How Much Acid to Lower Pool pH (Dry Acid or HCl)

Which acids can I choose in PoolDose?

Under Dosing, choose pH down. The Acid picker offers Sodium bisulfate (dry acid) and Hydrochloric acid, 33%. Hydrochloric acid is what is sold as muriatic acid in some countries; its strength varies by brand and region, and the app only knows the 33% figure, so read your jug before trusting the ml.

Make sure the pool volume is right in Pool volume (m³).
Open Dosing and pick pH down.
Choose the Acid you own.
Enter Measured pH and Target pH.
Read the amount under Dose: grams or kilograms for dry acid, millilitres or litres for the liquid.

Lowering pH in a 50 m³ pool
pH	Sodium bisulfate	Hydrochloric acid, 33%
7.8 to 7.4	3.00 kg	2.00 l
7.6 to 7.4 (a 0.2 drop)	1.50 kg	1.00 l

In US units a 13,000 gallon pool going from 7.8 to 7.4 shows 6.51 lb of dry acid or 66.6 fl oz of hydrochloric acid. The app switches to pounds at 16 oz and to gallons at 128 fl oz.

How do I add acid in stages?

Pouring the whole dose and re-testing an hour later, before the water has mixed, is an easy way to overshoot. A calmer routine: add about half with the pump running, wait a few hours, test, then decide on the rest. The app gives you the total for the move you typed in, not a schedule, so splitting it is up to you.

There is a shortcut that uses only what the app already does. To add half the dose, type a nearer target. In the 50 m³ pool, Measured pH 7.8 and Target pH 7.6 gives 1.50 kg of dry acid or 1.00 l of hydrochloric acid, which is half of the 3.00 kg or 2.00 l for 7.4. After the test, type the new reading as the measured value and repeat.

The two acids are not interchangeable by eye. For the same drop in a 50 m³ pool the dry acid is a weight (3.00 kg) and the liquid a volume (2.00 l), and each needs its own measuring jug or scale. Pick the one you really have in the shed under Acid before you read the Dose.

What do people get wrong when lowering pH?

Testing too soon. Give the pump a few hours to circulate the acid before you trust the next reading.
Dosing pH before checking alkalinity. If alkalinity is far off, pH will keep drifting back, so the Assistant order is alkalinity, then pH.
Pouring acid in one spot, or pouring water onto it. Follow the label for how to dilute and where to pour, and always put acid into water.
Chasing small differences. A strip is read by eye, so a gap of 0.1 pH is within what the Assistant treats as on target.

What if the pH is too low?

pH up works the other way, with soda ash (sodium carbonate) at 15 g per cubic metre per 0.1 pH. Going from 7.0 to 7.4 in 50 m³ is 3.00 kg.

What are the limits of the dose?

The rate is flat. It does not look at your alkalinity, and real water with high alkalinity resists a pH change more than the app expects.
Acid also lowers alkalinity, so re-test alkalinity after a big pH correction. The Assistant deals with alkalinity first and pH second.
Hydrochloric acid strength varies. Check the percentage on your jug against the 33% the app assumes.

Questions and answers

How much muriatic acid lowers pH in a pool?

At the 33% strength the app assumes, 10 ml per cubic metre for each 0.1 drop. If your jug is weaker than 33%, you need proportionally more, so read the label.

Can I use baking soda or vinegar to change pH?

The app does not cover them as pH tools. It doses sodium bisulfate and hydrochloric acid to lower pH and soda ash to raise it. Baking soda is sodium bicarbonate, which the app doses under Alkalinity, not as a pH fix.

Why does the dose seem big?

Because it is a flat rate for the whole volume and you are asking for the full move at once. Add half, circulate and test.

Does the dose change with alkalinity?

No. The app uses one rate regardless of alkalinity, which is a limit of the maths, not a fact about water.

How much acid to lower pool pH