Spectrophotometric determination of nickel(II), iron(II), boric acid and chloride in plating baths by flow injection analysis

Abstract

A simple, non-separation method for the determination of nickel(II) and iron(II) in an electroplating bath by flow injection analysis is described. Methods are also described for the determination of boric acid and chloride. Nickel(II)(0.17–0.24 M) is determined by direct spectrophotometry of the nickel(II) ion at 675 nm in the presence of 2.7–5.4 mM iron(II). Iron(II) is oxidised by persulphate ion to iron(III), which then forms a complex with thiocyanate ion. The absorbance of the intensely red [Fe(SCN)₂]⁺ species is measured at 505 nm. One hundred measurements h^–1 are possible with this system. In a second manifold boric acid is measured indirectly by the association reaction of 8-amino-1-hydroxynaphthalene-3,6-disulphonic acid with salicylaldehyde to form azomethine H in the presence of boric acid. A linear calibration graph is obtained in the range 0.05–0.8 M boric acid versus absorbance at 420 nm with a two-fold on-line dilution. The chloride concentration (0–1.7 M) is measured in the third and fourth manifolds. Chloride reacts with mercury(II) thiocyanate to form mercury(II) chloride, releasing thiocyanate ion which then complexes any iron(III) ion present in the reagen stream; the absorbance of the [Fe(SCN)₂]⁺ complex is measured at the absorbance maximum (480 nm). The analysis utilises a single-line manifold after a 500-fold manual dilution or after an on-line 380-fold effective dilution factor using a cascade dilution manifold to bring the concentration range of the analysis into the linear calibration region. Interferences by plating-bath components on each system are discussed.