How to Raise Total Alkalinity With Sodium Bicarbonate

Why this matters

Low total alkalinity is the reading behind the pool whose pH will not sit still: down after the acid, up after a rain, down again after a week of trichlor. It is also half of why soft-water plaster etches, because alkalinity is one of the four numbers in the saturation index. Bicarbonate is the gentlest product on the truck, which is exactly why it gets dosed carelessly: a whole bag dumped on a 60 ppm reading, a retest twenty minutes later near the return, and a pool that ends the month at 150 ppm and needs acid and aeration for weeks to come back down. This card is the one job of raising TA, sized right and put in so that the retest means something.

Start state: what you need and what to check first

Bring: sodium bicarbonate (baking soda, sold to the trade as an alkalinity increaser), a scale or a scoop you have weighed, the test kit, and goggles if there is any wind, because the fine powder drifts into eyes. Bicarbonate is a mild irritant, not a corrosive; rinse an eye with clean water if dust gets in, and follow the product label and Safety Data Sheet for anything more.

Check before you size the dose:

• Confirm the TA reading is real. Run the titration on a 25 mL sample (10 ppm per drop on most kits) from elbow depth away from returns, per 'How to Run the Weekly Water Test Panel at the Pool'. A strip reading of low alkalinity is a reason to titrate, not a reason to dose. If the reading surprises you, run it twice.
• The volume. Every pound is multiplied by gallons over 10,000. Use the service record, or work it out per 'How to Estimate Pool and Spa Gallons'.
• The rest of the panel. pH, calcium hardness and CYA all bear on this add. High pH plus high calcium is the combination where a bicarbonate add can cloud the water; very low pH (below the quick-reference range) is usually low TA showing itself and is often fixed by this add alone.
• Order on the deck. TA before pH, because the alkalinity sets how far pH moves for a given dose of anything. One product at a time, per 'The Dosing Order on the Deck: One Product at a Time'. If the pool also needs acid today, the acid is not going in at the same visit as a large bicarbonate add.

Why bicarbonate and not soda ash

Both raise TA and both raise pH; the difference is the ratio. A bicarbonate solution sits at about pH 8.3, so it adds alkalinity while nudging pH only slightly. Soda ash (sodium carbonate) in solution sits near pH 11.3, so per ppm of alkalinity added it moves pH far harder. That makes soda ash the pH product, for a pool whose alkalinity is already in range and whose pH is low, and bicarbonate the TA product. Using soda ash to fix a TA gap overshoots pH long before alkalinity arrives, and on a pool with any calcium it is the quickest way to cloud the water with calcium carbonate. Dose soda ash only from the pH side of the quick-reference card, and never as a substitute here.

Size the dose

The formula and factor belong to 'The Dose Per 10,000 Gallons Factor Card':

dose = (target minus reading) x (gallons / 10,000) x factor

The bicarbonate factor on that card is 1.4 lb per 10 ppm per 10,000 gallons. The target comes from 'Pool Chemistry Quick Reference'; within that range, a pool that runs trichlor (which consumes alkalinity steadily) sits better toward the upper half, and a salt pool or one with a spillway (both drive pH up) sits better toward the lower half.

The split rule. As a shop default, add no more than 20 ppm of TA in one visit when the gap is larger than 30 ppm, and retest before the next portion. Three reasons, in the order they cost you:

1. Overshoot is expensive to undo. TA comes down only through acid plus aeration, cycle by cycle across visits, per 'Muriatic Acid Handling and Dilution'. A half dose that turns out right is cheaper than a full dose that turns out 30 ppm high.
2. A volume error compounds. If the record says 22,000 gallons and the pool holds 18,000, the full dose overshoots by a fifth. The first half dose shows the miss before it is all in.
3. The pH response is easier to read in steps. Each add nudges pH up a little; split adds let you see that rise and stop before pH leaves the range.

The procedure

1. Pump on, valves set to circulate the whole pool, nobody in the water. The product should be carried away from where it lands and mixed into the full volume before the next test.

2. Weigh this visit's portion. Use a scale or a scoop whose weight you have checked once on a scale. Guessing pounds from a bag that says 12 lb on the front is how the full bag ends up in.

3. Broadcast it across the deep end, low and wide. Walk the deep end and throw it in a wide arc from close to the surface, so it lands across several square yards of water rather than in one pile. Bicarbonate dissolves readily; spread out, most of it is gone before it reaches the floor. Stand with the wind at your back so the dust goes out over the water. Not in the skimmer: a slug of powder through the pump and filter does the pool no good, and on a pool with an inline tablet chlorinator the skimmer is a path to the feeder, which the dosing-order card keeps clear of everything.

4. Brush the floor under the broadcast. Undissolved powder on the floor sits as a local high-pH patch and dissolves slowly. Brushing puts it into circulation. If you see a white haze spreading as it dissolves, calcium carbonate is forming; stop adding, brush, and check pH and calcium before the next portion.

5. Leave the pump running and the product to circulate. Retesting TA near the deep end ten minutes later reads the dissolving plume, high, and the next tech reads the diluted pool, lower, and the two disagree. Retest only after at least one full turnover, or on the next visit.

6. Retest TA and pH together. TA tells you whether the dose landed; pH tells you how much it lifted the water and whether the next portion needs anything in between.

Worked example: a 22,000 gallon trichlor pool

A plaster pool on a weekly route, 22,000 gallons on the service record, chlorinated by trichlor tablets in a floater. Readings: pH 7.3, TA 60 ppm (six drops on a 25 mL sample, run twice), calcium hardness 280 ppm, CYA 50 ppm, free chlorine 3.0 ppm. The target TA for this pool is 100 ppm, in the upper half of the quick-reference range because trichlor will keep eating at it.

Full correction. (100 minus 60) x (22,000 / 10,000) x 0.14 lb per ppm = 40 x 2.2 x 0.14 = 12.32 lb.

Split. The gap is 40 ppm, above the 30 ppm split threshold, so this visit adds 20 ppm: 20 x 2.2 x 0.14 = 6.16 lb. Call it 6.2 lb on the scale.

Second direction. 6.16 lb over 2.2 is 2.8 lb per 10,000 gallons; at 1.4 lb per 10 ppm that is 20 ppm. The full 12.32 lb is twice that, 40 ppm. The units cancel: lb per ppm per 10,000 gallons times ppm times (10,000 gallon units) leaves lb.

What pH should do. The pH is below the quick-reference range at 7.3, and no soda ash and no acid go in today. Bicarbonate added to water at 7.3 raises pH because it adds alkalinity without adding the dissolved carbon dioxide that sets pH against it; a 20 ppm rise on a 60 ppm base is a third more bicarbonate, which moves pH up by roughly a tenth. Expect 7.4 to 7.5 next week.

The broadcast. 6.2 lb thrown across the deep end in three arcs, pump on, floor brushed, no haze. The tablet floater is left alone.

Next visit. TA titrates at 80 ppm, pH 7.4. TA landed exactly as predicted at 60 plus 20, and pH rose by a tenth, so the gallons and the factor are confirmed. The second 6.2 lb goes in the same way.

The visit after. TA 100 ppm, pH 7.5. Both in range. The ticket shows each reading, the target, the gallons, the factor and the pounds, so a third tech could audit it.

What a full dump would have done. 12.3 lb on the first visit lands TA at 100 if the gallons are right. If the pool actually held 18,000 gallons, the same 12.32 lb is 12.32 / 1.8 = 6.84 lb per 10,000 gallons, about 49 ppm, landing TA near 109. Not a disaster here, but on a pool targeting the top of the range it would put TA over 120, and that takes acid plus aeration over several visits to fix. The split showed whether the record's volume was right after 6 lb, while half the bag was still in the truck.

What changes the call

• Trichlor or dichlor as the sanitizer. Both are acidic and pull TA down every week. A TA that drifts down 10 to 20 ppm a month on a tablet pool is the sanitizer, not a leak; dose on a routine rather than reacting each time. See 'What Each Chlorine Product Does to the Other Readings'.
• Liquid chlorine or cal-hypo as the sanitizer. Both raise pH as they are added; the acid that follows to correct pH consumes TA. The TA drift here is downstream of the acid, and a slightly lower TA target slows the cycle.
• High CYA. The TA titration counts cyanurate as alkalinity, so at 80 ppm or more CYA the carbonate alkalinity the plaster and pH care about is well below the reading. That correction lives in the saturation index; run it per 'How to Calculate the Saturation Index on the Pool Deck' before raising TA to "fix" balance.
• Rain and a heavy fill. Rainwater carries next to no alkalinity, so a pool that took a large top-up from rain or a soft source reads low across TA and calcium at once. Dose to the reading in front of you, not last week's.

Finish test

Accept the job when TA, retested after at least one turnover, sits in the quick-reference range within one titration drop of the predicted value, and pH is still inside its range. If pH has left the top of its range, lower it per 'How to Lower pH With Muriatic Acid Using the Acid Demand Test' at a later visit, accepting that this costs a few ppm of the TA you just added. If TA lands more than a drop short, check the volume on the record before adding more; landing short by the same fraction on bicarbonate and calcium says the gallons are wrong, not the product.

References

• See related: The Dose Per 10,000 Gallons Factor Card (formula, bicarbonate factor)
• See related: Pool Chemistry Quick Reference (TA and pH ranges)
• See related: Muriatic Acid Handling and Dilution (bringing TA back down)
• See related: The Dosing Order on the Deck: One Product at a Time; How to Calculate the Saturation Index on the Pool Deck
• Product label and Safety Data Sheet for the sodium bicarbonate in use