August 2008 Issue
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The Implications of pH
Some years ago, I was using a pool test kit to test the water in my pool before my family and relatives went swimming. My mother-in-law came up to me and asked what I was doing. Immediately after I told her that I was testing the pH and before I could also mention chlorine, she interrupted me and blurted out, “Oh yuck, I hope there isn’t any of THAT in this pool!”
Of course, everything has a pH, and we know that the pH is important in several aspects of pool operation. Maintaining the proper pH ensures that the chlorine is working at maximum efficiency. A low pH (usually 6.8 but even 7.0 in sensitive individuals) is irritating to the eyes of swimmers. The pH of water is probably the most important factor in whether water is in balance or not. Unbalanced water leads to scale formation or corrosion. Although scale and corrosion are not public health issues they can cause physical damage to the facility. pH is also, unfortunately, the most volatile and most easily affected component in water as compared to the other water balance parameters; total alkalinity, calcium hardness, TDS, and temperature. An increase or decrease in pH significantly affects the Langelier Saturation Index (LSI). Every 0.1 change in pH results in an identical 0.1 change to the LSI. But understanding what and how other variables and factors affect pH can help in predicting, dealing with, and keeping pH under control.
Because pH and total alkalinity are interrelated, the first obvious factor in controlling pH is to maintain total alkalinity between 60 ppm and 180 ppm. If the alkalinity is below 60 ppm, there is not enough buffering capacity to resist wide swings in pH in both directions when various chemicals are added. Even small additions of acid may lower the pH of the water below 7.0, while adding soda ash could raise the pH to over 8.3. When the pH is below 7.0, the water becomes very corrosive and aggressive to metal piping and to a pool plaster finish. Metal can be stripped or dissolved from pipes and equipment and dissolved into the water and create high concentrations of that metal in the water which can then stain a pool finish. Levels higher than 180 ppm of alkalinity can lead to scaling.
One will notice that after the pool water has been balanced to a pH of 7.5, and no other chemicals are being added, that the pH will most likely have risen a few days later. This is a natural phenomenon and is due to the loss of carbon dioxide (also known as CO2) from the water. CO2 in water is mainly created by the addition of acid, which converts alkalinity into CO2. The amount of CO2 in the water and its relationship to total alkalinity determines what the pH is, and the more CO2, the lower the pH. Whenever the pH is below 8.3, there is an excess of CO2 in the water. This excess CO2 tends to off gas into the atmosphere and reach equilibrium, which is known as Henry’s Law. During this process, the pH rises until it stabilizes at a pH of about 8.3, give or take, depending on the alkalinity. However, the pH should not rise above 8.4 unless other alkaline chemicals are added. Having a certain amount of CO2 in pool water is beneficial because it reduces the tendency of calcium carbonate or scale from developing.
The rate or speed of CO2 loss (and subsequently pH rise) is increased by several factors--high alkalinity, high water temperature, a high ratio of air-exposed surface to water volume--and the greatest effect is determined by the circulation and degree of aeration of the water. Water falling over a spillway or aeration in a spa increases the speed of CO2 loss and the pH rises. Operating air jets are a cause of the higher pH often observed in spas, even within minutes after balancing.
Another cause of CO2 loss can occur by the reaction of CO2 with pool plaster. While this reaction is greatest with new fresh plaster, it is lessened significantly as the plaster ages beyond 28 days. Many pool operators observe the minor difference in pH movement with a plaster pool as compared to a vinyl or fiberglass pool, which do not react with CO2.
Because the pH is usually rising in swimming pools and spas, acid is routinely added to maintain and keep the pH from going too high. As we all know, acid also lowers the alkalinity. So pool operators learn that after a certain period of time, the alkalinity level becomes low and needs to be raised again and again. Because of this continual cycle of adding acid to lower pH (and alkalinity) and then adding sodium bicarbonate to raise the alkalinity back up, some pools or spas have been installed with CO2 injection systems. The advantage of this process is that it eliminates the need for acid because adding CO2 to the water lowers the pH without affecting the alkalinity. CO2 does not raise alkalinity nor lower it. A continual dosage of CO2 can maintain a consistent pH without the need to restore alkalinity levels as when acid is used. It should be pointed out that there is a misconception that injecting CO2 into water raises the alkalinity level. This misconception is probably due to the fact that liquid chlorine bleach is most often used in conjunction with CO2 injection systems. Although it is a small amount thebleach adds alkalinity to the water. Every gallon of 12.5% bleach added to 10,000 gallons of water also adds about 1 ppm of alkalinity. Thus if the alkalinity climbs too high a stronger acid, such as muriatic acid, will occasionally be required to bring the alkalinity back in line.
The installation of a cover (non permeable types) inhibits CO2 loss from the pool water, and consequently, the pH in pools and spas that are covered will remain lower for longer periods of time. When these pools or spas with covers (or vinyl and fiberglass pools) are maintained with an acidic sanitizer, more sodium bicarbonate and soda ash may be needed to keep the pH up.
Finally, most sanitizers affect the pH and alkalinity. Acidic sanitizers such as chlorine gas, trichlor, dichlor (very small effect) and bromine will lower the pH and alkalinity. Alkaline sanitizers such as calcium hypochlorite, sodium hypochlorite, and lithium hypochlorite all raise the pH and alkalinity somewhat.
Kim Skinner
Kim Skinner has been working in the swimming pool business for 40 years. He began as a swimming pool plasterer and after five years got into the pool chemical service business. He is owner of Pool Chlor Inc. with offices throughout the Southwest. Mr. Skinner is also a partner of onBalance, a swimming pool research and consulting company. He has performed both laboratory and field research on water chemistry and its relationship with pool plaster. Mr. Skinner is author and co-author of several technical reports published in the Journal of the Swimming Pool & Spa Industry and has been a speaker at various swimming pool trade shows.
© 2008 by The Association of Pool & Spa Professionals. All rights reserved. No part of this publication may be reproduced or transmitted without written permission.
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