Outdoor Pool Spray

Everything you need to know about swimming pool chemicals

Why is maintaining the pH, chlorine and alkalinity of a pool so important?

It is vital to have a basic understanding of the chemicals you often hear mentioned regarding effective pool maintenance – regardless of whether your pool has already been fitted with a disinfection system. The three most important ones to remember are ‘pH’, ‘chlorine’ and ‘alkalinity’.


pH describes how acidic or alkaline the pool’s water is. The ‘pH scale’ is a two-way scale centred around the number 7. Water with a pH of 7 is said to be neutral, meaning that it is neither acidic nor alkaline. Values below this value are acidic, whereas values higher than this are alkaline. When it comes to swimming pools, the ideal pH is between the range 7.2 and 7.4.

Maintaining the pH between this range is important for a number of reasons. Firstly, a pH that is too high or too low could damage the fabric of pool equipment. In hard water areas, scale can form much more easily at higher pH levels, so this too could damage pool equipment. Moreover, the pH level can seriously impact bather comfort. If the pH is too high or too low, the bather could suffer from eye, nose or skin irritation.

Thanks to high-quality modern test kits and swimming pool chemicals, maintaining your pool’s pH is fairly straightforward. If the pH level falls too low, simply add Compass Pool’s pH Plus; if it becomes too high, use Compass Pool’s pH Minus. The amount you need to use will be clear from the dosage instructions on the label.

Do not add in a lot of chemical in one hit, otherwise the pH value will swing rapidly in one direction. Use small doses to get your pH level within the desired range both quickly and easily.


Chlorine is an extremely effective bactericide and oxidiser, hence why it is so commonly used across many water treatments. It is also the most widely available product for sanitising pool water.

Adding chlorine to your pool ensures that the pool’s water is properly sanitised and safe to swim in. Chlorine also leaves a residue in the water, so it maintains its germ-killing function even when your filter system isn’t on.

You will often come across the terms ‘total chlorine’, ‘free chlorine’ and ‘combined chlorine’ when it comes to pool maintenance, which can sometimes confuse pool owners. Here is what each term means:

  • Total chlorine. As the name implies, this indicates the total amount of chlorine contained in your pool water, but doesn’t tell you how effectively it is killing bacteria. The ideal total chlorine value should be between 8ppm and 2.5ppm.
  • Free chlorine. This is the most important value, as it relates to the chlorine component which is actually destroying bacteria. The free chlorine value should ideally be between 1ppm and 3ppm.
  • Combined chlorine. This 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 red eyes you typically saw at poorly run public pools before digital automatic control systems were introduced. This inactive element also inhibits free chlorine from working properly, therefore – the lower its volume is, the better.


Also described as ‘total alkalinity’ (TA), alkalinity 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.

In your pool, the ideal TA value falls between 100 and 150 ppm. Should your pool’s water fall outside of this range, here are some of the potential problems you could encounter:

Too high:

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

 Too low:

  • pH fluctuates wildly.
  • Potential corrosion.
  • Potential water discolouration and pool wall staining.

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, the TA is typically very low. This can be easily adjusted using Compass Pools TA Plus.

Once you have achieved the correct TA value, regularly measuring and adjusting the level is not normally necessary.

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