[UV-vis spectrometric characterization of the composition of active complex in the Ni-P plating solution]

A strong complex agent is normally used in the alkaline chemical plating solution for Ni-P plating in order to prevent Ni ion from precipitation by hydrolysis. To keep a stable pH condition, an NH3-NH4Cl buffer system is used. Traditionally, it is considered that NH3 does not participate in the complex because of the relatively low Ni-NH3 complexing constant, but some experimental results cannot be explained reasonably. Touhami etc. have proposed a ternary Ni-citrate-NH3 complex involved in the discharge process, however they cannot give the direct support on the presence of this complex in solution. In this paper, a UV-Vis spectrometric study was carried out to identify the nickel complex in the Ni-P plating solution, and the results indicated the presence of both Ni-cit binary complex and Ni-cit-ammonia ternary complex. After the systematic investigation of the dependence of UV-Vis spectra on the two ligands (cit and ammonia), the composition of this Ni-cit-ammonia ternary complex was supposed to be Ni(II)(C6H5O(7)3-)(NH3)3.