How is pH defined in relation to hydrogen ion concentration?

pH is defined as the negative logarithm (base 10) of the hydrogen ion concentration in a solution. This means that when the concentration of hydrogen ions increases, the pH value decreases, indicating a more acidic solution. Conversely, when the concentration of hydrogen ions decreases, the pH value increases, indicating a more basic or alkaline solution. This logarithmic relationship allows for a convenient way to express the acidity or alkalinity of a solution on a scale that typically ranges from 0 to 14.

Understanding this definition is crucial because it highlights how pH provides a measure of hydrogen ion concentration in a way that is more manageable than dealing directly with the concentration values, which can vary over several orders of magnitude. The logarithmic nature of the pH scale means that each unit change in pH represents a tenfold change in hydrogen ion concentration, reinforcing the significance of even small changes in pH in chemical and biological systems.

The other options either misrepresent the relationship of pH to hydrogen ions or are irrelevant to the direct definition of pH itself.