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alkalinity is corrected for cyanurate alkalinity by the following equation to yield carbonate alkalinity: Alkalinity corrected = Alkalinity total – 1/3 Cyanuric Acid Thus, if the test for total alkalinity shows 80 ppm, while the test for cyanuric acid shows 100, the carbonate alkalinity is roughly 47, which is too low.
80 – (.33 x 100) = 47 Note that the one third correction factor is pH dependent. That is because the cyanuric acid and cyanurate ion concentrations are also pH dependent. Therefore, it is best to use correction factors that account for this pH dependency. The cyanuric acid correction factors may be found in the accompanying table.
Don’t Forget To Subtract Out Borates
If borates are present in the pool water, it is also necessary to subtract them out from the total alkalinity because their presence is measured in the test.
Total Alkalinity results can be corrected for the presence of borate by the following procedure.
Begin by measuring the borate concentration. Then: 1. Select a borate factor (Bƒ) in the accompanying table based on measured pH: 2. Calculate the carbonate alkalinity (CAlk) from the measured total alkalinity (TA), measured borate (B), and selected borate factor (Bƒ): CAlk = TA – (B x Bƒ) For example: pH = 7.6; TA = 100 ppm; B = 50 ppm CAlk = TA – (B x Bƒ) Calk = 100 – (50 x 0.17) CAlk = 92 ppm Recommended Alkalinity Levels
For most pools, total alkalinity should be kept between 80-120 ppm, though pools using chlorine generated by saltwater generators often benefit from levels closer to 60-80 ppm to help control pH drift.
Pools sanitized with polyhexamethylene biguanide (PHMB) also need to maintain total alkalinity for water balance and clarity. The total alkalinity in biguanide pools should be between 80 and 150 ppm.
Changing Alkalinity
To reduce the alkalinity, add acid and then aerate the pool. Acid lowers the total alkalinity by converting bicarbonate to carbon dioxide. Aerating helps the carbon dioxide leave the pool so that when the pH returns to normal the bicarbonate ions are prevented from reforming.
Expose the water to as much air as possible. Run the fountain. Turn on the filter. Run the jets.
The steps are as follows: 1. Add acid to lower the pH to about 7. This lowers the total alkalinity by converting bicarbonate to dissolved carbon dioxide 2. Aerate until the pH rises to about 7.6. This is the only way to raise the pH without raising the total alkalinity 3. Repeat steps 1 and 2 until the desired total alkalinity has been reached.
To increase low alkalinity without a significant increase in pH, add sodium bicarbonate.
By maintaining proper total alkalinity, service pros can help keep pH stable, protect pool surfaces and equipment, and ensure water remains balanced according to the LSI, reducing the need for excessive chemical adjustments. Calcium Hardness
Calcium hardness is a critical factor in the Langelier Saturation Index (LSI), which determines whether pool water is balanced, corrosive, or scale-forming. It measures the amount of dissolved calcium in the water, typically expressed in parts per million. Proper calcium levels help maintain water stability and protect pool surfaces and equipment.
Calcium Hardness Affects LSI
It prevents Corrosive Water (Low LSI): If calcium hardness is too low (below 150-200 ppm), the water becomes aggressive and starts to dissolve calcium from plaster, grout, tile, and even corrodes heat exchangers in heaters. Over time, this can lead to etched surfaces, rough plaster, metal corrosion, and deterioration of pool materials.
It prevents Scale Formation (High LSI): If calcium hardness is too high (above 400-500 ppm), depending on the other water balance parameters, the water can become scaleforming, meaning excess calcium can precipitate out and form white, chalky deposits on pool surfaces, tile lines, heaters, and filters.
Scale buildup reduces water circulation efficiency and can clog plumbing and equipment, increasing maintenance costs.
It helps Maintain LSI Balance: The LSI considers calcium hardness alongside pH, total alkalinity, temperature, and TDS to determine whether water is in equilibrium.
Proper calcium levels (200-400 ppm for most pools) help keep LSI in the ideal range (-0.3 to +0.3), preventing both corrosive and scaling conditions.
Managing Calcium Hardness
Add calcium chloride to raise low calcium levels and prevent etching.
If calcium is too high, dilution (partial draining and refilling) has long been considered the best solution, but recently the research and consulting group onBalance has been promoting the use of soda ash — sodium carbonate — to precipitate it out.
Sequestering agents can help prevent calcium from precipitating but do not reduce hardness itself.
Maintaining balanced calcium hardness is essential for preserving swimming pool surfaces, protecting pool equipment, and ensuring stable water chemistry according to the LSI.
