Which of the following ions do NOT contribute to alkalinity?

Alkalinity refers to the capacity of water to resist changes in pH, primarily due to the presence of bicarbonate (HCO3-), carbonate (CO3-2), and hydroxide (OH-) ions. These ions can neutralize acids and play a crucial role in maintaining the pH balance in water systems.

Sulfate (SO4-2) does not contribute to alkalinity in the same way that the other ions do. Although it is an anion found in various water sources and can indicate the presence of certain pollutants or natural geological formations, it does not participate in the buffering capacity of water. Instead, sulfate ions do not react with acids to moderate pH fluctuations and are not involved in the foundational chemical processes that provide alkalinity. Therefore, sulfate's lack of involvement in neutralizing acids and moderating pH directly leads to its classification as a non-contributor to alkalinity, making it the correct choice in this context.

This distinction is essential when analyzing water chemistry since understanding what contributes to alkalinity helps in water treatment and environmental science.