Muriatic Acid Handling and Dilution

Purpose

This procedure guarantees that every acid addition is calculated from a measured alkalinity, poured in an order and a place that cannot produce chlorine gas or a plaster etch, and re-tested before anyone swims. Without it the same jug produces three outcomes on three trucks: a technician dosing by eye who burns a plaster finish, one who pours into a skimmer with the heater firing and sends concentrated acid through a copper heat exchanger, and one who tops off an acid vessel still holding hypochlorite residue and gasses himself at the pad. None of those needed a mistake anyone would have called reckless at the time.

Scope

Covers liquid hydrochloric acid sold to this trade as muriatic acid at both common strengths, used for pH and total alkalinity correction and for diluted equipment cleaning at the pad: filter housings, tile line, and the acid bath used on a salt cell.

It does not cover truck segregation and restraint, which the chemical storage and transport SOP owns, or a drain-and-acid-wash of a plaster shell, which carries its own containment and wastewater path. It does not cover dry acid (sodium bisulfate), a different product with a different dose and its own sulfate loading.

Roles and responsibilities

The shop's duty under 29 CFR 1910.1200 runs to its employees, so the SDS for the exact strength on the truck is available during the shift. Where an employee may be splashed with an injurious corrosive, 29 CFR 1910.151(c) requires suitable quick-drenching and eye-flushing facilities in the work area; ANSI/ISEA Z358.1, which binds you through your employer's adoption rather than by federal force, sets that flush at 15 minutes.

The dose is arithmetic, not a serving suggestion

Acid neutralizes alkalinity mole for mole, so knockdown per unit volume is fixed by strength:

Those come from the chemistry, not a chart: a gallon of the 31.45 percent product carries roughly 1.4 kg of hydrogen chloride, each mole consuming one equivalent of alkalinity, and an equivalent reported as calcium carbonate is 50 g. The second row is the one that bites. The 14.5 percent product delivers about 42 percent of the knockdown per unit volume, so a dose computed for one strength and poured from the other misses by better than a factor of two, in the direction nobody notices, because a light dose just looks like a stubborn pool.

Acid lowers pH and alkalinity together and nothing about the pour changes that. What separates a TA correction from a pH correction is what follows: aeration drives carbon dioxide off and raises pH without returning alkalinity, so a TA reduction is acid additions alternated with aeration, not one large pour. The claim that a column pour changes the split is trade folklore and this procedure does not rely on it.

Procedure

1. Clear the water and confirm circulation before you touch the closure. Nobody in the pool, nobody within splash reach of the pour line, pump running and water visibly moving at the returns. Accept when every return eyeball shows surface movement and the filter gauge sits at or above its clean baseline. Wrong is a closed valve, a dead-headed pump, or a pool full of kids while the parent waves you on. Do not dose still water: undiluted acid sinks and sits, and on plaster that is a permanent grey etch. Reschedule rather than pour with swimmers present.

2. Test and record pH, total alkalinity, calcium hardness and water temperature before any calculation. Fresh reagent or a calibrated photometer, sample from elbow depth away from the returns, four values written as you read them. Accept when the TA titration repeats within one drop on a second sample. Wrong is reagent past its bottle date, a surface scoop, or a test strip used as the basis of an acid dose: re-test with fresh reagent, because a bad TA reading is the commonest root of an over-acidified pool. Keep reagent bottles closed between drops so pad vapor cannot reach them.

3. Read the strength off the label in front of you and compute the dose from the table above. Never carry yesterday's number. Accept when the ticket shows pool volume, current TA, target TA, strength and computed quarts, all written before the jug is opened. Wrong is "about a gallon, same as last time": stop and compute, because the two strengths differ by better than a factor of two per unit volume and the shop stocks both. Cap any single addition at the volume that drops TA 30 ppm, then re-test, because you cannot un-pour acid.

4. Set out flush water, neutralizer and protection before the closure comes off, and put it on. Splash goggles plus face shield, acid-resistant gloves in nitrile or neoprene rather than latex, soda ash or sodium bicarbonate within arm's reach. Accept when every item is in hand, not in the truck. Wrong is goggles pushed up on a cap. Concentrated acid fumes hydrogen chloride at an open closure, an inhalation route rather than a contact one, so open the jug at arm's length with your face upwind and never inside a closed pump house or a screened enclosure.

5. When diluting for equipment work, water goes in the vessel first and acid goes into the water, always, in open air. Fill the marked vessel to its water line first, then add acid in a thin stream. Roughly ten parts water to one part acid for tile and housing work; for a salt cell, the ratio in that cell's manual, commonly nearer four or five to one. Accept when the vessel holds its full water volume before any acid moves. Wrong is acid in the bottom of a bucket about to receive water, which boils locally and throws concentrated acid back at whoever holds the hose: stop and start over. That vessel is acid-only and marked, because acid into hypochlorite residue makes chlorine gas with your head over it.

6. Pour the pool dose into deep water on the return side with the pump running, walking the pour along the perimeter. Hold the jug low so the stream enters rather than splashes and keep moving, so no one volume of water takes the whole dose. Accept when the pour ends with no cloud sitting on the floor and nothing on the coping. Wrong is a fixed-spot shallow-end pour, a pour into a skimmer, or a pour with the heater firing. Never the skimmer: undiluted acid then runs through the pump, filter and heat exchanger, which is how a copper exchanger pinholes two seasons early. If the heater is running, shut it down and let the pump finish its cooldown first.

7. Circulate a full turnover, then re-test pH and TA and record both. A turnover on a typical residential system is a few hours, so on a route the re-test is usually a return visit or a customer-read strip arranged in advance. Accept when TA lands within about 10 ppm of prediction and pH sits at or above 7.2. Wrong is TA that barely moved, or pH under 7.2. If TA barely moved, re-measure the pool from its dimensions before adding anything else, because the error is almost always the volume, not the acid.

8. Hold further acid whenever pH is at or below 7.2 and run the aeration cycle instead. Angle return eyeballs toward the surface, run a spillway, waterfall or air blower, and leave it running between visits. Accept when pH recovers to 7.4 or above with TA unchanged, which is what aeration should do. Wrong is adding the next increment anyway to finish today, which drives the water aggressive: sustained low pH with low alkalinity dissolves plaster, and the saturation index the trade uses for that condition does not care that you were on a schedule. Escalate when a pool will not recover between visits.

9. Rinse the vessel away from the pad, retorque the closure, write the ticket before you leave. Rinse water goes to the pool it was measured for, never to a storm drain or the pad's sanitary line. Accept when the closure is finger-tight plus a quarter turn and dry at the rim and the ticket carries every field below. Wrong is an unrinsed vessel back on the truck, which seeds the next gas event, and a dose remembered rather than written, which leaves the next technician guessing.

The record this produces

One dosing line per visit on the customer ticket, which becomes the pool's chemical history in the job record.

• Pre-dose readings: pH, TA, calcium hardness, water temperature, with the time taken.
• Pool volume used in the calculation, and whether it was measured or carried forward.
• Product strength and volume added, in label units, not in "jugs."
• Post-circulation re-test: pH and TA with the time, or a note that it falls to the next visit and why.
• Holds and escalations: any addition held at the pH floor and the aeration cycle set in its place.

The next technician reads the last three dosing lines before computing anything, because a pool needing acid every week has a fill-water or plaster-curing story one visit cannot see. The shop lead reads the holds monthly: a route with no holds recorded is not a route with perfect chemistry, it is a route where nobody re-tests. If a customer claims a service visit etched their plaster, these pH readings are the defense.

One acid visit, filled in

An 18,000 gallon plaster pool, mid-season. TA 190 ppm against a 100 ppm target, pH 7.9, calcium hardness 280 ppm, water 84 F. The complaint is that pH climbs back within days, which is the signature of high alkalinity rather than a pH problem.

Step 3 fails. The technician computes from the 31.45 percent row out of habit: 50 ppm per gallon per 10,000 gallons means one gallon in 18,000 gallons drops 27.8 ppm, so the 30 ppm cap is 1.08 gallons. Then he reads the label. The truck is carrying the 14.5 percent product this month. The stop rule is recompute, not pour: at 21 ppm per gallon per 10,000 gallons, one gallon here drops 11.7 ppm, and 30 divided by 11.7 is 2.6 gallons. Both figures go on the ticket, because the gap between 1.1 and 2.6 gallons is the whole reason step 3 exists.

Steps 4 through 6 pass: water in the vessel first for the tile work, goggles and shield on, dose walked along the deep-end return side with the heater already off and through its cooldown.

Step 7 partly fails. After one turnover, TA reads 162 ppm, within 10 ppm of the predicted 160, so the arithmetic closed. pH reads 7.0, under the 7.2 floor. Step 8 takes it: no second addition, eyeballs angled up, spillway left running, customer asked to leave the waterfall on until the next visit, hold recorded.

Six days later: pH 7.5, TA 160 ppm - pH up, alkalinity unmoved, which is aeration doing its job. A second 2.6 gallon addition brings the pool to TA 131 ppm, pH 7.3; a third lands TA 104 ppm at pH 7.4, inside target. A technician who poured the whole 7.7 gallon requirement at once would have driven pH into the low 6s against 280 ppm calcium hardness, and the etch would have been permanent before anybody re-tested.

References

• 29 CFR 1910.1200, Hazard Communication - container labeling and employee access to safety data sheets.
• 29 CFR 1910.151(c) - quick drenching and eye flushing where employees may be exposed to injurious corrosives; ANSI/ISEA Z358.1, in the edition your employer has adopted, for the 15-minute flush.
• The safety data sheet and container label for the exact strength on your truck - the only authority for strength, first aid and neutralization.
• Salt cell manufacturer's manual for permitted acid bath ratio and soak time on that cell.
• See related: the chemical storage and transport SOP for truck segregation and spill response; the shock dosing standard for oxidizer additions on the same visit.