The pH value, or hydrogen-ion activity, is a scale used to describe the acidity or alkalinity of an aqueous solution. More precisely, pH is related to the activity of hydrogen ions in the solution and is commonly expressed on a scale based on the negative logarithm of hydrogen-ion activity.

At 25°C, a pH below 7 is generally described as acidic, a pH above 7 as alkaline or basic, and a pH of 7 as neutral. The neutral point can vary with temperature, so pH 7 is not universally neutral at every temperature.

There are several methods for measuring pH. Acid-base indicators change color over particular pH ranges, allowing an approximate pH to be determined. pH test strips work on a similar principle and are available in different ranges and degrees of precision. For more accurate measurements, a pH meter uses a pH-sensitive electrode, commonly a glass electrode, to measure the electrical potential associated with hydrogen-ion activity. Properly calibrated laboratory pH meters can provide measurements to considerably greater precision than is normally required for household or field applications. Acid-base titration can also be used to determine the concentration of an acid or base and its neutralization capacity, although titration is not normally a direct method of measuring pH.

Because pH affects chemical and biological processes, controlling it is important in many fields:

In medicine, the pH of arterial blood in a healthy person is normally maintained at approximately 7.35 to 7.45. Significant deviations from this range can indicate acid-base disorders and may require medical attention. The pH of saliva can also vary considerably and may be measured in certain research and clinical contexts, but it is not by itself a general diagnostic test for disease.

In chemistry and chemical engineering, many chemical reactions are strongly affected by pH. Controlling pH can influence reaction rates, chemical equilibria, solubility, and the formation of desired products.

In agriculture, soil pH affects the availability of nutrients and the growth of plants. Different crops have different preferred soil-pH ranges. Tea plants, for example, generally grow well in acidic soils. Measuring and appropriately managing soil pH can therefore be important for agricultural production.

By Michael Chiu, MWS, CI
Previously published in Ming Pao Daily News – Updated and Revised in 2026