Maintenance and control of coating baths: Rapid and simultaneous hypophosphite and phosphite determination in a Ni-P deposition bath

In this work, a method based on UV-VIS spectrophotometry using the molybdenum blue reaction is described to monitor hypophosphite and phosphite levels in nickel electroless baths. Nickel is one of the most employed metals in the electroless coating industry and during the deposition process the metal is reduced from Ni (II) to Ni (0). Hypophosphite is a widely employed reducing agent, which is oxidized to phosphite. Therefore, the hypophosphite concentration should be monitored in order to replenish the bath while it is being consumed maintaining an optimum concentration in order to guarantee the desired coating. Phosphite, which could interfere with the coating process, must be monitored to ensure it does not exceed a certain concentration after which the bath stops working. Although iodometric titrations are the most commonly used methods to control hypophosphite and phosphite concentrations, these are time- and reagent -consuming. Thus, with the aim of developing a more appropriate methodology a colorimetric determination was carried out in this work. In certain conditions, the hypophosphite reacts with molybdenum, giving rise to a blue compound that can be measured between 390 and 1100 nm. The phosphite presence modifies the obtained spectra and that effect allows individual Partial Least Square (PLS) models to be built for each analyte. In this study, the calibration, crossvalidation and external prediction relative error were always lower than 8 %, and the relative standard deviation lower than 12 %. The models were applied to real samples from two different nickel electroless baths, and the results were compared and validated using ion chromatography (IC). No significant differences were observed between the proposed and the reference methods for either of the two analytes. Therefore, the colorimetric method could be an improvement over the methods currently used for the control of the two anions in coating baths in term of saving time, money and reagents, making the coating process more efficient.