Flow-injection potentiometric determination of iron (III) in vitamin formulations using a tubular ion-selective electrode in oxalic medium

The construction and analytical evaluation of a graphite-epoxy ion-selective electrode coated with an ion pair formed between [Fe(oxalate)₃]³⁻ and tricaprylylmethylammonium cation (Aliquat 336) in a poly(vinylchloride) (PVC) matrix was recently presented [14]. The same active material was used in a tubular electrode in a flow-injection system. The tubular electrodes were made using carbon rods with external diameter of 6 mm and internal holes of 1 mm with different lengths (2.0, 2.5, and 3.0 cm). A thin membrane film of [Aliquat 336]₃[Fe(oxalate)₃], dibutylphthalate (DBPh) in PVC was deposited 3–4 times directly in hole walls using a dropper. The effect of membrane composition and electrode length over the iron (III) electrode response [slope (mV/dec)] was initially evaluated in triplicate, at pH range from 4 to 8 and in 0.2 mol L⁻¹ of oxalate solution as carrier. The best response (−18.7 ± 0.3 mV/dec) was attained with the 5% m/m ion pair; 65% m/m DBPh and 30% m/m PVC and a 2.0 cm electrode length, in this pH range. The electrode shows a linear response to E (mV) vs. log [Fe(oxalate)³⁻₃ ] in the iron (III) concentration range from 3.2 × 10⁻⁵ to 1.0 × 10⁻² mol L⁻¹ and a useful lifetime of at least 700 determinations for each polymeric membrane used. The detection limit was 2.5 × 10⁻⁵ mol L⁻¹, and the relative standard deviation was less than 2% for a solution containing 5.0 × 10⁻⁴ mol L⁻¹ of iron (III) (n = 12). The sampling rate was 90 h⁻¹, and the results obtained with this procedure are in close agreement with those obtained using the AA spectrophotometry (r = 0.9999). © 1999 John Wiley & Sons, Inc. Lab Robotics and Automation 11: 163–168, 1999