Both lower pH and total alkalinity. The difference is in the form, the handling and what they leave behind in the water.
The short answer: muriatic acid is liquid hydrochloric acid with strong fumes; dry acid is sodium bisulfate granules. Both lower pH and alkalinity (Taylor). TroubleFreePool recommends muriatic acid and advises against dry acid in plaster pools and salt pools, because its sulfates can damage plaster and salt cell plates, and only draining removes them.
Part of: Pool pH and Alkalinity: How They Work Together, our overview of every pH and alkalinity fix.
Side by side
| Muriatic acid | Dry acid | |
|---|---|---|
| What it is | Hydrochloric acid solution (Sunnyside: 31.45%) | Sodium bisulfate (HTH pH Down: 92%) |
| Form | Liquid | Granules |
| Lowers | pH and alkalinity | pH and alkalinity |
| Handling | Corrosive liquid and vapor (label) | Granules; follow your product's label (HTH pH Down, for example, says to broadcast it over the deep end and never pre-dissolve it in a plastic bucket) |
| Sulfates | None (our note: hydrochloric acid contains no sulfate) | Adds sulfates, removed only by draining (TroubleFreePool) |
| Plaster or salt pools | Recommended by TroubleFreePool | Not recommended by TroubleFreePool |
| Example dose | About 1 pint per 10,000 gal (Sunnyside label) | 16 oz per 10,000 gal at pH 8.0 to 8.4 (HTH table) |
Doses are examples from one product each; always use the label of the product you buy.
When muriatic acid makes sense
For plaster pools, salt pools and regular pH control, following TroubleFreePool. It needs more care: the Sunnyside label warns of a corrosive liquid and vapor, says to avoid breathing the vapor and never to mix it with chlorine-type bleaches. See muriatic acid for pools for handling.
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Lowers pH and alkalinity Muriatic acid (hydrochloric acid) for poolsLiquid acid; strength differs per bottle, so the dose on your bottle's label is the one to follow. Shipping rules differ by product and state.
When dry acid makes sense
If you do not want to handle liquid acid and fumes, and your pool is vinyl or fiberglass without a salt system (our reading of TroubleFreePool's warning, which names plaster and salt pools). Watch the sulfates if you use it often.
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Lowers pH and alkalinity Dry acid (sodium bisulfate), sold as "pH Down"Granules instead of liquid. Check the label for sodium bisulfate and its dosage table.
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Protective gear Rubber gloves and safety gogglesThe acid label and the CDC both say to wear protective gear when you handle pool chemicals.
Sources
- Taylor Technologies: LSI calculator (both lower pH and alkalinity)
- TroubleFreePool: Lower total alkalinity (dry acid not recommended in plaster and salt pools, sulfates, draining)
- TroubleFreePool: ABCs of pool water chemistry (use muriatic acid, avoid dry acid in plaster or salt pools)
- Sunnyside Muriatic Acid label, 31.45% hydrochloric acid (31.45% hydrochloric acid, dose, hazards)
- HTH Pool Care pH Down (read 3 Nov 2026) (92% sodium bisulfate, dosage table, how to add)
Sources read on November 4, 2026. Product labels change: the label on the product you buy always wins.
