How to Lower pH With Muriatic Acid Using the Acid Demand Test

Why this matters

High pH is the most common correction on a plaster pool route, and the most common way to get it wrong is to dose by habit: the same jug-cap pour every week whatever the water says. Too little and the pool scales the heater and dulls the chlorine; too much and you have pushed soft water into the plaster for a week. The acid demand test is a titration of the water in your own comparator tube. It measures how much this pool's buffer resists the change, so the dose it gives already accounts for the alkalinity you would otherwise have to guess at. This card owns the pH correction. Acid handling, dilution for equipment work and the total alkalinity knockdown belong to 'Muriatic Acid Handling and Dilution'.

Start state: what you need and what to check first

Bring: the jug of muriatic acid, an acid-only measuring jug marked in fluid ounces, chemical splash goggles, acid-resistant gloves, the test kit with its phenol red pH reagent and acid demand reagent, and the kit's acid demand table (usually printed inside the lid or on the reagent instructions). Have a hose or a full bucket of clean water within reach before the cap comes off, so an eye or skin splash can be flushed immediately; flush an eye for 15 minutes and get it seen, per 'Chemical Exposure and First Aid Response'.

Check before you pour anything:

• The volume. The demand table gives acid per 10,000 gallons, and every ounce you pour is scaled by the pool's gallons. Use the service record, or work it out per 'How to Estimate Pool and Spa Gallons'.
• Circulation. Pump running, water visibly moving at the deep-end returns, nobody in the water. No flow, no acid: undiluted acid is denser than water, sinks, and sits on the floor where it etches a gray patch into plaster.
• Free chlorine. Phenol red reads falsely high, toward purple, when free chlorine is high (the trade rule of thumb puts the problem above about 10 ppm, and the reagent instructions state the limit for your kit). Right after a shock dose, neutralize the sample with the kit's chlorine neutralizer if it carries one, or wait and test another day.
• Total alkalinity. If TA is well above the range in 'Pool Chemistry Quick Reference', this week's high pH is coming back next week regardless. Correct pH today and plan the TA knockdown per the acid handling SOP rather than chasing the same pH every visit.
• Order on the deck. Acid goes in first, chlorine second, at a different return, per 'The Dosing Order on the Deck: One Product at a Time'. Do the pump-off work (baskets, pressure) before the first pour, so you are not shutting circulation off while the acid plume is still near the return.

The procedure

1. Read the strength on the jug before you open the kit. The trade strength is 31.45 percent (20 degrees Baume), and every demand table assumes it unless it says otherwise. The 14.5 percent "safer" product is common at retail and on some trucks, and the acid handling SOP's knockdown table puts it at about 42 percent of the stronger acid's effect per unit volume. What breaks if skipped: a dose computed for one strength and poured from the other misses by better than a factor of two, and the light dose just looks like a stubborn pool.

2. Take the pH sample at elbow depth, away from returns, and read it. Fill the comparator to the line, add the phenol red drops the kit calls for, cap, invert, read against good daylight. A sample from a return eyeball reads last visit's acid or this morning's chlorine, not the pool.

3. Set the target before you titrate. The target pH comes from 'Pool Chemistry Quick Reference'; on a plaster pool most shops aim for the middle of that range so the week's drift upward still ends inside it. Write the reading and the target on the ticket. If the reading is at the top of the comparator scale (8.2 on most kits), the true pH may be higher and the demand test will under-call the dose; treat the first dose as partial and retest after circulation.

4. Run the acid demand test in the same sample tube. Add the acid demand reagent one drop at a time, swirling after each drop, and count until the color matches the target pH on the comparator. Hold the dropper vertical so every drop is the same size. The count is the measurement. Stop counting at the drop that matches; one drop past is a different answer.

5. Convert the drop count with your kit's table, then scale to the pool. The table gives acid per 10,000 gallons for a given drop count and sample size. The shelf formula applies: acid = (table figure for this drop count) x (gallons / 10,000), rounded only at the end to what your measuring jug reads. Different kits use different sample sizes and reagent strengths, so a drop on one kit is not a drop on another; use the table that came with the reagent in your hand.

6. Measure the acid into the acid-only jug, outdoors, standing upwind. Pour from the big jug low, slowly, with the measuring jug on the deck rather than in your hand at face height. Muriatic acid fumes hydrogen chloride the moment the cap is off; the fume corrodes metal and burns airways, which is why the cap goes back on before you walk to the pool. The measuring jug never touches any other chemical.

7. Pour slowly into the stream of a deep-end return with the pump running. Pour straight from the acid-only measuring jug, the shelf's method in 'The Dosing Order on the Deck: One Product at a Time'; the red acid bucket is for diluting acid for equipment work, not for the pool dose. Kneel at the coping, hold the jug just above the surface so the acid enters rather than splashes, and pour along the return stream a foot or so out from the wall. Walking the pour along the deep end spreads it further. Never the skimmer: that sends concentrated acid through the pump, filter, heater (where it attacks a copper exchanger) and any inline chlorinator.

8. Rinse the measuring jug at the same return and stow it. Dip it in the pool stream, let the rinse go in, cap the big jug, wipe its shoulder dry, stow both in the acid bin. A wet drip on a jug shoulder is where acid meets chlorine on the truck.

9. Retest pH only after the acid has circulated. A pH read ten minutes after the pour, near the return, reads the plume. Retest after at least one full turnover, or on the next visit with the pump left on its schedule.

The artifact: a demand count turned into a dose

The figures here are illustrative: this kit's table reads about 3 fl oz of 31.45 percent acid per drop per 10,000 gallons on its sample size. Yours may read differently, so read your own card.

ACID DEMAND WORKSHEET          pool: 16,000 gal plaster
-----------------------------------------------------
jug label                31.45 pct muriatic
pH as found              7.8
target pH                7.5
demand drops to 7.5      3
table, per 10,000 gal    3 drops x 3 fl oz = 9 fl oz
scale                    16,000 / 10,000 = 1.6
dose                     9 x 1.6 = 14.4 fl oz
measured                 14 fl oz (jug marks)
poured at                R1, deep end, pump on
pH after 1 turnover      7.5

Worked example, carried through

A 16,000 gallon plaster pool at the weekly stop. Free chlorine 3.0 ppm, so phenol red can be trusted. pH reads 7.8, TA 100 ppm, CYA 40 ppm, water 80 F. The target is 7.5. Three drops of acid demand reagent bring the sample to the 7.5 color.

The dose. At 3 fl oz per drop per 10,000 gallons, three drops is 9 fl oz per 10,000 gallons. Times 1.6 is 14.4 fl oz. The measuring jug is marked in whole ounces, so 14 fl oz goes in.

Second direction. Start from what was poured: 14 fl oz over 1.6 is 8.75 fl oz per 10,000 gallons, over 3 fl oz per drop is 2.9 drops. That is the three drops counted, less the rounding. The units cancel both ways: fl oz per drop per 10,000 gallons times drops times (10,000 gallon units) leaves fl oz.

Sanity check against alkalinity. A pH correction should barely move TA. Using the knockdown figure in the acid handling SOP (about 50 ppm per gallon per 10,000 gallons for 31.45 percent), 14.4 fl oz is 0.11 gallon; in 16,000 gallons that is 0.11 x 50 / 1.6, about 3.5 ppm. TA should read 100 or 90 next week depending on where the 10 ppm drop step falls, which is what a pH-only dose looks like. If the arithmetic had come out at a full gallon for a 0.3 pH move, something upstream is wrong: the jug strength, the drop count or the volume.

If the jug had said 14.5 percent. The same pH change takes about 1 / 0.42, or 2.4 times the volume: 14.4 x 2.4 is about 34 fl oz. Pour 14 fl oz of the weak product by habit and pH lands near 7.7, the customer sees no change, and next week somebody pours a double dose on top.

Result. Retested after the pump's next turnover, pH reads 7.5. The ticket carries the jug strength, both pH readings, the drop count, the table figure, the gallons and the ounces poured, in that order, so the next tech can redo the arithmetic in ten seconds.

Where the call forks

• pH above the comparator's top. The demand test titrates from wherever the sample really is, but you cannot see where that is. Dose from the count as a first step, retest after circulation, and dose again; do not guess a larger pour.
• pH that rises back fast every week. High TA, aeration from a spillway or waterfall, a salt cell, or fresh plaster all push pH up. Repeated weekly acid on a high-TA pool is a TA problem; plan the knockdown with alternating acid and aeration per 'Muriatic Acid Handling and Dilution'. On a salt pool the cell is where pH runs highest; see 'Salt Cell Cleaning & Maintenance'.
• Low calcium or low TA alongside high pH. Run the saturation index before deciding how far to lower pH, per 'How to Calculate the Saturation Index on the Pool Deck'. Pushing pH to the bottom of its range on soft, low-alkalinity water takes the index negative on a plaster pool.
• Vinyl and fiberglass. No plaster to etch, but the liner and the fittings still suffer from a concentrated pour; the pour method does not change.

Finish test

Accept the job when pH, retested after at least one turnover from elbow depth away from the returns, reads the target or one comparator step from it, and the ticket shows the arithmetic. Two readings tell you more than one: if pH misses short by the same fraction two weeks running, check the gallons on the record before you change the drop math, because a volume error under-doses every product by the same share.

A reading well below target means the dose was sized from a falsely high pH (chlorine interference, a return-stream sample) or the table was read for the wrong sample size. Do not add base to fix it the same day unless pH is below the bottom of the quick-reference range; let aeration bring it up and log the cause.

References

• See related: Muriatic Acid Handling and Dilution (handling, strengths, TA knockdown)
• See related: The Dosing Order on the Deck: One Product at a Time
• See related: Pool Chemistry Quick Reference (target pH)
• See related: How to Calculate the Saturation Index on the Pool Deck; How to Estimate Pool and Spa Gallons
• Test kit reagent instructions and acid demand table; muriatic acid label and Safety Data Sheet