>pH13 in best case, but read the instructions.
If you have followed this synthesis and drop sulfuric acid into your solution at the end, then amphetamine sulfate will precipitate there until you reach pH6, anything below that means loss in yield and further impurities.
In general, the product should still be purified after synthesis.
Yes, tap water is also for drinking, why should that also be alkaline? How about going to a drugstore, getting distilled water and sodium hydroxide to make a lye and get your solution alkaline? But that is also in the instructions.