CPO Pool Water Chemistry Questions and Answers 2 — Questions and Answers
Question 1: What is the ideal pH range for a chlorinated swimming pool?
- 6.8 to 7.0
- 7.2 to 7.6 (Correct answer)
- 7.8 to 8.2
- 8.0 to 8.4
Correct answer: 7.2 to 7.6
The ideal pH range is 7.2 to 7.6, with 7.4 to 7.6 being optimal for chlorine effectiveness and swimmer comfort.
At pH 7.2, approximately 63 percent of free chlorine exists as hypochlorous acid (HOCl), the active sanitizing form. At pH 8.0, only about 21 percent is HOCl. Below pH 7.0 water becomes corrosive and irritating. Above pH 7.8, chlorine effectiveness drops significantly.
Question 2: What is the function of total alkalinity in pool water chemistry?
- To sanitize the water
- To act as a pH buffer resisting rapid pH changes (Correct answer)
- To increase calcium hardness
- To improve water clarity
Correct answer: To act as a pH buffer resisting rapid pH changes
Total alkalinity acts as a buffer system that resists rapid changes in pH. The ideal range is typically 80 to 120 ppm.
Total alkalinity measures the water's buffering capacity against pH changes. Low TA (below 60 ppm) causes pH bounce with erratic swings. High TA (above 150 ppm) makes pH stubbornly resistant to adjustment and promotes scaling. TA is raised with sodium bicarbonate and lowered with muriatic acid.
Question 3: What causes high pH in pool water and how should it be corrected?
- High chlorine, reduce chlorine feed
- Aeration, high-pH chemical additions, or high alkalinity; add muriatic acid or sodium bisulfate (Correct answer)
- Too many swimmers, reduce bather load
- Sunlight exposure, cover the pool
Correct answer: Aeration, high-pH chemical additions, or high alkalinity; add muriatic acid or sodium bisulfate
High pH results from aeration causing CO2 loss, use of high-pH chemicals like sodium hypochlorite, or high alkalinity. Correct with muriatic acid or sodium bisulfate.
Common causes include aeration from water features driving CO2 off, sodium hypochlorite (pH around 13), and high total alkalinity. For persistent pH rise from aeration, consider lowering the alkalinity target to 60 to 80 ppm to compensate.
Question 4: What is the recommended calcium hardness range for a plaster pool?
- 50 to 100 ppm
- 100 to 150 ppm
- 200 to 400 ppm (Correct answer)
- 500 to 700 ppm
Correct answer: 200 to 400 ppm
Calcium hardness should be maintained between 200 and 400 ppm for plaster pools to protect the surface from dissolution.
Below 200 ppm, water dissolves calcium from the plaster causing etching and roughness. Above 400 ppm, the risk of scale formation increases. Vinyl and fiberglass pools can tolerate lower levels of 150 to 250 ppm. Calcium hardness is increased with calcium chloride and can only be reduced by partial draining.
Question 5: What chemical is used to raise total alkalinity without significantly affecting pH?
- Soda ash (sodium carbonate)
- Sodium bicarbonate (baking soda) (Correct answer)
- Calcium chloride
- Muriatic acid
Correct answer: Sodium bicarbonate (baking soda)
Sodium bicarbonate raises total alkalinity with minimal impact on pH. Soda ash raises both pH and alkalinity significantly.
Sodium bicarbonate has a pH of approximately 8.3 in solution, causing relatively mild pH change. Soda ash (pH approximately 11.4) raises both pH and alkalinity significantly. Approximately 1.5 pounds of sodium bicarbonate per 10,000 gallons raises TA by approximately 10 ppm.
Question 6: Why does rain typically cause pool pH and alkalinity to drop?
- Rain adds chlorine to the pool
- Rainwater is naturally acidic and dilutes the pool's buffering capacity (Correct answer)
- Rain raises the water temperature
- Rain has no significant effect on pool chemistry
Correct answer: Rainwater is naturally acidic and dilutes the pool's buffering capacity
Rainwater is naturally acidic (pH approximately 5.0 to 5.5) and has zero alkalinity, so it lowers both pH and TA.
Rainwater has a natural pH of approximately 5.0 to 5.5 due to dissolved carbon dioxide forming carbonic acid. In polluted areas, acid rain can have pH as low as 4.0. Rainwater also has zero alkalinity and dilutes chemicals. Rain washes in phosphates, nitrates, and organic debris.
CPO Pool Water Chemistry Questions and Answers 2
What is the ideal pH range for a chlorinated swimming pool?