Effect of nitro-PAPS on the reduction of iron(III) with cobalt(II) and the flow-injection analysis of cobalt(II) in real samples

The reduction reaction of iron(III) with cobalt(II) easily occurs in the presence of 2-(5-nitro-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino) phenol (nitro-PAPS) at pH 3.5, in which iron(II)nitro-PAPS chelate is formed. The chelate has a specific absorption maximum at 790 nm. Cobalt(II) can thus be determined by measuring the absorbance of the complex at the above-mentioned wavelength. The redox reaction is introduced into a two-channel flow system for the determination of cobalt(II). The calibration curve is linear in the concentration range of 2.5 X 10(-7) M similar to 1 X 10(-5) M cobalt(II) with a sample throughput of 70 samples h(-1). The detection limit (S/N = 3) is 1 X 10(-7) M and the relative standard deviation for 10 determinations is 0.3% for 2.5 X 10(-6) M cobalt(II). The proposed flow-injection spectrophotometric method is applied to analyses of cobalt(II) in cobalt alloy (NIST SMR 862; High Temperature Alloy L 605) and pepperbush (NIES standard, No. 1) and to the indirect determination of vitamin B-12 in pharmaceuticals.