The stronger the vinegar the better when it cones to drain clearing. The chemical used to stop black and white photo development is acetic acid, which is the acid component of vinegar, plus an ingredient that turns purple when the ph goes too basic for it to work as photo stop. If you have a totally clogged drain and you pack some baking soda on top, then follow it up with concentrated photo stop you get a pipe clearing monster that beaches white foam that gradually turns purple.