Which chemical is added to raise alkalinity levels in swimming pool water?

Sodium bicarbonate is the correct chemical for raising alkalinity levels in swimming pool water due to its composition. It acts as a buffer and helps stabilize the pH of the water, making it less acidic. Maintaining ideal alkalinity is crucial because it helps prevent sudden changes in pH, which could lead to issues with water balance, corrosion, and scaling in the pool.

When sodium bicarbonate is added to the water, it dissolves and increases the total alkalinity, effectively enhancing the water's ability to resist pH fluctuations. This is essential for swimmer comfort and the longevity of pool equipment and surfaces.

Other chemicals listed serve different purposes: muriatic acid is used to lower pH and alkalinity, sodium thiosulfate is primarily a chlorine neutralizer, and hydrogen peroxide is typically utilized as an oxidizer or algaecide. Therefore, sodium bicarbonate is specifically the right choice for adjusting alkalinity in pool water.