Alkalinity testing isn’t just another way to add cost and confusion to a service technician’s routine. It’s actually meant to save time and effort, ensuring that the pH stays put and the water remains balanced. But its true definition has become muddled in misinformation.
“The thing that’s probably most misunderstood [about alkalinity] in the industry is the belief that cyanurate isn’t alkalinity,” says Que Hales, general manager for Pool Chlor in Tucson, Ariz. “Total alkalinity means total alkalinity, regardless of what form it’s in.”
Every buffer has a base and an acid conjugate with varying levels, dependent on the pH. Just because the percentage of some buffers are in an acid form at traditional pool pH levels doesn’t mean they are ineffective in preventing pH bounce.
Most techs understand bicarbonate alkalinity, which is often the easiest to maintain. In fact, it is the predominant, naturally occurring form of alkalinity in tap water. However, cyanurate (from cyanuric acid) is also a common buffer in the water, whether techs are cognizant of its effects or not. Borate also can be used as a buffer.
Utilizing each of these types of alkalinity can help techs steer the pH of a pool with increased precision. The following article discusses bicarbonate, cyanurate and borate alkalinity and the application for each. While total alkalinity may be glossed over by hasty techs, it is essential to a well-maintained pool.
Although there are a handful of buffers available to techs, focusing on bicarbonate alkalinity is by far the most popular option. It is generally the cheapest, most convenient path to a steady pH. In addition to the naturally occurring bicarbonate alkalinity from water, techs typically add this kind of alkalinity in the form of sodium bicarb to increase TA, or if they are aiming to raise the pH as well, soda ash.
“If the goal is to just change the alkalinity, we use bicarb,” notes Robert Lowry, owner of Lowry Consulting in Jasper, Ga., and author of several IPSSA training manuals. “Bicarbonate alkalinity is easier to control and it’s a lot cheaper.”
A major component of the convenience of using a bicarbonate buffering system is its fast results. A substance like sodium bicarbonate will dissolve quickly in the water and provide an immediate buffer. It is also less likely to cloud calcium-rich water than the less expensive, but stronger soda ash.
Bicarbonate is an excellent buffer for mid-range pH levels, making it ideal for pools. Water saturated with bicarbonate has a pH of 8.3, so when bicarb is placed in a pool with a lower pH, a small percentage of the bicarbonate is changed to carbonic acid. If any acid is introduced to the pool, it will consume alkalinity, changing even more of the bicarbonate to carbonic acid. But the pH should remain stable for any moderate acid dose.
Ironically, the peak buffering point of bicarbonate is at a pH of 6.2, well below the industry-recommended range of 7.2-7.8.
“In a practical sense, if you have water with pH in the 7 range and you start pouring acid in there, [bicarbonate alkalinity] will immediately resist the pH going down to a certain extent, but its peak resistance — the point at which it will push back the hardest — is 6.2,” Hales explains.
This doesn’t mean bicarbonate alkalinity won’t work effectively under normal pool conditions. However, if a tech is troubleshooting a pool or spa with a pH that tends to ride too low, he may want to consider alternate buffering options.
Moreover, the trouble with the acid conjugate of bicarbonate — carbonic acid — is that it easily comes out of the water as carbon dioxide. This off-gassing, which typically is associated with spas because of heat and aeration, could eventually eliminate all the carbonic acid.
“In a spa, when we aerate it and turn it over a lot and heat it, we’re doing all the things [contrary to] holding CO2 in water,” Hales says. “We’re forcing it out, and the result is that the pH goes up.”
Provided there are no other buffers in the water, the pH will drift up to 8.3 when all the carbonic acid and carbon dioxide is gone.
One of the common mistakes a pool tech makes is to eliminate cyanuric acid (stabilizer) from a total alkalinity (i.e., buffer strength) reading. This practice completely disregards the transformation of cyanuric acid into cyanurate as the pH rises. Cyanurate and cyanuric acid work together as a buffer, and may be a welcome buffering enhancement for certain applications.
Furthermore, cyanuric acid has a peak pH resistance of 6.8, enabling techs to create a higher buffering zone for the pool or spa if necessary.
“If acid needs to be added to a pool to reduce alkalinity, the pH also takes a nosedive,” Hales notes. “If a pool with 160 ppm of bicarbonate alkalinity at a pH of 8.2 gets enough acid added to lower the alkalinity to 120, the pH will typically drop to 6.8 before it begins to rebound as CO2 offgasses.”
However, if that same 160 ppm of total alkalinity had been accompanied by a 100 ppm cyanuric acid reading, adding the exact same amount of acid (and thus still reducing the total alkalinity to 120) would have caused the pH to only drop to 7.3, thus protecting the plaster better, he adds. (See accompanying buffer chart.)
If enough acid is added to push the pH past the peak cyanurate resistance of 6.8, the pool’s bicarbonate alkalinity will still provide additional buffering through its greatest strength zone at 6.2. Cyanurates and bicarbonates can thusly work together as a dual buffering system.
Of course, in the above example, although the total alkalinity was 140 ppm in either scenario, in the first case the bicarbonate portion of the total was also 140 ppm, where in the second scenario the bicarbonate portion was only around 110 ppm (with the other 30 ppm being cyanurate).
Also, unlike carbonic acid, cyanuric acid will not offgas from the water. Even as spa jets aerate the water and carbonate dioxide is released, the cyanurate alkalinity will stick around.
However, this is both a blessing and a curse.
“[Besides dilution], the only thing you can do with cyanuric acid is watch it go up,” Lowry explains.
While bicarbonate is much easier to adjust, cyanuric acid can only be removed by partially or completely draining and refilling the pool. This is an especially common situation if the pool is using dichlor or trichlor, which continually add to the cyanuric acid levels.
Although there is an obvious advantage in buffering when the cyanurate is present, service techs must be mindful that when calculating Saturation Index values, only bicarbonate alkalinity is included.
Furthermore, the tests to determine how much of the pool’s total alkalinity is cyanuric can be very subjective.
“The problem is you have varying amounts of it in the water, and the easy and economical tests for it are not real accurate,” Lowry notes.
Poolside readings are often done with a visual turbidimetric test, which can have significant variances depending on the user. Now, handheld photometers are much more affordable, making the tests more accurate and uniform, Lowry adds.
Finally, techs can’t become over-reliant on cyanuric acid as a buffer. Even as it stabilizes the chlorine, high cyanuric levels will reduce the speed at which chlorine can kill germs. Still, used in moderation, it can be an excellent compliment to bicarbonate alkalinity.
Another alternative to strictly bicarbonate alkalinity is using borax, which can serve as both an algastat and a buffer. Borax has an even higher peak resistance level than cyanurates and bicarbonates.
“There’s all kinds of people using tetraborate as a buffer and swear by it,” says Ellen Meyer, technology manager at Arch Chemicals’ Water Products in Smyrna, Ga.
At normal pool pH levels, the vast majority of borax (or tetraborate) will transform into boric acid. However, this doesn’t mean it isn’t capable of effective buffering if coupled with other buffers like bicarbonate and cyanurate. In fact, borax may be ideal for trichlor and gas chlorine pools, whose creation of hydrochloric acid continually drives down the pH and alkalinity.
“[A tech] with a gas chlorinator… might put borax in the water to ensure the pH can’t dip even down as low as it would with only the cyanurate and bicarbonate in there,” Hales says.
In fact, borate can be used in congress with cyanurate and bicarbonate to create three different layers of buffering.
However, techs should be careful not to engineer a buffer that encourages too high of a pH.
“I don’t believe in a pH closer to 8, it just makes the sanitizer less effective,” says Alan Schuster, president of Aqualab Systems in Apex, N.C. “Chlorine’s effectiveness is not subject to debate — it rises and falls with the pH.”
And trying to reduce the pH level for more effective sanitation can be especially difficult in water that’s been heavily treated with borax.
Still, some techs like having options, especially with low-pH water that tends to drift toward the 7.0 mark.