pH is a measure of how acidic or alkaline a solution is. The diagram below shows what happens to an enzyme when denaturation occurs. Each enzyme has an optimum pH but it also has a working range ...

Acid phosphatase hydrolyses phosphate ester bonds under an optimum pH of 4.8 and an optimum temperature ... Para-nitrophenol is yellow at alkaline pH and its concentration can be measured at 405 nm.