Blue Cyanide Copper Bath

Q. My cyanide copper plating bath has turned deep blue from the earlier pale blue color we have seen in the past. The bath consists of copper cyanide, sodium cyanide and Rochelle salts. What is going on and what should I do to solve the problem? L. M.

A. This is a question I like because it harkens back to my days as a chemistry student! Let’s start at the top! Rochelle salt is potassium sodium tartrate, which can form complexes with various metals. Rochelle salt is used in the cyanide copper plating bath to aid in anode corrosion and tends to give a denser, smoother deposit. Copper forms a complex with the tartrate to give a deep blue color.

In a typical cyanide copper plating bath, you will see a little of this blue color in the area around the anode. Normally this color will disappear as the tartrate complex is converted to the cyanide complex. In your case, the color is not disappearing which means that your plating bath does not contain enough free cyanide. Add more free cyanide to the plating bath and monitor the concentration on a more frequent basis.