Total alkalinity refers to the ability of the pool water to resist a change in pH. The key purpose total alkalinity serves is to help control the pH in the pool. It does this by acting as a buffer so that when materials are added to a pool that would otherwise cause the pH to go up or down, these changes are managed and do not result in severe changes to pool water balance.
When a substance is added to pool water that could affect the pH, total alkalinity will react to neutralize it and help keep the pH in the desired range. Total alkalinity does not determine what the pH will be, but rather acts to help keep the pH in the range desired.
Total alkalinity is measured in parts per million (ppm) using a total alkalinity test kit. It is best kept in the range of 80-120 ppm. When the total alkalinity value is less than 80 ppm, the water can become aggressive and the pH can swing easily upward and downward and back again. If the value is higher than 120 ppm the water can become cloudy and scale forming and the pH will tend to drift upward.
In adjusting total alkalinity downward, the same acids used to lower pH are employed. When reducing total alkalinity, it is best to add small amounts of acid, either liquid or dry, over a period of several days as opposed to making large adjustments rapidly. Adding too much acid at once may result in lowering the pH so severely that corrosion of pool surfaces and equipment may result. When raising total alkalinity, alkalinity increaser is the chemical of choice. Adding the required amount in recommended increments over a few hours with the pool circulating is suggested. Please be aware that some clouding could occur.
On occasion it is possible, especially in freshly filled pools, to find that both total alkalinity and pH need to be adjusted. Typically, if one factor is high or low, the other will be as well. It is not unusual, however, to have a condition where one factor is high and the other is very low. In such a situation, adjusting the wrong factor first may cause a significant problem with the other, or worse - cause a problem such as corrosion of equipment or precipitation of calcium. If this occurs with a freshly filled pool, it may be worthwhile to wait about 24 hours before making any adjustments. This wait will generally result in some natural balancing of the water without added chemicals. This process is commonly referred to as allowing the water to come into equilibrium. If additional adjustment is still needed, it will require far less time or chemicals.
In cases where the pH is low and the total alkalinity is high, raise the pH first into the normal range of 7.2 - 7.8 and then lower the total alkalinity. When the total alkalinity is low and the pH high, raise the total alkalinity first and then reduce the pH.
In all cases, never add acid to the pool water if the pH is less than 7.2, even if the total alkalinity is high. Instead wait for the pH to rise first before proceeding. If the pH does not come up by itself after a day or two you will need to add some pH Up before proceeding.