Pool Water Chemistry: The Complete Guide for Pool Service Professionals
Why Chemistry Balance Matters
Water chemistry in a swimming pool exists in a complex equilibrium. When any single parameter falls out of range it typically affects others. Low pH accelerates chlorine consumption. High calcium combined with high alkalinity causes scale. Understanding each parameter and how they interact is the foundation of effective pool service.
The Core Parameters
Free Chlorine
Free chlorine is the active sanitizer in pool water. The ideal range is 1 to 3 parts per million for a traditionally chlorinated pool and 1 to 3 parts per million for a salt system pool. Below 1 ppm sanitizer effectiveness drops. Above 10 ppm the water can be irritating to swimmers and may bleach vinyl surfaces.
Chlorine consumption accelerates with high sunlight exposure, warm water, heavy bather load, and low cyanuric acid levels. Weekly service visits should verify free chlorine is within range and add chlorine as needed to maintain the target.
pH
pH measures how acidic or basic the water is on a scale of 0 to 14. Pool water should be maintained between 7.2 and 7.6. Below 7.2 the water is acidic and will irritate eyes, corrode metal equipment, and cause concrete surfaces to etch over time. Above 7.8 chlorine effectiveness drops significantly and scale formation becomes more likely.
pH tends to rise over time in most pools due to outgassing of carbon dioxide, addition of chlorine products, and swimmer activity. Muriatic acid or sodium bisulfate are used to reduce pH. Sodium carbonate or sodium bicarbonate raise it.
Total Alkalinity
Total alkalinity measures the water's capacity to resist pH change. The ideal range is 80 to 120 ppm. When alkalinity is too low, pH is unstable and will swing significantly between readings. When alkalinity is too high, pH becomes difficult to adjust and scale formation is more likely.
Sodium bicarbonate raises total alkalinity. Muriatic acid reduces it, though reducing alkalinity also affects pH and requires careful adjustment.
Calcium Hardness
Calcium hardness measures the amount of dissolved calcium in the water. The ideal range is 200 to 400 ppm. Water below this range is aggressive and will pull calcium from plaster and grout surfaces, causing etching and deterioration over time. Water above this range combined with high pH and alkalinity causes scale on surfaces and in equipment.
Calcium hardness only increases through evaporation and can be reduced by partial draining and dilution with fresh water. Calcium chloride is used to raise calcium hardness when it falls below range.
Cyanuric Acid
Cyanuric acid, also called stabilizer or conditioner, protects chlorine from degradation by sunlight. Without it, free chlorine in an outdoor pool can be depleted by UV exposure in a matter of hours. The ideal range is 30 to 50 ppm for traditionally chlorinated pools.
Cyanuric acid accumulates over time and can only be reduced by dilution. Very high cyanuric acid levels, sometimes called chlorine lock, reduce the effectiveness of chlorine even when free chlorine reads within range. The total dissolved solids level in the pool should be monitored and a partial drain performed when stabilizer becomes excessive.
Salt
Salt pools use a salt chlorine generator to produce chlorine from dissolved sodium chloride. The ideal salt range varies by equipment manufacturer but is typically 2,700 to 3,400 ppm. Below the minimum the chlorinator will not produce adequate chlorine. Above the maximum some equipment will shut down to prevent damage.
Salt levels are measured by the chlorinator's flow cell and should be verified with an independent test. Salt accumulates through evaporation and decreases when water is added or drainage occurs.
Reading Chemistry as a System
Individual chemistry readings should always be interpreted in the context of other readings. A pH of 7.8 combined with high calcium hardness and high alkalinity presents a higher scale risk than the same pH with lower values in those parameters. Water balance calculations such as the Langelier Saturation Index combine multiple readings to give a single corrosion or scale tendency score. You can run your numbers in our free LSI calculator or get full dosing in the pool chemistry calculator.
Common Chemistry Problems and Corrections
Green Water
Green water is caused by algae growth, which is typically the result of inadequate free chlorine or excessive phosphates. Treatment involves raising free chlorine to shock levels, brushing all surfaces, and running the filter continuously until the water clears. Phosphate remover can be used to remove the nutrient source that feeds algae.
Cloudy Water
Cloudy water without green tint is typically caused by inadequate filtration, high total dissolved solids, or chemistry imbalance. Checking and correcting pH, alkalinity, and calcium hardness and ensuring adequate filter run time resolves most cloudiness.
Tracking Chemistry Over Time
Recording chemistry readings at every service visit creates a history that reveals patterns in how each pool behaves. Pools with heavy tree cover may consistently show low pH due to organic load. Pools with older equipment may show rising calcium hardness as scaling compounds build up. Chemistry history is a diagnostic tool as much as a compliance record.