A Method for Measuring the Etching Strength of Ferric Chloride Solutions.

The etching strength of a ferric chloride bath for making copper photoengravings can be measured by the potential of the cell (Pt; Fe+++, Fe++) standard//(Fe+++, Fe++; Pt) unknown. This cell has been studied for solutions having copper concentrations in the range of 0.5 ounce per gallon to 2.5 ounces per gallon. Within this range, conventional redox cells are almost completely insensitive to even fairly large changes in copper content, a difficulty which is overcome by the cell described. By proper choice of standard solution, the method can be extended to greater concentrations of copper. The potentials determined experimentally agree well with potentials calculated from the Nernst equation. The method is being used experimentally in halftone work for predicting etching time.