pH measurement is used in a wide variety of applications: agriculture, wastewater treatment, industrial processes, environmental monitoring, and in research and development. pH is a measure of the acidity or alkalinity of a solution. The pH value states the relative quantity of hydrogen ions (H+) contained in a solution. The greater the concentration of H+, the more acidic the solution, and the lower the pH. In this relationship, pH is defined as the negative logarithm of hydrogen activity.