Dissolution of metallurgical or ore samples containing iron often requires the use of hydrochloric acid.
Iron(II) is readily titrated with potassium permanganate in strong acid solution.
Unfortunately, oxidation of chloride ions will also occur. This problem is overcome by
using the 'Zimmerman–Rheinhardt' reagent which contains manganese(II) ions and phosphoric acid.
The former displace the Mn(VII)–Mn(II) equilibrium and lower its reduction potential whilst the
phosphoric acid preferentially complexes Fe(III), lowering the reduction potential for Fe(III)–Fe(II).
Hence, the potassium permanganate, having its reduction potential lowered, stoichiometrically oxidizes
Fe(II) without interacting with the chloride ions.
Among the most important indirect methods of analysis which employ redox reactions are the
bromination procedures for the determination of aromatic amines, phenols, and other compounds which
undergo stoichiometric bromine substitution or addition. Bromine may be liberated quantitatively by
the acidification of a bromate-bromide solution mixed with the sample. The excess, unreacted bromine
can then be determined by reaction with iodide ions to liberate iodine, followed by titration of the
iodine with sodium thiosulphate. An interesting extension of the bromination method employs 8-
hydroxyquinoline (oxine) to effect a separation of a metal by solvent extraction or precipitation. The
metal-oxine complex can then be determined by bromine substitution.
Complexometric Titrations
Complex-forming reactions find a wide utility in chemical analysis and have been used in titrimetric
procedures for many years. Recently most attention has been concentrated on the use of
ethylenediaminetetraacetic acid (EDTA) and consideration of this reagent in some detail is important.
This study will also be useful in that it illustrates nearly all aspects of the use of inorganic and organic
complexing agents in titrimetry.
Ethylenediaminetetraacetic Acid (EDTA)
In its unreacted form EDTA is a tetrabasic acid represented by
A useful abbreviation is H 4 Y with H 3 Y–, H 2 Y^2 – , etc. referring to the various ions derived from the
successive stages of dissociation. For practical purposes, the disodium salt Na 2 H 2 Y is preferred as a
reagent. This salt has a distinctly higher solubility than the parent acid and avoids the high alkali-