Phenol red is a water-soluble dye, named phenolsulfonphthalein. It is used as a colour indicator in tests, to show the degree of acid or base in a solution. Phenol red is added as a reagent to water-based solutions, changing colour from yellow to red as the pH changes
The pH is a logarithmic scale (which is a nonlinear scale used for a large range of positive multiples of some quantity. Common uses include earthquake strength, sound loudness, light intensity, and pH of solutions), which ranges from 0 to 14.
The optimum pH value for swimming pools water is between pH 6.8 and 7.6 for chlorine base disinfections. This narrow target range is necessary for the disinfection process to run efficiently, for the comfort of the swimmers and the general condition of the pool structure, fixtures and fittings.
A pH of 7.0 is neutral and if the value is above this, the water is alkaline i.e. it contains more alkaline components than acid components. Conversely, if the pH value is below 7.0 it contains more acid components than alkaline ones and there for acidic.
If the pH level is too high (>8.0) the effectiveness of the disinfection process will be reduced, and the water could become cloudy and scale formation can be promoted.
If the pH is too low (<6.8) eye and skin irritation can occur and the pool fittings could be damage by corrosion.
To avoid swings in the pH levels caused by addition of treatment chemical it is necessary to main an acceptable level of Alkalinity in the water. This is usually about 100mg/l and it made up of dissolved carbonates, bicarbonates and hydroxides. Theses have the effect of acting as a buffer to prevent big changes in pH upon addition of acids or alkalis to the water.
To rise the pH level
- Sodium carbonate or Soda ash, also called pH plus. This has a pH of around 10 and will also rise the alkalinity level of the water.
- Sodium hydroxide or caustic soda- this is very alkaline with a pH of 14 and must be handle with care. This will raise the alkalinity level of the water.
- Dolomitic Filter Material has an effect of raising the pH but is not dosed separately as a chemical.
To lower the pH level.
- Sodium Hydrogen Sulphate, Sodium Bisulphate or dry acid. Also called pH minus. This is a free-flowing crystalline power or granules which in solution has a pH of 1. It dissolves in some water in a plastic container and then sprinkled around the pool. It adds sulphate to the water.
- Hydrochloric acid also known as muriatic acid. This is an economical pH reducer, but handling can be a problem, the concentrated acid (32%) is highly corrosive. Commercial grades should not be used as their iron content may be high and this could cause discolouration and cloudiness in the water. It’s advisable to dilute the concentrated acid by adding water in a plastic container before sprinkling around the pool.
- Carbon Dioxide, this a gas and requires special equipment. When combining with the pool water, carbon dioxide forms carbonic acid which reduces the pH level and adds alkalinity to the water. This has distinct advantages in soft water areas where natural alkalinity if supply of water is low, but in hard water areas it is not suitable as it will raise the alkalinity to unacceptable levels. C02 works best at reducing pH value when the total alkalinity of the mains water is less than 150mg/l as Ca2=CO3 and hardness is less than 300 mg/l as Ca2CO3.
For the managment of swimming pools, phenol red reagents are used to test a pools pH levels, which can help monitor and maintain an acceptable pH range in the pool water. You can get the phenol red reagent in most swimming pool test kits and also you can buy the reagent separately.