"Since I prefer not to use the urine when it is acid...."
I drink apple cider vinegar when I think that my system is too acid. It's cheap, easy and it works for me.
Just found this today. They feed urea to cattle but limit the quantity. If they get urea poisoning from ingesting too much urea, they recommend giving them vinegar! Sounds like it should work for fasting on urine too.
http://www.ext.colostate.edu/Pubs/livestk/01608.html
"Vinegar is a helpful emergency treatment for urea poisoning if the animal is treated before tetany develops."