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It is important to maintain the correct pH for proper sanitation, water clarity, and bather comfort. Dynamic factors, such as aeration and hot water temperatures, can make pH control more difficult in spas than in swimming pools.

Why Does pH Tend to Rise in a Spa?

This question requires knowledge of "chemical equilibrium." Chemical equilibrium describes the material balance of a given chemical reaction. In the case of total alkalinity there is a balance between mildly basic carbonates and mildly acidic materials formed in the water when a bicarbonate buffer reaction occurs. The following reactions describe how chemical "buffers" are formed and how pH is controlled through proper alkalinity.


An initial addition of SpaGuard Balance Pak 100 raises alkalinity and pH. The addition of acid, in the form of muriatic or granular acid, adds hydrogen ions to the water. Remember, pH is the concentration of hydrogen ions in the water, so this addition will lower the pH. However, instead of just "bouncing" to a low pH, the acid reacts with some of the bicarbonate to form carbon dioxide. Carbon dioxide is a gas, but, dissolved in water, it forms "carbonic acid," a weak acid which acts to lower the pH. Now, if the correct amounts of acid have been added, the pH is balanced between 7.4-7.6. Whenever acidic materials are added to the water a portion of it reacts with the bicarbonate to form carbon dioxide and, thus, carbonic acid. This reaction helps control and stabilize pH.


Why is a Spa Different to a Swimming Pool?

Two things occur which tend to increase pH. First, when water is hot, any gases in that water become less soluble and can "gas off." In combination with aeration, the air moving through the water will carry off much more carbon dioxide into the atmosphere. Our "equilibrium" is no longer balanced by carbonic acid, because the carbon dioxide which forms it is no longer in the water. Any acid which you might add cannot react fast enough to redistribute bicarbonate back to carbonic acid. Thus, high pH bicarbonates are present without any balancing factors to help control the pH.


If you add acid to a hot and aerating spa, the pH may decrease temporarily, but it will quickly increase again until you destroy enough alkalinity, BELOW a proper amount for pH control, and then pH drops quickly and uncontrollably. Second, calcium is LESS soluble in hot water, thus calcium, along with carbonates, can more readily form scale or calcium carbonate in hot water. The carbonate comes from bicarbonate, part of the high pH side of our balance. Carbonates have an even higher pH than bicarbonates and thus the pH can remain high.


Bather load also helps buffer the pH in this situation. People add perspiration and other slightly acidic materials to the water which help prevent pH increase during use, but this variable factor is often slight and pH generally increases over time.


Maintaining Correct pH in Spas

You do the same thing as you would in a swimming pool - you add acidic chemicals - with some general guidelines.

  • Always add chemicals with CIRCULATION ON, but AIR OFF.
  • Use aeration ONLY when the spa is being used.
  • If possible, lower the temperature of the spa when it is not being used to combat pH increase.
  • After the spa is used, the pH is usually high. Turn off the aeration, allow the spa to "rebalance" for an hour, then check pH and sanitiser and adjust if necessary.


Points to Remember

  • pH has a tendency to remain high (up to 8.2) in hot, aerated and properly balanced water due to complex chemical reactions. The pH balance is easily upset in these conditions.
  • pH can be controlled if certain guidelines are followed for chemical additions.
  • pH adjustments may need to be done after every use.
  • As in any water situation, awareness of proper maintenance, sanitation, and draining schedules will help keep the water looking clean and inviting.

The above information is supplied by Bio-Lab and represents its best interpretation of available technical information at the time of preparation. The sole purpose is to supply factual information to Bio-Lab customers. It is not to be taken out of context nor used as support for any other claim not made herein.