In this article, we explain in detail what pH is, how to measure it and how to act to lower or raise it. pH plays a crucial role in maintaining healthy, balanced water in your pool. It varies according to a number of factors, so you need to check it regularly.
The pH of swimming pool water can be measured using test strips, test kits, electronic pH meters or connected water analysers. For a chlorinated swimming pool, the ideal value is between 7.2 and 7.6. A pH that is too low makes the water acidic and irritating, whilst a pH that is too high reduces the effectiveness of the disinfectant. To adjust it, a pH+ or pH− product is used, as required. Regular testing is essential to maintain balanced water.
The pH is a measure of the acidity of water, indicating the concentration of hydrogen ions. The pH scale ranges from 0 to 14. A value of 7 is considered neutral, and water below 7 is acidic. Above 7, it is alkaline.
When the pH is not optimal (too high or too low), a number of problems quickly arise:
Maintaining a stable pH helps prevent these imbalances and ensures an optimal swimming experience.
Maintaining the correct pH balance in your swimming pool or spa is of the utmost importance because of its impact on the water’s disinfection.
pH balance affects:
The recommended pH differs according to your disinfection method:
For chlorine disinfection, the ideal pH is between 7.0 and 7.4
For bromine or salt disinfection, the ideal pH is between 7.2 and 7.6.
pH is never completely stable. Several factors influence its change:
Rainfall, outside temperature, use of the pool (frequency and number of bathers) and the dosage of treatment products all have an effect on the pH of the water.
Understanding these variations enables us to anticipate the necessary adjustments.
Fluctuations in pH throughout the day are therefore entirely natural. It is estimated that a daily variation:
The addition of liquid chlorine (shock treatment or a dosing pump) or a salt chlorinator causes the pH to rise.
The addition of stabilised chlorine and bromine causes the pH to drop slightly.
Sweat, urine (even in small quantities) and organic matter are acidic, but they turn into ammonia when they come into contact with water, causing the pH to rise.
When there are lots of people swimming, there’s diving, playing, churning water and splashing. The water loses CO2 and the pH rises.
The change in pH throughout the day is due to the day-night cycle, which is caused by the effect of light on photosynthesis in microorganisms and algae. Consequently, in the early morning, the pH tends to be lower. It rises as the day progresses, reaching a peak in the evening. At night, the pH drops again when photosynthesis stops due to the lack of light.
Contrary to popular belief, rain is generally acidic and lowers the pH.
As the water temperature rises, it promotes the release of CO₂ and the growth of algae, and causes the pH to rise.
Tap water is often treated to make it slightly alkaline and generally raises the pH. Well water or rainwater is often acidic and lowers the pH.
There are several methods for measuring pH, with varying degrees of accuracy.
| measurement accuracy | frequency of measurements | constraints | |
| strips | low | one-off (manual) | rough estimate sensitivity to humidity and heat |
| electronic pH testers | good | one-off (manual) | probe calibration |
| connected water analysers | good | continuous (automatic) | probe calibration |
To measure the pH of your swimming pool, you can use pH test strips. They are extremely sensitive to moisture and light. As soon as you open the bottle, the air in the room begins to degrade the test strips. A test strip that has ‘gone off’ due to moisture will give completely inaccurate results.
If you choose this testing system, we recommend that you read the instructions carefully, as an incorrect test may lead to inaccurate results and cause you to use chemicals unnecessarily.
More accurate than test strips, the liquid reagents (in sealed vials) or tablets (in aluminium blister packs) included in the kits are much better protected against oxidation and external damage.
Test strips and kits rely on a chemical reaction that changes the colour of the sample. They are easy to use and provide a quick result, but the results are only approximate.
There are also electronic pH testers, also known as pH–meters that display the readings on a digital screen. They are very easy to use: simply dip the probe into the pond water to measure the pH of your pond water. However, they need to be calibrated very regularly, and you mustn’t forget to change the batteries.
For more reliable and frequent data than test strips and electronic testers, you can use a smart water analyser, such as ICO Pool, which automatically takes measurements every hour. In addition, smart water analysers measure other parameters, including temperature, pH, ORP (water disinfection) and TDS. Your pH readings and those of other parameters are sent to you at regular intervals, making it easier to manage your water parameters. These connected analysers also send you recommendations to help you maintain your pool, such as suggesting the right product to use.
To find out more about the different pH measuring instruments on the market, you can read this comparative of pH testers.
You should test the pH regularly, i.e. at least once a week. You should, of course, do so more frequently depending on how often you swim, the number of swimmers, and the weather. This is because the weather affects water quality, particularly the pH.
The pH should be measured at least twice a week. Once again, this is not a hard and fast rule, as it depends on how you use your pool, but also on the qualité de filtration de l’eau. Just bear in mind that water temperature is one of the factors that affects pH. In other words, the warmer the water, the more it deteriorates, so you need to monitor the water parameters more closely.
You should measure the pH if you have opted for a hivernage actif. In this case, check the pH once a month. However, passive overwintering does not require testing of the pH or any other parameters, as your pond is inactive.
This is a question often asked by pool owners. As we saw earlier, the pH level fluctuates throughout the day in two distinct cycles: day and night.
We recommend establishing a routine for testing your pool’s pH. Choose a time of day – in the morning or in the evening – when there are no swimmers in the pool, so that you can obtain consistent results from one test to the next. This routine is also recommended for testing all water parameters.
An incorrect pH level (too low or too high) can have a detrimental effect on bathing water quality and compromise the effectiveness of disinfectants such as chlorine or bromine. It can also affect the pool’s equipment.
| pH too low (<7) | pH too high (>7.6) | |
| Water quality | – Severe irritation of the eyes and mucous membranes – Skin that feels tight and itchy. – Unpleasant smell of chlorine (formation of chloramines) | – Eye irritation – Dry skin |
| Swimming pool equipment | – Corrosion of metal parts – Deterioration of the seals and liner | – Severe limescale build-up: walls, filter, electrolyser |
| Effectiveness of treatment products | – Chlorine is highly reactive but unstable. Excessive use of the product | – Chlorine becomes ineffective |
| Organic growth | – Growth of fungi and bacteria | – Rapid proliferation of algae |
| Apparence de l’eau | – The water becomes cloudy or milky. |
There are several factors that can cause low pH levels, such as rain, well water or rainwater, and over-dosing with pH- adjusters.
A low pH level can also have a detrimental effect on bathing water quality and the maintenance of pool equipment:
A high pH level can be caused by a number of factors: tap water with a high pH, frequent swimming, over-dosing with pH+, or an increase in water temperature.
A high pH level can have harmful effects on your pond. For example, it can cause:
A pH that is too high (above 7.6) reduces the effectiveness of the disinfectant.
To lower the pH: add pH− gradually and check regularly
If the pH is still too high, repeat the process, adding smaller amounts of pH- until the desired level is reached.
A pH that is too low (below 7) leads to excessive use of disinfectant and can cause corrosion of equipment and skin irritation for bathers.
To raise the pH: add pH+ gradually, then wait a few hours before measuring the pH again
If the pH is still too low, you can repeat the process by adding smaller amounts of pH+, testing regularly to ensure you do not exceed the desired pH level.
Be sure to follow the manufacturer's instructions carefully regarding quantities to be used, and wear protective equipment such as gloves and safety glasses when handling chemicals. Safety is paramount when handling chemicals. If you have any doubts or are having difficulty adjusting your pool's pH, don't hesitate to consult a pool care professional for advice specific to your situation.
Dans ces 2 cas, il est nécessaire de vérifier le TAC. il est soit trop haut (>200), et le pH reste stable malgré l’ajout de pH+ ou pH-, soit trop bas (>100), le pH fait le yo-yo. Avant de corriger le pH, il faut au préalable ajuster le TAC.
If the pH of the water does not change after adding pH– or pH+, this indicates that the water is buffered. It is the total alkalinity (TAC) – which is too high (>200) – that prevents the pH from regulating itself. The TAC should normally be between 80 and 150 mg/litre.
To treat your pool water in this situation, you must therefore address the TAC issue to restore a balanced pH level by adding pH-reducer, waiting 2 to 3 hours, and then running the filter. pH-reducer can be added in powder form (sodium bisulphate) or as a liquid (hydrochloric acid).
Quand le pH de la piscine fait le yo-yo, il est fort probable que le TAC soit trop bas <100. Dans ce cas, ajouter du TAC+ et activer la filtration.
A stable pH level depends on regular and accurate monitoring
Best practices:
pH is one of the most important factors in maintaining clean, clear water. A pH that is too acidic or too alkaline can damage equipment. It is therefore important to be aware of the factors that cause pH fluctuations so that you can anticipate them, or to recognise the tell-tale signs, such as eye irritation or skin discomfort.
In conclusion, pH is undoubtedly the most important parameter that all pool owners should monitor on a regular basis.
