CPO Disinfection and Sanitizers 3 — Questions and Answers
Question 1: At a pool pH of 7.5, approximately what percentage of free chlorine exists as the effective hypochlorous acid (HOCl) form?
- About 97%
- About 75%
- About 50% (Correct answer)
- About 21%
Correct answer: About 50%
At pH 7.5, the equilibrium between HOCl and OCl⁻ is approximately 50/50, making pH 7.5 a balance point for disinfection efficiency.
Question 2: Why is cyanuric acid (CYA) added to outdoor pools?
- To increase calcium hardness
- To lower total alkalinity
- To protect chlorine from ultraviolet (UV) sunlight degradation (Correct answer)
- To raise pH
Correct answer: To protect chlorine from ultraviolet (UV) sunlight degradation
CYA bonds to free chlorine, shielding it from UV radiation and extending its useful life in outdoor pools.
Question 3: What is the generally accepted maximum recommended level of cyanuric acid in a swimming pool?
- 10 ppm
- 40 ppm
- 100 ppm (Correct answer)
- 250 ppm
Correct answer: 100 ppm
Above 100 ppm, cyanuric acid over-stabilizes chlorine, significantly reducing its disinfection effectiveness (chlorine lock).
Question 4: What is the only reliable way to reduce excessively high cyanuric acid levels in a pool?
- Add muriatic acid
- Superchlorinate the pool
- Partially drain and refill with fresh water (Correct answer)
- Run the filter continuously for 24 hours
Correct answer: Partially drain and refill with fresh water
CYA does not degrade or filter out; diluting the water by partial drain-and-refill is the only practical way to lower it.
Question 5: When pool pH drops below 7.2, what is the primary operational concern?
- Chloramine formation increases
- Algae growth accelerates
- Corrosion of equipment, surfaces, and swimmer discomfort (Correct answer)
- Cyanuric acid breaks down
Correct answer: Corrosion of equipment, surfaces, and swimmer discomfort
Low pH (acidic conditions) corrodes metal fittings, etches plaster, degrades gaskets, and causes eye/skin irritation.
Question 6: What is the primary function of total alkalinity (TA) in pool water chemistry?
- Disinfect the water
- Buffer pH against rapid changes (Correct answer)
- Reduce calcium hardness
- Prevent chloramine formation
Correct answer: Buffer pH against rapid changes
Total alkalinity acts as a pH buffer, resisting sudden swings in pH caused by added chemicals or bather load.
Question 7: At a high pH of 8.2, free chlorine's disinfecting effectiveness is reduced because:
- HOCl converts to inert chlorine gas
- The hypochlorite ion (OCl⁻) dominates and is far less effective than HOCl (Correct answer)
- Cyanuric acid breaks down the chlorine
- Total alkalinity neutralizes the chlorine
Correct answer: The hypochlorite ion (OCl⁻) dominates and is far less effective than HOCl
At pH 8.2, over 90% of free chlorine shifts to the OCl⁻ ion form, which is approximately 100 times less effective at killing pathogens than HOCl.
CPO Disinfection and Sanitizers 3
At a pool pH of 7.5, approximately what percentage of free chlorine exists as the effective hypochlorous acid (HOCl) form?