well, i just guess, urine ?
Why urine? Our urine has uric acid.
If you do a proper search on the internet, and you should get the answer.
Just to help you a bit more:
Unpolluted rain is mildly acidic because of weak carbonic acid arising when carbon dioxide dissociates in water.
Tomato contains ascorbic acid.
You are left with black coffee and seawater.