Pool Chemistry & Water Balance Practice Questions

Explanation: The ideal pool pH range is 7.2 to 7.8. This range maximizes chlorine effectiveness, prevents corrosion of equipment and surfaces, and minimizes eye and skin irritation. pH below 7.2 is too acidic (corrosive); pH above 7.8 significantly reduces free chlorine efficacy.

Explanation: Public swimming pools should maintain 1.0 to 3.0 ppm free chlorine residual (many health codes require a minimum of 1.0 ppm at all times). Adequate free chlorine inactivates pathogens including Cryptosporidium, E. coli, and Giardia.

Explanation: Total alkalinity should be maintained between 80 to 120 ppm. Alkalinity acts as a pH buffer, preventing rapid pH swings. Low alkalinity causes pH to fluctuate wildly; high alkalinity makes pH difficult to adjust and can cause cloudiness.

Explanation: Chloramines form when free chlorine reacts with nitrogen-containing compounds introduced by bathers — primarily urea from sweat and urine. Chloramines are less effective disinfectants than free chlorine and cause the characteristic chlorine odor and eye irritation often associated with pools.

Explanation: Calcium hardness for plaster pools should be maintained at 200 to 400 ppm. Water with insufficient calcium hardness is corrosive and will leach calcium from plaster surfaces, causing pitting and etching. Water with excessive calcium hardness causes scaling and cloudy water.

Explanation: Cyanuric acid (CYA) should be maintained at 30 to 50 ppm for outdoor pools. Within this range, CYA effectively protects free chlorine from UV degradation while still allowing chlorine to inactivate pathogens efficiently. Many health codes cap CYA at 100 ppm; above that level, chlorine effectiveness is significantly impaired, potentially requiring a partial drain-and-refill to lower CYA concentration.

Explanation: ORP measures the electrical potential (in millivolts) of pool water to oxidize (disinfect) contaminants. It reflects the combined disinfection power of free chlorine and pH — since HOCl (the active form of chlorine) is more predominant at lower pH, ORP rises as pH decreases. An ORP of 650 to 750 mV is generally considered adequate for pathogen control. ORP controllers can automate chlorine dosing, but free chlorine and pH must still be verified with chemical testing.

Explanation: Combined chlorine (chloramines) of 0.8 ppm significantly exceeds the 0.2 ppm threshold that triggers corrective action. To reach breakpoint chlorination and destroy chloramines, the free chlorine must be raised to at least 10 times the combined chlorine level: 10 x 0.8 = 8.0 ppm free chlorine needed. Shocking with calcium hypochlorite, sodium hypochlorite, or non-chlorine oxidizer will oxidize and eliminate the chloramines, eliminating the associated eye irritation and odor.

Explanation: The Langelier Saturation Index (LSI) is a calculated value using pH, total alkalinity, calcium hardness, water temperature, and TDS to predict whether water is corrosive (negative LSI, tends to dissolve plaster and corrode metal), balanced (LSI near 0), or scale-forming (positive LSI, tends to deposit calcium carbonate). The target LSI for pools is typically -0.3 to +0.3. Operators use LSI calculations to make informed decisions about chemical adjustments.

Explanation: Increase needed = 100 - 60 = 40 ppm. Using the rule: 1.4 lbs per 10 ppm per 10,000 gallons. For 100,000 gallons: 1.4 x 10 (pool-volumes) x 4 (40 ppm / 10 ppm increments) = 56 lbs of sodium bicarbonate. Always add chemicals in divided doses, allow circulation between additions, and retest before adding more. Sodium bicarbonate raises alkalinity with minimal effect on pH, making it the preferred alkalinity increaser.