In preparing etchants employing sulfuric acid, it is beneficial to dilute the concentrated acid with at least part of the water first, followed by a standing period to dissipate the appreciable heat of dilution. Or the concentrated acid can be replaced with an equivalent amount of 50% or 60% acid. The sulfuric acid should be added last, after the other materials are in solution. The formulas shown are typical of this type of product. The theoretical amount of sulfuric acid, required to utilize the chromic acid present, will be seen to range from about 50% ot the calculated amount to about 75% of this figure. Three moles of sulfuric acid are required per mole of chromic acid anhydride. However, if the theoretical amount of sulfuric acid is used, the etchant tends to run wild. Where anything above 75% theoretical sulfuric acid is used, the etchant must be tailor-made to fit a specific etching job.
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