Pool pH Keeps Dropping

Balanced chemicals in a swimming pool are an extremely important element of having a clean and healthy pool and avoiding recurring and costly problems with the swimming pool, the pool equipment, and swimmer comfort. Keeping the pH in the proper range is a must and not a difficult task with a little practice. A common problem that occurs when trying to get the pH in the proper range for a swimming pool of 7.2 to 7.8 is a recurring drop in pH. The pool owner will make adjustments to raise the pH, which will bring the pH in line in the short term. But over a period of a week or two the pH will drop back down to an unhealthy level. There are a couple of simple reasons the pool pH keeps dropping.

Total Alkalinity Is Too Low - total alkalinity is the measure of the ability of the swimming pool water to resist changes in pH. In other words, if the total alkalinity of the pool water is in the proper range of 80 to 120 ppm, the pH of the pool water will stabilize and be much easier to manage. But if the total alkalinity is below the lowest recommended level, the pH will tend to follow and drop to undesirable levels. Therefore this is the first step in finding out why the pH keeps dropping in the pool is to test the total alkalinity of the pool water. If the pH is repeatedly dropping, the alkalinity is probably too low.

Alkalinity Adjustment for Stabilizer Level - if the standard total alkalinity test shows the proper range off 80 – 120 ppm, and the pH continues to drop, the level of stabilizer (cyanuric acid) in the pool might be on the high side. When a test kit is used to test the alkalinity of pool water, the results include both the carbonate alkalinity and cyanurate alkalinity. Since water balance calculations only use the carbonate alkalinity portion, an adjustment to the test results needs to be made. Test the cyanuric acid level or have it tested at a local pool store. As a general rule of thumb for pool water within recommended pH and cyanuric acid levels, subtract one third of the cyanuric acid level from the alkalinity test results to get the true alkalinity level. For example, if the test results show an alkalinity of 85 and the cyanuric acid level test shows 60 ppm, one third of 60, or 20 would be subtracted from 85 to get a true alkalinity of 65. An alkalinity of 65 would be below the recommended low value of 80. Since pH tends to follow alkalinity and the alkalinity is too low, this could explain the drop in pH.

Sanitizer Has Low pH - some sanitizers have a very low pH value and can drive the pH down when used. For instance, trichlor tabs or trichlor granular chlorine has a pH of 2.8 – 3.5. While there are advantages of using trichlor chlorine, the disadvantages include slowly driving the pH and alkalinity down because of their low pH.

When the pH in a swimming pool is too low, many pool problems can occur including swimmer discomfort, etching of the pool plaster, metals in the pool dissolving and causing stains on the plaster, increased chlorine usage, and many other pool problems. Getting a handle on the alkalinity is the first step to take in stabilizing the pH and having well balanced and stable swimming pool water.

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