Water Chemistry
15% of this examSanitization and water balance: what chlorine does in its free and combined forms, how stabilizer and product choice change the chemistry, and how pH, alkalinity, calcium hardness, temperature, and TDS combine into balanced — versus corrosive or scaling — water. Dosing math is label arithmetic scaled to pool volume.
Core concepts
Free vs combined, and the breakpoint fix
Free chlorine sanitizes; combined chlorine (chloramines) causes the 'chlorine' smell and eye sting. The remedy is breakpoint chlorination: shock with roughly ten times the combined chlorine level, all at once — an insufficient dose can leave or even raise chloramines.
Know your chlorine products
Trichlor and dichlor are stabilized (they carry cyanuric acid, which builds up with continued use); liquid sodium hypochlorite and cal-hypo are unstabilized. CYA shields chlorine from sunlight outdoors, but too much blunts its killing power. And always add acid to water, never water to acid.
Balance is five dials, one index
The saturation index combines pH, total alkalinity, calcium hardness, temperature, and dissolved solids. Negative = aggressive water that etches plaster (low calcium hardness is a classic cause); positive = scale-forming. pH stays in the 7.2–7.8 band with alkalinity as the buffer that keeps it from bouncing. TDS only ever climbs — the fix is partial drain and dilution.
Test and dose by the numbers
DPD testing distinguishes free from combined chlorine; OTO shows only total. Dosing math scales the label: (pool volume ÷ label volume) × label dose × ppm desired — e.g., 2 lb per 10,000 gal per ppm becomes 12 lb to raise a 30,000-gal pool 2 ppm.
Key facts to know cold
Where it lives in your books
The real exam is open book. Knowing which book — and which tab — answers this domain is worth as much as memorizing it.
Lookup strategy
- · Tab the handbook's water balance table and saturation index — chemistry questions about corrosive vs scaling water are answered straight from it.
- · Dosing questions always state the label rate; resist memorized doses and scale the number given in the question.
Reading isn't learning — retrieval is.
21 questions in this domain, each with an explanation and source.