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Why is pH important?

What is pH?
pH is a number between 0 and 14 that indicates how acidic or basic a solution is. Pure distilled water has a pH of 7.0 and is neither acidic nor basic. Water with a pH of lower than 7.0 is said to be acidic, and the smaller the number, the more acidic the water is. In the opposite, water with a pH greater than 7.0 is basic, and the larger the number, the more basic the water.
The scale between 0 and 14 is also logarithmic, this means that, pH 8 is 10 times more basic (alkaline) than pH 7 and pH 9 is 100 times more basic than pH 7. In the reverse pH 6 is 10 times more acidic than pH 7 and ph 5 is 100 times more acidic than pH 7 and so on. So you can see that, what appear to be small variations in the range are in fact more serious than one would assume from the variation in the numbers. E.g. pH 3.5 is 10 000 times more acid than an ideal pH of 7.5 for pool water, even though one would have said there is only a difference of 4 in simple mathematics.

How does it affect my pool?

All that having been said, we can now say simply pH is a reading that indicates how acidic or basic (alkaline) your pool water is. The ideal pH range for pools is between 7.2 and 7.6. A reading below the range is acidic and a reading above the range is base. The most unstable and probably the most important component of water balance is pH. Without elaborating at this point, suffice it to say that it's important because it has an impact on your sanitiser effectiveness, your pool surfaces and equipment -- and you as the bather.
Here are the most common problems with both high and low pH levels:
High pH Readings

Poor sanitiser efficiency
Cloudy water
Shorter filter runs
Scale formation
Skin and eye irritation
Low pH Readings

Poor sanitiser efficiency
Etched or stained plaster
Corroded metals/equipment
Skin and eye irritation
Destruction of total alkalinity

pH is a very complicated subject, and the above explanation has been deliberately simplified to give sufficient, accurate information for swimming pool owners. Chemists would not accept it as complete, but it will do nicely just for this discussion. It is important to remember that pool pH levels are affected by the many impurities that find their way into the pool water including top up water, leaves, dust, cosmetics, perspiration, and all too often, urine (especially if your dog swims). All these substances have their own pH levels, which is why they affect the swimming pool pH levels

How do I "fix" my pool pH?

To correct a low pH reading situation, you would add soda ash or sodium bicarbonate; suitable products are available from your pool professional. If the pH reading is too high i.e. above 7.6, to bring the pH level down in pools, you'll need to add some pool acid (dry acid is safer but you may also use Hydrochloric acid). Here, you need to take special care and follow your pool professional's advice. Soda ash and sodium bicarbonate can be added to a pool by dissolving the powder in a bucket of water and pouring the solution directly into the pool water; or by broadcasting the dry powder over the water surface. The amount required varies greatly from pool to pool, it is therefore advisable to have your pool professional test the water and advise on quantities and products to use. Hydrochloric acid (HCI) may be added to a pool if the pool owner is sufficiently aware of the dangers and precautions of handling acid. Dry acid can be poured directly into the pool when no one is swimming. Acids may also affect the total alkalinity of water. When making adjustments to the pH level, ensure the pump is running before adding any chemicals to your pool. Once the water has been allowed to re-circulate for a few hours, retest again to see if further action is needed. Note: Before adding any chemicals to treat your pool, adjust your pH levels as required. When adjusting pH always consider impact to total alkalinity. The use of both alkaline and basic to refer to high pH, and to products that raise the pH, may cause some confusion with the Total alkalinity of water. The difference between total alkalinity and pH is the subject of a future post. Accurate control of the pH of swimming pool water is essential. The effects of pH upon flocculants, bactericides, algae growth, equipment, maintenance and bather comfort is substantial and is often incorrectly attributed to other factors such as chlorine levels.

So, why is pH important?

1. The pH value affects the amount of free available chlorine (Hypochlorous acid) that is formed, and therefore determines the effectiveness of the chlorine to keep your pool water safe and hygienic.
At pH 6.5, 90% of the chlorine will be Hypochlorous acid
At pH 7.5, 50% of the chlorine will be Hypochlorous acid
At pH 8.0, only 20% of the chlorine will be Hypochlorous acid

Unfortunately you cannot run your pool at pH 6.5 - as it would then be acidic enough to corrode the metal parts in your pool equipment and it is just too far from the human body's pH of 7.4 to be comfortable for us to swim in. The compromise is therefore to obtain the best effectiveness of chlorine while ensuring bather comfort. (I.e. 7.2 to 7.6) Remember, if you let the pH drift out of this range, you will have to use much more chlorine to get adequate sanitation. 2. Bather comfort. At high pH, the water will make your eyes sting and possibly give you skin irritations. At low pH the impact on bathers is even more pronounced and red stinging eyes and chaffed skin in younger bathers may occur. 3. At high pH there are two dangers. a. The danger of scale forming on your pool surfaces, pipe work and fittings. This is because at a pH of around 8.0, the calcium in the water combines with carbonates in the water (Calcium carbonate or scale). b. Calcium carbonate can form into tiny particles and float around in the water giving it a cloudy, turbid appearance. 4. A low pH can corrode metals, damaging expensive equipment and leaving metal oxides to stain pool surfaces. Under certain conditions the precipitated (particulate) metals can even tint your blonde hair. Pools constructed with plaster are far more prone to wear and tear damages from low pH, and even if you cannot see it happen, tiny pit holes can form in the structure of the pool which opens up perfect breeding grounds for micro organisms and algae to form.


If you test nothing else always ensure that you are able to test pH & chlorine and that these tests are performed regularly and adjust accordingly. Effective ph control is far more economical and efficient than having to face; what would be required when your pH has been left unchecked and has been far out of range for extended periods of time.