Which of the following is an example of an acid cation?

An acid cation is a cation that can contribute to the acidity of a solution, and hydrogen ions (H+) are the most prominent example of such cations. When H+ ions are present in a solution, they can lower the pH, thereby making the solution more acidic. This is fundamental in various chemical reactions and processes in soil chemistry, where the concentration of H+ can affect nutrient availability, microbial activity, and overall soil health.

In contrast, other cations like calcium (Ca2+), sodium (Na+), and potassium (K+) do not directly contribute to acidity in the same way. While they are all important nutrients in plant growth and impact soil properties, they do not produce an increase in hydrogen ion concentration in the solution, which is central to defining an acid cation.