I don't know if OP is still active but in case anyone else needs this information:
4.5~8 is the general range for urine pH. Generally it stays within this range because of limitations of the kidney, but there are some exceptions. Namely, a urea-splitting organism infection will generate ammonia from urea and cause Urine pH to exceed 8.
The acid-base equilibrium of urine is established by H+ ATPase (secretion into urine) and HCO3- ATPAse (reabsorption from urine). The minimum and maximum mentioned are based on the actions of these two functioning features.
The lower limit (4.5) is usually caused by some form of acidosis and full function of the H+ ATPase function; and the upper limit is a combination of H+ ATPase failure and HCO3- ATPase failure (which can happen for many reasons).
Although I couldn't find the specific numbers for the HCO3- ATPase, I presume that calculating the maximum Urine pH would be some equilibrium calculation assuming both the H+ ATPase and HCO3- ATPase fail.