i'm not scientist, but from what i understand from people in "the know"
it is the caffic acid in the coffee that developers
the soda changes the ph ( adds fog too ) and the vit c from what i understand
increases speed and contrast ...

reinhold knows way more than me: http://caffenol.blogspot.com/

it has to be robusta coffee, arabica coffee doesn't have very much ( if any ) caffic acid.
the robusta coffee is usually the cheap stuff that sells in instant coffee, not in boutique coffee shop.

if you are interested ( and have a coffee pot! ) i have a bunch of robusta beans ( GREEN! ) and roasted
that works even better than instant ( for me at least ) ...
instant is mostly robusta, but still often times is "cut" with arabica so it tastes better ...

have fun!