Physical characteristics of ferrous complex
The colorimetric reagent for iron, ferrozine or disodium salt of 3-(2-pyridyl)-5,6-bis(4-phenylsulfonic acid)-1,2,4-triazine have similar sensitivity to the 2,4,6-tris(2-pyridyl)-1,3,5-triazine (commonly known as TPTZ) and costs substantially less than other commercial. The ferrozine method is recommended for ferrous measurement rather than the TPTZ method because of its lower autoreduction rate and measurements are not delayed beyond the standard color development time needed. The two major advantages of the use of ferrozine for ferrous measurement are its sensitivity and its low cost.
In 1970, Stookey reported the synthesis of ferrozine which complexed with ferrous iron to form a tris ferrozine/iron, Fe(Fz)3 complex. At wavelength of 562 nm, the molar absorptivity is 27900 and the Beer-Lambert law is obeyed to around 4 mg/liter of Fe.
Spectrophotometric titration has showed that ferrozine forms the expected tris complex with ferrous iron. The end point for the titration of 9×10-6mole of ferrozine occurs at 3×10-6 mole of ferrous ion. The magenta Fe(ligand)32+ species will generate completely in aqueous solution between the pH values of 4 and 9. Once generated between these values, the complex is stable.