The pH of a solution is the negative logarithm of its hydrogen ion concentration, not the hydroxide ion concentration.

Here's the breakdown:

* pH: A measure of the acidity or alkalinity of a solution.

* Hydrogen ions (H+): The ions responsible for acidity.

* Hydroxide ions (OH-): The ions responsible for alkalinity.

The relationship between pH and hydroxide ion concentration is through the pOH scale:

* pOH: The negative logarithm of the hydroxide ion concentration.

* Relationship between pH and pOH: pH + pOH = 14 (at 25°C)

Example:

If the pH of a solution is 3, then the pOH is 11 (14 - 3 = 11). You can then calculate the hydroxide ion concentration from the pOH:

[OH-] = 10^(-pOH) = 10^(-11) M

In summary:

* pH: Measures hydrogen ion concentration (acidity)

* pOH: Measures hydroxide ion concentration (alkalinity)

* pH and pOH are related through the equation: pH + pOH = 14