Which chemical reagent is used to indicate the pH value in pool water?

Phenol red is the correct chemical reagent used to indicate the pH value in pool water. It is a pH indicator that changes color depending on the acidity or alkalinity of the water. In the pH range typical of swimming pools, phenol red transitions from yellow in acidic conditions (pH below 6.8) to red in alkaline conditions (pH above 8.2), allowing pool operators to easily assess whether the water's pH is within the ideal range of 7.2 to 7.8 for safe and comfortable swimming conditions.

The other choices are not relevant to pH testing. Pseudomonas blue is not a standard indicator for pH and is more related to bacterial presence. The DPD solution is used primarily to measure free chlorine levels in water, helping ensure that the pool is properly sanitized. Chlorine yellow is a less common term and typically relates to chlorine color tests, not pH measurement. Thus, the significance of phenol red lies in its effectiveness and reliability as a pH indicator for maintaining pool water quality.