What determines the pH of a solution?

The pH of a solution is fundamentally determined by the concentration of hydrogen ions, also referred to as protons, in that solution. pH is defined mathematically as the negative logarithm of the hydrogen ion concentration, given by the formula pH = -log[H+]. This means that as the concentration of hydrogen ions increases, the pH value decreases, leading to a more acidic solution. Conversely, if the concentration of hydrogen ions decreases, the pH increases and the solution becomes more basic or alkaline.

Although factors like hydroxide concentration, temperature, and concentration of salts can influence the behavior of a solution, they do not directly define its pH. Hydroxide concentration is indeed related to acidity and basicity, impacting the overall relationship of pH, but it is the hydrogen ions that are the primary determinant of the solution's pH value. Temperature can affect the ionization of water and other solutes, thus indirectly influencing pH, but it doesn't dictate it. Salts might affect solution properties, such as ionic strength, but they don't singularly determine pH. Therefore, the concentration of protons is the direct factor defining the pH of a solution.