The Analysis of Free Acid in Used Ferric Chloride

The analysis of hydrochloric acid, commonly re­ferred to as “free acid”, in used ferric chloride is of con­sequence for a number of inter-related reasons which help to maintain etching process control. Free acid helps to solubilize the metal that is etched during the etching process. It prevents ferrous chloride from pre­cipitating out of solution, and participates in the oxida­tion of this material back to ferric chloride using the oxygen in the air. Lastly, free acid prevents the build­up of complex ferrous salts, such as Fe(OH)Cl, the ef­fects of which are not wholly understood but are presumed to be largely detrimental to the etching pro­cess.