pH — getting it right

Macquatics Training
By John McKenny
Monday, 25 July, 2016



pH — getting it right

In this article, the second in a series of non-technical articles, John McKenny* discusses pH: what it is, why it’s important and how to manage it. Keep your costs down and your pool clean and safe for swimming by following ‘Good water matters’ in each publication.

What is pH? Simplified, pH — little ‘p’ and capital ‘H’ — is the measure of the activity of an acid or alkali in a liquid, such as pool water. The pH test actually measures the amount of hydrogen ions in the liquid.

Regardless of what the other tests are reading, the pH should always be between 7.35 and 7.45.

The pH scale measures from 0 to 14, with 0 being most acidic and 14 being most alkaline (see Figure 1).

Figure 1: Image by Edward Stevens (Own work) [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

Figure 1: Image by Edward Stevens (own work), CC BY 3.0, via Wikimedia Commons.

The pool chemicals that are normally used for the correction of pH are strong acids such as hydrochloric acid, sulfuric acid and sodium bisulfate. These are the acids used in swimming pools to lower the pH. Examples of strong alkalis used in pools are sodium carbonate (soda ash) and sodium bicarbonate (bicarb).

Pool chemical Approximate pH
Sulfuric acid 2.7
Trichlor 2.7
Hydrochloric acid 3
Cyanuric acid 4+
Distilled water 7 — pH neutral
Ideal pool water 7.4 +/- 0.5
Human tears 7.4 +/- 1
Sodium bicarbonate 8.5
Calcium chloride 8–10
Sodium carbonate 10.5
Calcium hypochlorite 11–12
Sodium hypochlorite 13

Figure 2: Approximate pH values of some important pool liquids and chemicals.

I mentioned above that the pH should always be between 7.35 and 7.45. The reasons for this are:

1. Swimmer comfort

It’s now established that the pH of tear drops from the eyes of normal healthy people is about pH 7.4. Surely, then, if we have the swimming pool water at this pH, swimmers shouldn’t get sore eyes from high or low pH water.

Trials I’ve done on this have concluded that the comfortable range for pH in pool water is 7.6 to 7.2. You can expect severe eye irritation once the pH reaches 8+ or goes below 2.

2. Chlorine activity

Whilst the activity of bromine in pools is only slightly affected by various pH changes, chlorine is greatly affected. At pH 8, only 20% of the chlorine in the water is sanitising, whilst at 7.4, 60% is sanitising (this is a dumbed-down, easy-to-follow description of chlorine acting in water at various pH levels). Lowering the pH alone can increase the chlorine’s activity, as many pool operators have found.

3. Less chlorine odour

At the recommended pH levels above, and with the increased chlorine activity, there will be less so-called ‘chlorine odour’ from the pool, as the now-activated chlorine kills off contaminants quicker than at higher pH levels. This chlorine odour, although smelling exactly like chlorine, is not actually chlorine, but the result of the chlorine acting on contaminants in the pool water and producing chloramines, which have this characteristic chlorine smell.

If you have a pool, especially an indoor pool, and you have this chloramine odour, maintain your pH between 7.35 and 7.45 and keep the chlorine at about 3 mg/L (free chlorine, that is) and you should end up with better water and air quality. Total dissolved solids (TDS), bather loads, water balance and possibly other things will all affect the water and air quality too, and sometimes these have to be addressed to get an improvement with indoor pool environments. This pH and chlorine test level will apply equally to an outdoor pool too, but chloramines tend to hang around in indoor pools and can make conditions uncomfortable.

This is one area of enquiry I regularly receive, but it’s usually easily fixed and I’m just a phone call away and always available and happy to give free advice.

4. Less algae

Most algae is extremely chlorine-sensitive, so having the pH in the right range will allow for maximum chlorine activity and will kill off any algae. If the pH and chlorine tests are at the right levels, as mentioned in the previous paragraph, then if algae remains, quite often there’s a problem with pool ‘dead spots’. If algae is noticed in the pool, wait until the pool is empty of people, obtain the correct protective equipment and a small quantity of calcium hypochlorite (powdered chlorine), sprinkle it dry over the area where you noticed the algae, and whammo! You’ve freed your pool of algae!

5. Water balance

It’s easier to maintain your water balance correctly when you start off with the 7.35–7.45 pH range. Water balance done correctly is vital for proper water quality management. This will be discussed and explained in a later article.

So there are five sound reasons for maintaining the correct pH. It’s normal for those who don’t test and adjust it regularly to end up having important water quality problems.

How much acid is needed to reduce your pH?

Firstly, it’s necessary to buy a cheap and basic water test kit. They’re easy to use and reasonably accurate. Don’t buy any acid yet — you may not need any.

Do the simple pH test on the water. If you’re using liquid or powdered chlorine, your pH will be either okay or most likely above pH 7.6.

If you have a smallish backyard-type pool (up to about 80,000 L) and the pH is, say, 7.8, have the circulation pump going, then using gloves, a face shield, etc, add 500 mL of acid (hydrochloric) directly to the pool along the side. Do this when there is no one about, especially children, and do yourself a favour and have a hose with running water close by to rinse off and dilute any spills.

After 5–7 hours of water filtering, do another pH test. If, for instance, the pH is now 7.4, you know that 500 mL of hydrochloric acid will lower the water in your pool by pH 0.4. Now you’re controlling your pool’s chemical addition correctly, which will lead to safe and comfortable swimming. It’s that easy!

Other pools will be different, depending on volume, initial pH test, other chemicals in use and the source and test of your pool’s water supply (most pools will have town water, but out west, creeks, rivers, tanks, dams and bores are accessed to fill and run the pools).

With pH being so important for proper water quality management, ensure that you keep on top of it by test and correct chemical addition. If you have any problems after that, I’m available through my website, www.macquaticstraining.com, with free and no-obligation assistance.

What’s next?

Stayed tuned for the next instalment of ‘Good water matters’, which will focus on accurate water testing.

*John McKenny has managed and leased swim centres for more than 30 years. He commenced TAFE teaching in swim centre operations and management 25 years ago and continues today. John is the author of The Complete Swimming Pool Handbook and The Leisure Pool and Spa Handbook. Obligation-free advice is given any time.

Image credit: ©Shawn Hempel/Dollar Photo Club

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