Phenol Red Tablets used for testing the pH of swimming pool and spa bath water. The phenol red tablets shown are foil wrapped for convenient dosing and optimum storage

 phenol red tablet

Phenol red tablet general information

Phenol red tablets are based on a pH sensitive dye which varies in colour between yellow at pH 6.8 and red at pH 8.4. The optimum range for swimming pool water is pH 7.2 to 7.8.

Phenol Red is available as a liquid, powder but most frequently and conveniently in a tablet form.

Phenol red tablet use in domestic pools

A widely used type of test method for domestic pools is the dip cell or pooltester. This consists of a small plastic block with one or more cells into which the water sample is added. Coloured scales are provided next to each cell, showing the value of the measurement against the colour. In addition to pH, these are often used, with the correct tablet, to measure chlorine, bromine and alkalinity.

 pool testing kit
A typical pooltester kit using tablets

The complete test method, which should be carried out daily when the pool is in use, is as follows:

Do not sample the water with anything breakable especially glass

  • Remove the cap and rinse out the testing cell with the water to be tested
  • Fill the cells with the sample water directly from the pool
  • Place a rapid dissolving phenol red tablet in the pH test cell. Place other tablets such as DPD1 for free chlorine, in the other cell(s) as appropriate
  • Replace the cap and shake to dissolve the tablets
  • Hold the cell up to the light, ideally a north natural light. Avoid greenish fluorescent lights, and evening light which has a red cast to it
  • Within 15 seconds read the values
  • Make the necessary chemical adjustments to the water and retest

Phenol red tablet use in commercial pools

 disc comparator with phenol red tablets
A disc comparator using phenol red tablets for pH testing. A full range of tablets and tests are available for all pool water tests

 
Hand held type of photometer which measures up to 5 parameters
  
Portable photometer using phenol red tablets and other types to measure up to 9 parameters. A 25 parameter version is also available

In commercial pools where a "duty of care" exists, a far stricter regime of water testing needs to be employed. Chlorine and pH testing should take place when the pool opens, when it closes and intermittently throughout the day depending upon bather load. Frequently 2 hourly testing can be justified. Above this manual testing becomes a full time job and some sort of automatic system will be used.

Many pools have used comparators which employ different colour discs with which to compare the colour generated by the tablet. Some use natural light and some artificial light sources, but nevertheless they still rely on the operator's eyesight and lighting conditions to obtain the reading. The latest instruments known as photometers use a fixed wavelength of light to examine the water sample as coloured by the phenol red tablet or other types of test. The operator merely has to select the test required by pressing a key pad, crush the right tablet in the sample and directly read off a digital numeric value. The typical test method for pH using phenol red tablets would be:

Do not sample the water with anything breakable especially glass

  • Collect a sample by filling a clean plastic bottle below the water surface excluding any air
  • Test as soon as possible to avoid deterioration of the sample
  • Switch on photometer and select the pH key
  • Rinse the test tube with the sample water
  • Fill the tube up to the 10ml mark with the sample water. Add a photometer grade Phenol red tablet. Mix to dissolve
  • Set the wavelength to 520nm
  • Optionally cover the tube with the light shield if working in bright light
  • Note the pH reading immediately

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