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What is the SI unit of pH?

Published in pH Unit 1 min read

pH has no unit. It is a logarithmic scale representing the concentration of hydrogen ions (H⁺) in a solution. Because it's a logarithmic expression (the negative logarithm of the hydrogen ion activity), it's a dimensionless quantity.

Understanding pH

  • Definition: pH is defined as the negative logarithm (base 10) of the hydrogen ion activity.
  • Scale: The pH scale ranges from 0 to 14, with 7 being neutral. Values below 7 indicate acidity, while values above 7 indicate alkalinity.
  • Logarithmic Nature: The logarithmic nature means a change of one pH unit represents a tenfold change in hydrogen ion concentration. For example, a solution with a pH of 3 is ten times more acidic than a solution with a pH of 4.

While pH meters and buffer solutions might be calibrated using SI units for other parameters (like temperature or voltage), the pH value itself remains unitless. References explicitly state that pH is a dimensionless quantity because it's a logarithmic scale, making it inherently without units. The mention of "SI units" in some product descriptions, such as buffer solutions, refers to the units of the concentration of the buffer solution, not the pH value itself.