If a pool operator finds that the calcium hardness is low, which chemical should be used to raise it?

When a pool operator finds that the calcium hardness level in the water is low, the best chemical to use for raising it is calcium chloride. This compound directly adds calcium to the pool water, which is essential for maintaining the proper balance in water chemistry and preventing issues such as corrosion of pool surfaces and equipment.

Calcium hardness refers to the concentration of dissolved calcium in pool water, which contributes to the overall water balance. Adequate calcium levels are important not just for swimmers' comfort but also for maintaining the integrity of plaster surfaces and preventing metal corrosion. By introducing calcium chloride, you increase the calcium levels effectively and address potential problems associated with low hardness, such as cloudy water and etched surfaces.

While the other chemicals mentioned serve their own purposes, they do not raise calcium hardness. Muriatic acid is typically used for lowering pH and alkalinity. Sodium bicarbonate is used to increase total alkalinity but does not have a significant impact on calcium hardness. Sodium thiosulfate is primarily used as a dechlorinator and does not affect calcium levels. Therefore, calcium chloride is the appropriate choice for raising calcium hardness in pool water.