PH

pH indicates how acidic or basic the water is. In a simple sense, pH is a measure of the hydrogen ion concentration ( [H+] ) in your pool water. The pH scale is logarithmic and reversed, which means it measures things in orders of magnitude and smaller numbers are a representative of larger hydrogen ion concentrations. For example, a pH of 7 means there’s ten times as many hydrogen ions in solution as compared to a pH of 8. The pH scale runs from 0 (highly acidic) to 14 (extremely alkaline).

Pool water is typically maintained in a pH range of 7.2 to 7.8 while most test kits can read values as low as 6.8 and as high as 8.2. pH values outside of those extremes are typically measured using an electronic pH probe. However, for the purposes of pool water maintenance, using the standard colorimetric test for pH is more than sufficient in terms of the precision and accuracy needed.

Measuring and maintaining an appropriate pH level is important for several reasons - bather comfort, pool surface quality and equipment longevity. The acidity or basicity of the pool water directly impacts the comfort level of the swimmer. If the pool water's pH is extremely outside the normal ranges (7.2-7.8), bathers can experience discomfort ranging from eye irritation to itchy skin.

As well, some of your pool equipment contains metallic components and the rates of corrosion of those metallic components are directly related to pH. pH is the single most important factor when it comes to metal corrosion and stability in pool water.

Similarly, plaster pools need to have their pH balanced properly (along with other water parameters like total alkalinity and calcium hardness) in order to maintain the integrity and long life of their pool surface.

While low pH (less than 7.0) can be detrimental to pool equipment and surfaces, high pH can be equally as damaging by promoting the formation of calcium and metal scale. Calcium scale can occur in any pool type and can lead to rough pool surfaces and damage to pool equipment like heaters and salt-water chlorine generators.

The pH of most pool’s will constantly drift over time by either increasing (very common) or decreasing (less likely). Rarely does pH stay stable for an entire swim season. Most of the chemicals you add to your pool will affect pH in some way as well as the fill water you add to maintain a proper water level.

The age of the pool surface can affect pH - a newly plastered pool will often have rising pH and large acid demand for as long as 18 months after plastering and fill. It is critical to test pH on a very regular basis (sometimes daily) until you get to know your pool water.

There is no “optimal” pH value - despite the often cited, but bogus claim that 7.4 is the right pH because of human tears. The optimal pH value for any pool is the pH that your pool stays the most stable at. For some pools, the water will settle at a pH value of 7.7 and seem to hang there for long periods of time. For other pools, a pH of 7.5 seems to “stick”.

Trying to force your pool water to a specific pH value often leads to cyclical dosing of chemicals (eg, adding acid to reduce pH, then baking to offset the lost alkalinity which in turn leads to pH rise and then further acid additions…the “pool chemical yo-yo”) and frustration on the part of the pool owner. Simply determining what pH your pool water prefers to settle at and using the least amount of chemicals to get it there is often the easiest and least costly approach to pool care.