As I understood acidic urine could be a detox symptom. If somebody`s urine is getting acidic during fasting that could be means she/he has lots of toxins in their system. They said if you notice that your urine is getting acidic during fasting take sodium bicarbonate/baking soda.
If you notice that the urine is acidic I think that could be means the detoxification is under progress and I think it supposed to be get better after a while. These toxins will leave your body after a while if you countinue the fast.
My opinion is that you have to do a fast until you notice that your urine is no longer acidic.