Black oxidation develops on white gold

When I work out white gold jewelry pieces I get that blackish oxidation on the worked out items. I have tried to dip the pieces when hot in diluted sulfuric acid, but the work seem not to whiten out easily, and that blackish sooty color often remains on certain parts. What is the best whitening solution can I use to have the worked out items clean and free from basic oxides?

You need to protect the surface of your jewelry from oxidation before you heat it. Make a solution of fine boric acid ⇦ on eBay or Amazon [affil link] and alcohol to dip your jewelry, let dry and this will create a protective film which your diluted acid will dissolve after your soldering operation.