Plating baths can become contaminated in many ways from many sources, and it may become necessary to remove those contaminants to purify the plating solution.

There are a number different ways to do that based on what the plating solution is and what is contaminating it, but one method often applicable is 'dummy plating'.

Recognize that different metals have different 'potentials'. Some will plate out at very low voltage and some need more voltage before they will plate out. So, if the contaminating metal will plate out at a lower voltage than the metal we are plating with, it may be possible to apply that by applying a low voltage on a scrap piece of metal we can plate out the contaminant without wasting a significant amount of metal from the plating solution.

As an example, it is fairly common to electroplate copper onto a component, and to follow that with nickel plating -- which can cause the nickel plating solution to become contaminated with copper. Purifying a nickel plating solution by dummy plating the copper out of it is probably the most common application of dummy plating.