An ammonia bath is the one most widely used for depositing Pd-Ni alloy coatings. The art...

An ammonia bath is the one most widely used for depositing Pd-Ni alloy coatings. The article “Modelling of Palladium and Nickel in an Ammonia Bath in a Rotary Device” (Plating and Surface Finishing, 1997: 102–104) reported on an investigation into how bath-composition characteristics affect coating properties. Consider the following data on x₁ = Pd concentration (g/dm3), x₂ = Ni concentration (g/dm³), x₃ = pH, x4 = temperature (?C), x₅ = cathode current density (A/dm²), and y = palladium content (%) of the coating.

a. Fit the first-order model with five predictors and assess its utility. Do all the predictors appear to be important?

b. Fit the complete second-order model and assess its utility.

c. Does the group of second-order predictors (interaction and quadratic) appear to provide more useful information about y than is contributed by the first-order predictors? Carry out an appropriate test of hypotheses.

d. The authors of the cited article recommended the use of all five first-order predictors plus the additional predictor x₆ = (pH)² . Fit this model. Do all six predictors appear to be important?