PH - Further Reading

What is pH?

The pH scale measures how acidic an object is. Objects that are not very acidic are called basic. The scale ranges from zero (the most acidic) to 14 (the most basic).

As you can see from the pH scale above, pure water has a pH value of 7. This value is considered neutral—neither acidic nor basic.

Normal, clean rain has a pH value of 5.0 and 5.5, slightly acidic, and a TA of 0. However, when rain combines with sulfur dioxide or nitrogen oxides—produced by power plants and automobiles—the rain becomes much more acidic. Typical acid rain has a pH value of 4.0. A decrease in pH values from 5.0 to 4.0 means that the acidity is 10 times greater.

What Does pH in a Pool Mean?

pH indicates how acidic or basic the water is. Simply, pH measures your pool water's hydrogen ion concentration ( [H+] ). The pH scale is logarithmic and reversed, which means it measures things in orders of magnitude, and smaller numbers are representative of larger hydrogen ion concentrations. For example, a pH of 7 means 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).

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 pool water maintenance, using the standard colorimetric test for pH is more than sufficient in terms of the precision and accuracy needed.

When testing, make sure that your Phenol Red matches your comparator:

• With Phenol Red, R-0004, you should use a Taylor 9056 Comparator.
• With Phenol Red, R-1104, you should use a Taylor 9781 (Taylor 9781) or a Tftestkits.net comparator (Tftestkits.net comparator)

An electronic pH probe measures pH values outside of that range.

Taylor pH Test is Unreliable When FC > 10

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, 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 first neutralize the sanitizer by adding a chlorine neutralizer (i.e., sodium thiosulfate). However, thiosulfate solutions have a high pH and, if heavily used, may cause a false higher sample pH.

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

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

Testing pH When FC is > 10

One way to correct the chlorine interference is by adding one drop of R-0007 to a 50 mL sample and adding that treated sample to the comparator. However, adding R-0007 to high FC water is known to cause changes in pH, rendering the test results useless.

You DO NOT want to use R-0007 drops with the Taylor phenol red. When sodium thiosulfate reacts with chlorine, several dechlorinating reactions 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.

A better way to test pH in high FC water (below 20ppm) is to mix a 1:1 solution of pool water with DISTILLED water. Then test pH. Doing that will cut the FC by half but have almost no effect on pH (due to the much higher alkalinity of pool water than distilled water).

Distilled water is typically at a pH of 6.9-7.0. It has zero carbonate alkalinity and no mineral hardness. Diluting a pool water sample with it doesn’t change anything since the alkalinity of the pool water will resist any changes to pH introduced by the distilled water. That’s why it works better than adding R-0007 because the thiosulfate in R-0007 chemically reduces the chlorine, but that reaction causes a shift in pH.

Adjusting pH

How to Raise pH

The best way to raise pH is by aeration of the water. That will increase your pH gradually without 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 as it will raise the pH and raise the TA a lot.

If TA is below 50, 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.

pH can also be lowered with CO2 injection, although CO2 injection does not lower TA at the same time as acid does.

Why is Maintaining Pool pH Range Important?

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

Also, 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 most critical factor in preventing metal corrosion.

Similarly, plaster pools must have their pH balanced (along with other water parameters like total alkalinity and calcium hardness) to maintain their pool surface's integrity and long life.

While low pH (less than 7.0) can be detrimental to pool equipment and surfaces, high pH can be equally damaging by promoting calcium and metal scale formation. Calcium scale can occur in any pool type, leading 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 and 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 filling. It is critical to test pH regularly (sometimes daily) until you get to know your pool water.

Know your Fill Water pH

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

Most tap water has a 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 source, it can be perfect for pools or 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 correct 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 of 7.7 and seem to hang there for a long 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 chemicals to get it are often the most straightforward and least costly approaches to pool care.

pH Control for Spas

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

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, 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 slight lye excess from the manufacturing process. Still, 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), the pH rise would need to be offset by lowering the pH with acid. This is why TFP requires a pool owner to adjust their pH down to 7.2 before 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 a definitive amount that adding liquid chlorine will raise pH?

There is no simplistic answer regarding pH calculations because pH depends significantly 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.

For example - at a pH of 7.5, CYA 50ppm, TA 80ppm, adding 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 obvious on a phenol red pH test. Pool Math used to call out the pH change when one added chlorine, but because of programming limitations, it was not accurate, and it would confuse people who were trying to make FC adjustments to their water.

Does Pool pH affect 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 the point of being irrelevant.

pH affects the active chlorine (hypochlorous acid) level 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.

Specifically, with no CYA in the water, going from a pH of 7.5 to 8.0, the active chlorine level drops 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 where a large amount of chlorine is added, and the pH will rise significantly. Before beginning the SLAM Process, we recommend lowering the pH before a SLAM, especially when there's a lot of FC due to high CYA levels.

Operating Swimming Pools at High pH -- How, and Why?

Operating Swimming Pools at High pH -- How, and Why? is a paper from 1998 by Ben Powell describing when a pool can be let to find its natural pH.

The preface says:

If you read this page, and go away thinking that high pH is good, read it again. It's not that simple. Let me repeat: IT'S NOT THAT SIMPLE!! The ideas here are really not for pool beginners.

This page attempts to correct the simplistic pool industry idea that high pH is always BAD, but I don't want to replace that oversimplification with another one.

High pH is NOT always BAD! But, high pH is NOT always GOOD, either!