Palintest AP130 pH Phenol Red Reagent Tablets 250 per box. The Palintest Phenol Red test uses a tablet reagent containing the precise amount of phenol red indicator required for the test. Phenol red reacts in water at different pH values over the range 6.8 - 8.4 to produce a distinctive range of colours from yellow to red. The colour of the test solution is indicative of the pH value and is measured using a Palintest Photometer.