SPN (Swimming Pool News) December 2021

Page 39


Delving Deeper – Why pH Is The Most Important Parameter In follow-up to the water testing article in the October issue, Luke Griffiths, STA’s Pool Plant Qualification Development Manager, delves deeper into the importance of pH WHAT IS PH? pH is the power of hydrogen within the water. The pH scale refers to how acidic or alkaline a substance is. The pH scale runs from 0 to 14. pH zero is water containing no hydroxyl ions, so is most acidic. At the other extreme is pH 14, where the water contains no hydrogen ions and so is as alkaline as it is possible to be. Neutral water has a pH of 7. The pH scale is logarithmic. This means that reducing the pH by one unit means a ten-fold increase in the concentration of hydrogen ions. So, increasing the pH from 7 to 8 involves a ten times reduction in the concentration of hydrogen ions. So, although the difference between pH 7 and pH 8 might not sound much, it represents a very large difference in the chemical composition of the water. The Hydrogen Ion is acidic, whereas the Hydroxyl Ion is alkaline. Therefore, whether a solution is acid or alkali depends on the ratio of these two different ions. If water has equal quantities of these Ions then we get a neutral pH of 7. The level of pH is dependent on the concentration of these Ions in a solution or our pool and spa water. This pH reading therefore determines whether a substance is neutral, acid or alkali.

WHY MEASURE PH? • Bather comfort: pH hugely affects the comfort of bathers within the pool or spa, bearing in mind the pH of eye fluid is approximately 7.5 and fluctuations away from this pH can have a radical effect on the eye and therefore bather comfort. If the pH was to fall below 6, then the acidic solution has the potential to dissolve teeth. The skin also becomes very sensitive when the pH drifts above 8 and below 7. If the pH goes above 8 then there is a potential for the lipids on the skin to be broken down, leaving the hair follicles open to infection. • Maintenance: If the water becomes acidic (below a pH of 7) then corrosion can occur on plant room equipment, pool tank grout, tiles and screed which can be costly. At low pH, water can even dissolve copper/metal

pH has a critical role in ensuring the water in facilities does not become excessively corrosive or scale forming

pipe work and metal pump parts. When the pH starts to rise above 8 then the water can transform to scale forming where lime scale is likely to accumulate, and potentially cause blockages, especially in equipment such as heat exchangers. It is interesting to note that under corrosive conditions, calcium can be leached out of grout and appear as deposition. Sometimes this is misinterpreted as deposition, whereas it is the opposite. pH therefore has a critical role in ensuring the water in our facility does not become excessively corrosive or scale forming. • Flocculation: Most coagulants work best at pH 7.3-7.4. Altering the pH will change the distribution of electrical charges. The pH can also have a strong effect on the degree of hydrolysis of the coagulant, which will affect its performance. • Effectiveness of Chlorine: Free chlorine, which is measured via the undertaking of a DPD1 test, is broken down to the two forms of chlorine that co-exist − Hypochlorous Acid & Hypochlorite Ion. These two compounds are what make up our free chlorine; the Hypochlorous Acid is the very effective disinfectant, whereas the Hypochlorite Ion is the weak disinfectant. To break down and eliminate the threat of bacteria we want as much Hypochlorous Acid as possible. The percentage uptake of these two chemical compounds is dependable on the pH of the pool water. The graph above details the effect pH has on the percentage

...although the difference between pH 7 and pH 8 might not sound much, it represents a very large difference in the chemical composition of the water” www.swimmingpoolnews.co.uk 37_SPN_December_21_STA.indd 37

production of the Hypochlorous Acid and the Hypochlorite Ion. As the graph demonstrates, at a pH of 8.0 only 20% of the chlorine is in the effective form of Hypochlorous Acid. The closer the pH is to 7.0, the higher the production of the Hypochlorous Acid. Therefore, without managing pH our chlorine can become ineffective at killing bacteria and ensuring safe bathing water is provided.

WHAT PH DO WE WANT? For pools PWTAG recommend a pH value of between 7.0 and 7.4 to be the range when using chlorine-based disinfectants and ideally 7.0 to 7.2 for a better disinfectant effect. Spas and interactive water features should maintain pH values between 7.0 and 7.6 pH in spas and interactive water features can elevate due to aeration and agitation which can cause an increase in pH as CO2 escapes. All the above factors make it imperative to ensure that pH is managed and within parameters. This ensures bather comfort, effective flocculation, and maintenance, as well as chlorine efficiency to control bacteria within the pool. STA 01922 645097 www.sta.co.uk STA’s Level 3 Award in Pool Plant Operations qualification, which is CIMSPA professionally endorsed, covers everything operators need to know to ensure safe, clear and hygienic water practices are adhered to in pools, spas and interactive water features. For more advice, contact STA on the number above, visit our website or email luke.griffiths@safetytrainingawards.co.uk

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