The pH value, also known as the hydrogen ion concentration index or acidity and alkalinity, is a scale measuring the activity of hydrogen ions in a solution—in other words, it is the standard by which the acidity or alkalinity of a solution is generally measured.

Generally, when the pH is less than 7, the solution is acidic; when the pH is greater than 7, the solution is alkaline; and when the pH is equal to 7, the solution is neutral.

There are many methods for measuring the pH of a solution: Add a pH indicator to the solution being tested; different indicators change color at different pH levels, and the pH range can be determined by observing the indicator’s color change. Titration can also be used to establish a precise pH standard. Using pH test strips—which come in both general-purpose and precision varieties—involves dipping a glass rod into the solution to be tested and applying a small amount to the strip; the pH value can then be determined by comparing the color change on the strip to a color chart. Using a pH meter—an instrument designed to measure the pH of a solution—which employs a pH-selective electrode (such as a glass electrode) to determine the pH value; a pH meter can provide readings accurate to two decimal places.

As defined, pH is a measure of a solution’s acidity or alkalinity. In many fields, it is necessary to control the acidity or alkalinity of a solution, and knowing the pH value is essential in these contexts:

• In medicine, the pH of human blood typically ranges between 7.35 and 7.45;
• any deviation from this range is considered a pathological condition. The pH of saliva is also used to assess medical conditions.
• In chemistry and chemical engineering, many chemical reactions must occur at specific pH levels; otherwise, the desired products cannot be obtained.
• In agriculture, many plants prefer acidic or alkaline soil—for example, tea cultivation. Controlling soil pH can promote better growth in cultivated plants.

By Michael Chiu, MWS, CI
Previously published in Ming Pao Daily News