A complex relationship: perfect water balance is the ultimate goal of any service technician. Understanding the close affiliation between pH and total alkalinity will go along way toward helping you achieve it - service
Many variables come into play when a service technician attempts to balance water in a swimming pool. However, the relation ship between pH and total alkalinity (TA) is the most instrumental.
"If the total alkalinity is in the right range, pH won't be so volatile," says Neil Lowry, Ph.D., owner of Lowry and Associates, a pool consulting firm in Toronto. "If your pH is bouncing, chances are the TA is too low. [TA] is the key to controlling the pH."
If a service tech doesn't understand this relationship, poor water balance can result.
"TA is the governor of pH," says Lowry, who also is a member of the National Spa & Pool Institute's Chemical Treatment and Processing Committee. "Never try to change the pH unless the TA is in the fight range. The first correction is always TA. The last thing you change is pH. And you may never have to change it if the TA is OK."
Here's a look at exactly what pH and TA are, how they relate and interact and what service techs can do to keep it all under control.
What is pH?
pH is defined as a measure of water's acidity or alkalinity. This is done by determining the power of hydrogen ions (H+) in the water. These ions are measured on a logarithmic scale from 0-14, with 0 being the most acidic and 14 the most basic or alkaline.
In simplified terms, "pH is actually counting how many hydrogen ions are in the water," says Que Hales, editor of Journal of the Swimming Pool and Spa Industry in Tucson, Ariz.
The National Spa & Pool Institute recommends a pH range between 7.2 and 7.8, slightly on the base side of neutral. This is a comfortable range for humans as well as the optimal pH range for chlorine to do its job.
Keep in mind that extraneous factors from both people and Mother Nature can alter pH levels. Things such as pool parties with high bather loads or heavy rainstorms, especially when the rain tends to be on the acidic side, all have an impact. In the wake of such events, techs need to check water chemistry variables as soon as possible.
If the pH is allowed to dip below the 7.2 minimum standard, the water becomes more acidic or, as it is sometimes known, aggressive. This can lead to equipment corrosion, pool surface damage and inefficient sanitizer use.
If the pH rises above the 8.3 level, the pool may experience scale and high turbidity. In either case, skin and eye irritation are sometimes reported.
It is important for techs to monitor pH levels because the number drastically impacts the Saturation Index equation--a formula used to diagnose water's chemical balance.
"If pH does change, for whatever reason, the Saturation Index changes," Lowry says.
That's because although both TA and pH figure into the Saturation index, pH has a larger impact on it. Lowry explains that the formula calls for use of the actual pH figure, while the TA is represented by a factor. For example, a TA reading of 150 ppm uses a factor of 2.2 in the formula. A reading of 200 ppm is represented by a factor of 2.3. In fact, TA ranges from 100 to 300 ppm are represented by factors ranging only from 2.0 to 2.5. Consequently, it takes a major shift in TA ppm to affect the results of the Saturation Index formula.
On the other hand, the Saturation Index uses the actual pH reading in the formula--not a factor--so the change in the pH will have a profound effect on the outcome.
"Whatever the pH changes, the index will change in the exact same amount. It's a 1:1 factor," Lowry says. "If you want to change the Saturation Index, lower or raise the pH."
When pH levels stray, service techs often employ muriatic acid to lower it and sodium bicarbonate or soda ash to raise it.
However, it's not always that simple. If all the variables are not in their proper ranges as well, particularly total alkalinity, it can be difficult keeping the pH within ideal limits.
The TA/pH relationship
Total alkalinity has an important relationship with pH in swimming pool water. But first, what exactly is TA?
Total alkalinity is essentially a natural buffering system that helps to stabilize pH readings. "If pH is counting how many hydrogen ions in the water, TA is counting how many things in the water are capable of absorbing hydrogen," says Hales. "Total alkalinity is made up of things that can take on hydrogen."
At higher pH ranges, total alkalinity is the combined presence of hydroxide ions (OH-), carbonate ions (CO3) and bicarbonate ions (HCO3).
However, Lowry says that in swimming pools and hot tubs, where the pH range shouldn't stray too far from the recommended mid-7 range, TA remains strictly a bicarbonate.
"[TA] is nearly 100 percent bicarbonate [in pools] unless the pH is in the 9.0 range or better," he says. "You can define TA as the sum of alkaline salts and, in essence, the main one is bicarbonate."
When TA levels are too high or too low, it affects overall water balance. For example, with water that has very low TA levels, the pH is understabilized and will change dramatically whenever even small additions of chemical are made. Water becomes more corrosive to plaster and metal fittings and heater elements. In conditions of excessive bicarbonate alkalinity, the pH is overstabilized and tends to remain around 8.3. The pH level is likely to bounce back to the 8.3 range even after acid is added. With TA and pH in such high ranges, dull, turbid water also can result.
According to its Basic Pool & Spa Technology textbook, NSPI guidelines recommend a TA level of 80-100 ppm when using calcium, lithium or sodium hypochlorite disinfectants and 100-120 ppm for vessels using sodium dichlor, trichlor, chlorine gas or bromine disinfectants. The latter are high in acidity, and the higher TA ranges will help prevent the pH from plummeting.
"At a given alkalinity level, the pH wants to lean toward a particular range," says Hales. "The higher the TA, the greater the capability of it taking [the pH] into a particular range and keeping it there. The lower the TA, the less ability it has to do either of those things. So, when TA increases and gets above recommended ranges, it can be difficult to even adjust pH. If TA goes below acceptable ranges, the pH then becomes capable of `wild mood swings' because there is insufficient TA to lock it in."
According to Hales, when bicarbonate is used to raise pH levels, it will always try to push pH toward 8.3. "The more you add, the more it zeros in on 8.3," he says. "But if you use soda ash, it will try to push the pH toward 10."
The practical side of this for service techs is knowing which product to use and when. Soda ash raises total alkalinity nearly twice as much as bicarbonate. The downside is that it's good to use soda ash only if you want to raise the pH as well, says Hales. "You get twice the bang for your buck with soda ash. If it's OK for the pH to shoot up along with the TA, then use it," he says.
However if the pH is already in the range you want, use bicarbonate (baking soda) because, as Hales says, "it won't do nearly as much to the pH."
Steadying the reins
Controlling pH has been made somewhat easier by the number of buffeting products available to service technicians and pool owners today. The most common buffers are what Lowry calls "the salt of a weak acid," and include sodium bicarbonate, cyanuric acid and borate.
These buffering systems work to neutralize any acid or base added to the water without altering pH levels.
If an acidic sanitizer gets added to the pool water, the buffer system's salt or "anion" (the alkaline part of the system) will neutralize the acid before it even registers on the pH scale. On the other hand, if a base product is introduced, the weak acid portion will absorb any additional alkalinity before a change in the pH can happen.
One of the best examples of a buffering system is human blood. Blood functions best at certain pH levels. Any deviation in this and the body's proteins begin to lose their ability to function. Whenever an imbalance in the blood is detected, more of the missing element is automatically produced to prevent life-threatening conditions.
In pool water buffering systems, rather than producing more of what is missing, the buffer absorbs the excess alkalinity or acid before it can affect pH levels.
A group effort
Even with these products, a constant and consistent monitoring of the pool water elements is key to keeping the water balanced and the pH in line.
Between the water's total alkalinity and any buffering system that's added, service techs stand a better chance in the never-ending battle for balanced water. A buffering system will resist pH bounce as long as there is the salt of a weak acid in the water.
Together, they work to keep pH levels from following a natural inclination to move with every little chemical addition or change in conditions.