Spectrophotometric determination of iron(III) with chrome azurol s or eriochrome cyanine r and some cationic surfactants

Sensitive spectrophotometric methods for the determination of iron(III), based on ternary complexes with a triphenylmethane reagent, chrome azurol S (CAS) or eriochrome cyanine R (ECR), and cetyltrimethylammonium (CTA) or cetylpyridinium (CP) ions, are described. For the system Fe—CAS—CTA, the molar absorptivity is 1.35 × 10 5 l mol -1 cm -1 at 645 nm; for Fe—ECR—CTA it is 1.28 × 10 5 l mol -1 cm -1 at 635 nm. Maximum absorbance is attained (at about pH 4) when the molar ratio (to iron) or CAS or ECR is about 20 and that of CTA or CP is 60–80. Citrate, tartrate, oxalate and EDTA interfere. Interference by metals (e.g. Be, Al, Ga, In, Sc, Zr, Th) can be eliminated by preliminary extraction of iron(III) as thiocyanate complex. The method was successfully applied to determining traces of iron in analytical-grade sodium hydroxide.