Phenol Red

Test Method Technical Information


Tests For: pH Value of Water and Aqueous Solutions
Tests Range: : 6.8–8.4
Reagent Chemistry Used: Phenol Red
Basis of Test Method: Industry Accepted Method

Testing for pH

pH measurement is one of the tests most frequently carried out on water and aqueous solutions. The phenol red indicator method provides a simple colorimetric means of pH determination for neutral and slightly alkaline waters over the range 6.8–8.4 units. The Phenol Red pH test is particularly applicable to testing swimming pools and spas. If your pool’s temperature is between 27 and 39 °C, the NSF calibration should be used as this is specific to pool and spa water. For samples 26 °C or less, the standard calibration should be used.

Reagent Chemistry

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.


Phenol red tablets contain a dechlorinating agent so that the test can be carried out in water containing normal levels of chlorine or other disinfectant residuals.
The colour range of the phenol red test is yellow, through orange, to red. The formation of an intense purple coloration shows that the indicator has been affected by high chlorine or other disinfectant residuals. In such cases the result should be disregarded.

Best Practice Advice for Testing

  • Phenol red does not show any further colour change at pH values below 6.8 or above 8.4. Note therefore that when such values are recorded this could indicate that the sample has a much lower or much higher pH value.
  • Ionic strength, temperature and other water factors may have an effect on pH readings. .