You cannot reverse toning/acidification. There are products you can buy to prevent further acidification such as http://www.subwaystamp.com/ARCHIVAL...o/ZPTWATTAM/http://www.talasonline.com/deacidification
I assume that a sodium bicarbonate solution would also work but am unsure if this is an approved archival method.
I would add to rwarde post that lignin does not need light to generate acids in paper. Once any buffering has been depleted, lignin can break down in the dark too (i.e. with constant variance in temperatures and RH).