What should a CPO add to adjust the pH in a pool with a reading of 7.2?

In a pool with a pH reading of 7.2, it is important to understand that this level is on the lower end of the ideal pH range, which typically sits between 7.4 and 7.6. To raise the pH to a more optimal level, sodium carbonate, commonly known as soda ash, is the appropriate choice.

Sodium carbonate acts as a pH increaser and effectively raises the pH level of pool water. When added to the water, it interacts with hydrogen ions and helps in balancing the pH back into the desired range. This adjustment is crucial for maintaining the comfort of swimmers and ensuring the effectiveness of chlorine as a disinfectant.

Other options may not be suitable for this specific situation. For example, citric acid is used to lower pH, calcium hardness is related to water hardness levels, and an alkalinity increaser would primarily boost total alkalinity rather than directly adjusting pH. Hence, for correcting low pH levels in a pool, sodium carbonate is the best choice, making it the correct answer.