosphomolybdate and organic compounds (Trichloroacetic acid, dehydroacetic acid, syclamic acid, saccharin, salicylic acid, alkylbenzenesulfonic acid, maleic acid, pentachlorophenol, tetraphenylborate, and chloranilic acid)
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