Acid and base demand

Problem set: 6 worked problems

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Watch for The base side is computable: raising alkalinity or hardness uses the ppm-to-weight formula times a product factor. The acid side is not: lowering pH or alkalinity depends on the water's buffering, so the dose comes from an acid-demand titration test, not a formula. Adding acid by guess overshoots and turns the water corrosive.

1. How much sodium bicarbonate raises 20,000 gallons by 10 ppm of total alkalinity? (Product factor 1.679.)

   • A About 0.99 lb
   • B About 28 lb
   • C About 1.67 lb
   • D About 2.80 lb sodium bicarbonate
   Show the worked solution

   Correct answer: D. About 2.80 lb sodium bicarbonate

   Pure weight = 20,000 x 10 x 8.34 / 1,000,000 = 1.67 lb; multiply by the sodium-bicarbonate factor 1.679 = 2.80 lb. Forgetting the factor leaves 1.67 lb; dividing by it gives 0.99 lb.

   Source: Public relationship: alkalinity-raising dose = (gallons x ppm x 8.34 / 1,000,000) x 1.679

2. How much anhydrous calcium chloride raises 30,000 gallons by 20 ppm of calcium hardness? (Product factor 1.109.)

   • A About 5.0 lb
   • B About 4.51 lb
   • C About 55 lb
   • D About 5.55 lb calcium chloride
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   Correct answer: D. About 5.55 lb calcium chloride

   Pure weight = 30,000 x 20 x 8.34 / 1,000,000 = 5.0 lb; times the calcium-chloride factor 1.109 = 5.55 lb. Skipping the factor leaves 5.0 lb; dividing gives 4.51 lb.

   Source: Public relationship: hardness-raising dose = (gallons x ppm x 8.34 / 1,000,000) x 1.109

3. How do you determine the acid dose needed to lower pH or total alkalinity?

   • A Use the ppm-to-weight formula with an acid factor
   • B Match the sodium-bicarbonate dose
   • C Multiply the volume by 8.34
   • D Run an acid-demand titration test and add acid in steps
   Show the worked solution

   Correct answer: D. Run an acid-demand titration test and add acid in steps

   There is no single formula for acid demand because it depends on the water's buffering capacity, which varies pool to pool. Operators use an acid-demand titration test to find the dose, then add acid gradually and retest.

   Source: Public operating practice: acid demand is set by titration, not a fixed formula

4. Why can't the acid dose be computed from one formula the way an alkalinity dose can?

   • A Acid has no measurable strength
   • B Lowering pH does not change alkalinity
   • C Acid is added by weight, not volume
   • D The dose depends on the water's buffering, which varies pool to pool
   Show the worked solution

   Correct answer: D. The dose depends on the water's buffering, which varies pool to pool

   Buffering resists pH change by an amount that depends on the water's current alkalinity and chemistry, so the same acid moves different pools differently. That is why acid demand is titrated rather than calculated, while a bicarbonate dose follows the fixed ppm-to-weight relationship.

   Source: Public chemistry: buffering capacity determines acid demand and varies by water

5. How much sodium bicarbonate raises 50,000 gallons by 10 ppm of total alkalinity? (Product factor 1.679.)

   • A About 4.17 lb
   • B About 2.48 lb
   • C About 0.70 lb
   • D About 7.0 lb sodium bicarbonate
   Show the worked solution

   Correct answer: D. About 7.0 lb sodium bicarbonate

   Pure weight = 50,000 x 10 x 8.34 / 1,000,000 = 4.17 lb; times 1.679 = 7.0 lb. This mirrors the handy benchmark that about 0.83 lb of pure chemical moves 100,000 gallons by 1 ppm.

   Source: Public relationship: alkalinity-raising dose = (gallons x ppm x 8.34 / 1,000,000) x 1.679

6. What is the risk of adding acid by estimate instead of by an acid-demand test?

   • A Alkalinity rises instead of falling
   • B The pool scales up
   • C Nothing, acid self-corrects
   • D You risk overshooting and driving the water corrosive
   Show the worked solution

   Correct answer: D. You risk overshooting and driving the water corrosive

   Guessing an acid dose can push pH and alkalinity far below target, sending the saturation index negative and making the water corrosive to plaster and metal. Titrating and adding in steps keeps you from overshooting.

   Source: Public operating practice: overshooting acid drives the LSI corrosive