PH

What Does pH in a Pool Mean?

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).

What is the Correct pH Range for Pools?

Pool water is typically maintained in a pH range of 7.2 to 7.8.

How Do You Test a Pool's pH?

While most test kits can read values as low as 6.8 and as high as 8.2. 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.

pH values outside of that range is typically measured using an electronic pH probe.

Why is Maintaining Pool pH Range Important?

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.

Why Doesn't Pool pH Stay Stable?

The pH of most pools 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.

What is the Optimal Pool ph?

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 at which your pool stays the most stable. 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 (e.g., adding acid to reduce pH, then baking soda 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.

What is the Effect of Adding Liquid Chlorine on pH?

It is a myth that liquid chlorine will raise pH. The myth is based on a misunderstanding of chlorine chemistry.

Adding liquid chlorine to water can cause a temporary increase in pH which is usually offset by the chlorine reacting with organics and biological matter which are acidic (creates a proton) reactions. Thus, on balance, the net chlorine reactions are pH neutral.

Most retail and commercial liquid chlorine products contain a small excess of lye from the manufacturing process but this amount of OH- is minimal and does not change the pH with normal levels of liquid chlorine use.

If one were to add significant amounts of liquid chlorine (for example, raising the pool water chlorine concentration to SLAM FC levels), then the pH rise would need to be offset by an initial lower of the pH with acid. This is why TFP requires a pool owner to adjust their pH down to 7.2 prior to starting the SLAM Process.

Adding liquid chlorine DOES increase the pH at first, however as it's used up the acid that's created lowers it back down to where it was before......thus equaling each other out making it pH neutral.

Does Pool pH have an Effect on the Chlorine's Ability to Sanitize Water?

In a pool with zero CYA, chlorine is more effective at lower pH, but this relationship is much less with CYA in the water to point of being irrelevant.

pH does affect the active chlorine (hypochlorous acid) level that is responsible for most of the disinfection of the water. However, when Cyanuric Acid (CYA) is in the water, it buffers this active chlorine so that pH has far less of an effect on it.

To be specific, with no CYA in the water, going from a pH of 7.5 to 8.0 has the active chlorine level drop by 50%. With CYA in the water, it drops by only 15%. Going the other direction, going from a pH of 7.5 to 7.0 the active chlorine level with no CYA increases by a little more than 50% but with CYA in the water it increases by 35%.

So we generally ignore the pH effect on chlorine except for a SLAM because the larger amounts of chlorine have the pH rise significantly so we recommend lowering the pH first before a SLAM, especially when it's a lot of FC due to high CYA levels.