Usually in the morning the pH of urine is moderately acid, it's value varying between 6.5 and 7. In the evening, urine tends to become slightly alkaline, with it's pH's values varying between 7.5 and 8.
The acid profile of urine is caused by the presence of free H+, discarded by cells in blood, to preserve the acid or alkaline environment and to stabilize the pH.
However, the pH of urine may fall below 6 or it may rise above 8.5, depending on the acidity or alkalinity of extracellular fluid.
When urine pH is below 6, the body struggles to keep the pH values in normal, hence it releases electrolytes for this purpose. An acid environment can lead to formation of uric acid stones in kidneys. The pH of urine can be decreased by food or drugs, such as sal ammoniac.