What does pH measure in a solution?

pH is a measure of the concentration of hydronium ions (H₃O⁺) in a solution. It is defined on a logarithmic scale, where a lower pH indicates a higher concentration of hydronium ions, corresponding to more acidic solutions, while a higher pH indicates a lower concentration of these ions and thus more basic or alkaline solutions. The formula for calculating pH is:

[ \text{pH} = -\log[\text{H}_3\text{O}^+] ]

This relationship shows how the pH value directly reflects the concentration of hydronium ions in the solution, allowing chemists to determine the acidity or basicity of the solution.

In contrast, the other options do not relate to the pH measurement in this way. The concentration of hydroxide ions, while important in understanding acidity and basicity, is not what pH directly measures. The density of the solution is a physical property unrelated to hydrogen ion concentration, and temperature, while it can affect pH readings indirectly, does not enter into the definition of pH itself.