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pH of Pool Water Difficult to Control

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Earlier this week we received a message from 'Vince' asking, "Why does our pH go up and down so much all the time?  Then when we add pH Down it goes through the floor, when we add pH Up it goes through the roof and when we add chlorine it goes through the floor all over again!  It never stays neutral.  Help, please?  Thx"

Hello, Vince, and thank you for the question.  A couple main factors impact pH and/or the stability of pH levels in water:

  • Total Alkalinity - Think of this a mood stabilizer for pH.  If your total alkalinity level is too low your water's pH level can run wild and will get affected by the addition of pretty much any thing that has a higher or lower pH than the water.  By the same token, having too high a total alkalinity level will, for lack of better terminology, weigh the pH level down and lake it very difficult to change.
  • pH of Chlorine -- Judging by the fact your pool's pH level goes down when you add chlorine we suspect that you use a stabilized chlorine tablet as your main sanitizer/disinfectant.  Most Trichlor chlorine tablets have a pH down around 3 so when added to water with low alkalinity it makes complete sense that the pH of the water would shoot straight down.

So what should you do next?  Simple:  Test your water's total alkalinity level with a product such as the WaterWorks pH & Total Alkalinity Test Strip which has a very easy test procedure and very short test time.  Other features of this convenient water testing tool include:

  • pH: 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0
  • Total Alkalinity: 0, 80, 120, 180, 240, 360 ppm (mg/L)
  • Manufacturer Part Number: 480005
  • No MSDS required.
  • Obtain accurate, reproducible pH test results in under one minute.
  • This product proudly manufactured in the United States by Industrial Test Systems, Inc.

OK, so now you know your water has a low alkalinity reading.  Now what?  Simple:  Fix it by adding an alkalinity increasing product (usually a powder) commonly sold in pool supply stores until the level gets up around 80 to 120 parts per million (ppm).

After the chemical has circulated its way into the pool water and the alkalinity reading registers as normal (between 80 and 120 parts per million), take a pH reading and then use the correct pH adjusting product(s) to bring the pH into the range you desire (usually between 7.2 and 7.8).

Won't adding chlorine still affect the pH, though?

Yes, it will, but a proper total alkalinity level helps the water resist the urge to change its pH to the level of the chlorine.  Shocking a pool will definitely still cause the water's pH level to change but as the chlorine level drops over time, the water's pH should drift back to its normal range if the water has its total alkalinity concentration in the proper range.

WaterWorks pH & Total Alkalinity Test Strips
WaterWorks pH & Total Alkalinity Test Strips
WaterWorks Extended Range pH Test Strips
WaterWorks Extended Range pH Test Strips