G’day Karla; I read the essay in the URL
http://www.polymetaal.nl/beguin/mape/edinburgh_etch.htm And note
that anhydrous citric acid is specified as an addition to ferric
chloride etchant for copper, etc… In MY OPINION I see no reason why
the citric acid should be anhydrous. There is a monohydrate of citric
acid, and this is most probably the citric acid which is found in the
supermarkets, and I don’t think the fact that it may not be anhydrous
would not matter all that much After all you are going to add it to
a water solution anyway. It seems to me that the object of adding
citric acid is to lower the pH and add a few more hydrogen ions. I
would try a little and see. Citric acid is very cheap.
To be honest, I wasn’t keen on the style of that writer - it seemed a
bit on the pompous side (academic?) I suspect it was translated or
written by a German.
Cheers for now,
John Burgess; @John_Burgess2 of Mapua, Nelson NZ