PH - Further Reading

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.

Taylor pH Test is Unreliable When FC > 10

The pH can be inaccurately high when FC is above 10. Above 10ppm FC, you start see the effects of chlorphenol red formation but between 10ppm and 20ppm the reaction is fairly slow. You can still do the pH test if you add the drops and do the reading waiting 10 to 15 secs. Above 25ppm FC, the chlorphenol red formation is too fast.[1]

However, there is an additional work-around that will still give accurate results on pH - take a pool water sample that has high FC (below 20ppm) and dilute it 1:1 with distilled water. Distilled water has very low to near zero alkalinity and a pH of about 7.0 or so. Because of that low alkalinity, there will be almost no shift in pH upon dilution but a reduction in half of the FC level. You then read your pH normally.

The Taylor R-0004 pH Indicator solution has a proprietary combination of chlorine neutralizers already in it that try and keep the pH the same when chlorine gets neutralized, but there's only enough to neutralize roughly 10 ppm FC or so of chlorine. They didn't want to add more since they didn't want to adversely affect results by this neutralizer solution (i.e. too much could affect the pH). Handling 0-10 ppm FC seemed reasonable.[2]

The quote from Taylor's Pool & Spa Water Chemistry booklet on the issue of a high FC causing a false high pH reading is as follows:

FALSE READINGS: high levels of chlorine (usually > 10 ppm) will quickly and completely convert phenol red into another pH indicator (chlorphenol red). This new indicator is a dark purple when the water's pH is above 6.6. Unfortunately, some pool operators mistake the purple color for dark red and think the pool water is very alkaline and wrongly add acid to the pool.

When a sanitizer level is not extreme, only some of the phenol red may convert to chlorphenol red. However, purple + orange (for example, pH 7.4) = red. This error is more subtle as no purple color is observed and the operator does not suspect that a false high pH reading has been produced. Some operators neutralize the sanitizer first by adding a drop of chlorine neutralizer (i.e. sodium thiosulfate). However, thiosulfate solutions have a high pH and, if heavily used, may cause a false higher sample pH.

You can correct the chlorine interference by adding 1 drop of R-0007 to a 50 mL sample; then add that treated sample to the comparator.[3]

In general you DO NOT want to use R-0007 drops with the Taylor phenol red. When sodium thiosulfate reacts with chlorine, there are several dechlorinating reactions that can occur and none of them are pH neutral. That means if you add the R-0007, you will shift the pH of the resulting solutions.

Adjusting pH

How to Raise pH

The best way to raise pH is by aeration of the water. That will raise your pH gradually while not affecting your TA.

If TA is 50 or above, bring your pH up to 7.4 or so using 20 mule team Borax.....soda ash is not a good choice.

If TA is below 50, then bring the TA up to 50-60 using baking soda and THEN raise your pH to around 7.4 using 20 Mule Team Borax.

How to Lower pH

The best way to lower pH is with Muriatic Acid.

Dry acid can be used to lower pH. However dry acid contains sulfates which will accumulate in the water. Dry acid should not be used in plaster pools or pools with a SWG.

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.

Know your Fill Water pH

Every pool owner should know what their fill water pH and TA and CH is. Just about every pool has some level of evaporation and needs fill water occasionally. Knowing the chemistry in your fill water lets you anticipate the way it will change your pool chemistry.

Most tap water has pH between 6.5 and 8.5, but some municipal water suppliers may raise the pH of water as high as 9 to prevent plumbing corrosion.

If your fill water comes from a well or other non-municipal water scource it can be very good for pools or very bad.

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.

pH Control for Spas

TFP recommends dropping the TA to 50ppm and then adding 50ppm borates to the water. The combination of lower TA and an additional high pH buffer makes pH control a lot easier.[4]

Bioguard Optimizer is just borax. It will strongly raise pH and should be added in small, quarter batches with acid additions to counter the pH rise.

BioGuard Optimizer PLUS is boric acid which does not lower pH much.

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.[5]

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.

Is there an definitive amount that adding liquid chlorine will raise pH?

There is no simplistic answer when it comes to pH calculations because pH depends a lot on what chemical species are in that water that can buffer pH. Carbonates, cyanurates and borates all exist in solution and will modify pH calculations beyond simply accounting for a specific amount of hydroxide ion (OH-) added or taken away.[6]

For example - at a pH of 7.5, CYA 50ppm, TA 80ppm, the addition of one gallon of bleach to 10,000 gallons of pool water will increase pH by +0.13. If you decrease the TA to 60ppm, the pH change is +0.17 and if you change the TA to 120ppm, the pH increase drops to +0.09. None of those changes would be particularly visible on a phenol red pH test. Pool Math use to call out the pH change when one added chlorine but because of programming limitations, it really was not accurate and it would confuse people who were trying to make FC adjustments to their water.

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.[7]

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.[8]

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.