About Pool Chemicals

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Could you explain a little more about pH, chlorine and alkalinity?

Regardless of whether you have a fully-automatic or semi-automatic disinfection system for your pool, it is still very important to have a basic understanding of some key chemical terms you will hear mention of often in regard to the safe and hygienic maintenance of pool water chemistry; most important of those are ‘pH’, ‘chlorine’ and ‘alkalinity’


This term is used to describe how acidic or alkaline the water is. It is a two-way scale centred on the number 7. Water with a pH of 7 is said to be ‘pH neutral’, i.e. the water is neither acidic nor alkaline. Values lower than 7 are acidic, and values higher than 7 are alkaline. In a swimming pool, the ideal pH value is ever so slightly alkaline, between 7.2 and 7.4.

It is important that the pH is maintained between 7.2 and 7.4 for a number of reasons. Firstly, a pH that is too high or too low may start to damage the fabric of pool equipment. In hard water areas, scale forms much more readily at higher pH levels, and this too is damaging to pool equipment. It is important too for bather comfort. If the pH is too high or too low, irritation to eyes, nose and skin can occur. Lastly, it’s also important for the performance of your sanitiser (i.e. chlorine), which kills germs and oxidises impurities more effectively at the correct pH.

Controlling your pH value is easy with quality modern test kits and swimming pool chemicals. If the pH level in your pool is too low, you need to add Compass Pools pH Plus; and if it is too high, add Compass Pools pH Minus, according to the simple and easy to follow dosage instructions on the label.

When adjusting your pH value, follow the instructions and adjust it in stages if necessary. Do not be tempted to add in a lot of chemical in one hit, otherwise the pH value may swing rapidly in the other direction and may need to be corrected again.


Chlorine is an extremely effective bactericide and oxidiser. It’s properties have been well-documented for many, many years and it is commonly used in many water treatment processes, including in plants which treat our drinking water supplies. It is also by far and away the most widely available and used product for sanitising pool water. Adding chlorine to your pool to the correct levels ensures that the water is properly sanitised and safe and hygienic to swim in. Another property of chlorine that makes it ideal for use in a pool environment is that it leaves a residual in the water, so it’s always killing germs, even when your filter system is not operating.

To maintain safe and hygienic water conditions, the (free) chlorine value should be between 1ppm and 3 ppm (parts per million).

You may come across the terms ‘total chlorine’, ‘free chlorine’ and ‘combined chlorine’, which sometimes confuse pool owners. This is what they mean.

  • Total chlorine: indicates, as the name implies, the total amount of chlorine contained in your pool water, but says nothing about how effectively it is killing germs. The total chlorine value should be between 0.8ppm and 2.5ppm.
  • Free chlorine: This is the most important value, and is the one we are concerned with, as it relates to the chlorine component which is actually working and destroying bacteria. The free chlorine value, as discussed, should be between 1ppm and 3ppm.
  • Combined chlorine: describes chlorine which has combined with impurities in the water and is inactive in terms of its sanitising power. It is also associated with the acrid smell and red eyes associated with poorly run public pools of years gone by before the introduction of digital automatic control systems. This inactive element also inhibits free chlorine from working properly. There is no recommended value for this, the lower it is, the better. A weekly backwash reduces the build-up and prevents too high a concentration of combined chlorine in your pool.

Alkalinity of your pool:

Alkalinity, also described as ‘total alkalinity’ (TA) is often confused with pH, as it sounds very similar to the word ‘alkaline’. They are linked though, as the TA value governs how stable the pH is, and therefore how easy it is to maintain the pH.

The ideal TA value is between 100 and 150 ppm.

Here are potential problems if the TA is incorrect:

Too high:

  • Difficult to adjust the pH
  • Cloudy water
  • Calcium deposits
  • Chlorine does not work properly

 Too low.

  • PH fluctuates wildly
  • Possible corrosion
  • Possible discoloration of the water and potential staining of pool walls.

Both the pH and TA values need to be correct for your pool water to be sanitised properly. In hard water areas, the TA is usually at or near correct levels. However, in soft water areas, TA is normally very low. It can be adjusted upwards easily by the addition of Compass Pools TA Plus. Once the TA value is correct, regular measurement and adjustment is normally not necessary.

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