Which device measures the hydrogen ion activity in a solution?

The pH meter is specifically designed to measure the hydrogen ion activity in a solution, which is essential for determining the acidity or alkalinity of that solution. It functions by using electrodes to detect the concentration of hydrogen ions, giving a precise reading that reflects the solution's pH level. This measurement is crucial in various applications, including water quality testing, chemical analysis, and environmental monitoring, as pH can significantly impact the behavior of chemical compounds in a solution.

In contrast, a conductivity meter measures the ability of a solution to conduct electricity, which is influenced by the presence of ions but does not provide specific information about hydrogen ion concentration. A flow meter measures the rate at which fluid flows through a system, valuable for understanding water distribution but unrelated to pH measurement. A pressure gauge monitors the pressure of gases or liquids in a system, serving different purposes that do not include determining acidity or alkalinity.