pH adjustment chemicals for water treatment are 10 acids and alkalis, from sulfuric acid and carbon dioxide to caustic soda, lime and soda ash, that operators dose to move pH and alkalinity into the range each treatment step or discharge permit needs. Each one changes alkalinity by a different amount per mg/L (ppm), so which one fits which water?
They form 4 groups (acids such as sulfuric acid, also spelled sulphuric acid, hydroxide alkalis, carbonate alkalis and ammonia) and one of the 16 families of water treatment chemicals.
Caustic soda adds 1.25 mg/L of alkalinity as CaCO3 per mg/L, and the US secondary pH range is 6.5 to 8.5.
Every value on this page is checked against primary sources (NSF listings, eCFR, EPA, AWWA, CEN, WHO); see our methodology and fact-checking process.
What Is pH Adjustment in Water Treatment?#
pH adjustment in water treatment is the controlled dosing of an acid, such as sulfuric acid or carbon dioxide, or an alkali, such as caustic soda, lime or soda ash, to hold pH, and often alkalinity, at a target set by the process, the distribution system or a discharge permit. Natural-water pH follows the carbonate system (WHO), so several adjusters also change alkalinity.
Target values by system are listed under pH in water treatment.
Why do treatment plants adjust pH?#
Treatment plants adjust pH for 4 reasons: coagulation, lead and copper corrosion control, cooling and membrane scale balance, and wastewater discharge. The 4 reasons are listed below.
- Coagulation: acid or alkali sets pH, never extra coagulant (Irish EPA).
- Corrosion control: lead target pH 8.8 to 10 (US EPA OCCT, 2016).
- Cooling: acid to pH 8.7 cuts alkalinity at 15 cycles from 600 to 310 ppm as CaCO3 (Veolia).
- Discharge: POTW effluent pH 6.0 to 9.0 (40 CFR 133.102(c)).
Coagulation pH is compared under coagulants for water treatment.
How Do pH Adjustment Chemicals Work?#
pH adjustment chemicals work in 2 ways: acids and alkalis neutralize each other to shift pH, and carbonates and CO2 also change alkalinity and dissolved inorganic carbon, which hold the new pH.
Neutralization, alkalinity and dissolved inorganic carbon#
Each mg/L of caustic soda adds 1.25 mg/L of alkalinity as CaCO3, each mg/L of sulfuric acid removes 1.02 mg/L, and carbon dioxide lowers pH without changing alkalinity (US EPA OCCT, 2016; acid value calculated). Carbon dioxide adds 0.27 mg C/L of dissolved inorganic carbon (DIC) per mg/L instead.
P and M end points are explained under alkalinity of water.
Why hydroxides overshoot in poorly buffered water#
Hydroxide alkalis overshoot in soft, poorly buffered water because they add hydroxide without carbonate buffer, while soda ash and sodium bicarbonate add buffer and hold pH firmly (US EPA OCCT, 2016). Saturated lime reaches pH 12.45 at 25 °C (77 °F); magnesium hydroxide is self-buffering.
10 pH Adjustment Chemicals in 4 Groups#
The 10 pH adjustment chemicals used in water treatment fall into 4 groups: 3 acids that lower pH, 4 hydroxide alkalis, 2 carbonate alkalis and ammonia, the volatile alkali of steam cycles. Table 1 compares the groups.
| Group | Members | Direction | Effect on alkalinity | Maximum pH of the chemical (NLA, 25 °C) | Typical role |
|---|---|---|---|---|---|
| Acids | sulfuric acid, hydrochloric acid, carbon dioxide | lower pH | mineral acids remove it; CO2 leaves it unchanged | n/a | cooling and RO feed pH, neutralizing alkaline waste, recarbonation |
| Hydroxide alkalis | sodium hydroxide, hydrated lime, quicklime, magnesium hydroxide (magnesium oxide as a section) | raise pH | adds the most per mg/L | caustic 14; lime 12.45; magnesium hydroxide self-buffering | corrosion control, softening, neutralizing acid waste |
| Carbonate alkalis | soda ash, sodium bicarbonate | raise pH (bicarbonate slightly) | adds alkalinity and DIC | soda ash above 11 | buffering, corrosion control, noncarbonate hardness removal |
| Ammonia | ammonia (anhydrous and aqueous) | raise pH | n/a | n/a | chloramination, all-volatile steam-cycle pH |
1. Acids that lower pH (3)#
Acids lower pH; this group has 3 chemicals: sulfuric acid, hydrochloric acid and CO2.
Sulfuric acid#
Sulfuric acid (H2SO4) is the default acid for cooling-tower alkalinity control and wastewater neutralization, supplied as 93 % (66 Bé) or 98 % acid.
Hydrochloric acid#
Hydrochloric acid (HCl), supplied at 31.45 % (20 Bé) or 35.21 % (22 Bé), replaces sulfuric acid where calcium sulfate limits cooling cycles.
Carbon dioxide (CO2)#
Carbon dioxide (CO2) lowers pH without removing alkalinity, which makes it the recarbonation chemical after lime softening.
2. Hydroxide alkalis (4)#
Hydroxide alkalis add hydroxide; this group has 4 chemicals: caustic soda, hydrated lime, quicklime and magnesium hydroxide.
Sodium hydroxide (caustic soda)#
Sodium hydroxide (NaOH, caustic soda) is the fastest-acting alkali, sold mainly as a 50 % liquid that crystallizes near 12 °C (54 °F).
Hydrated lime#
Hydrated lime (Ca(OH)2) is the softening and neutralization alkali, fed as 95 to 98 % powder or slurry of about 30 to 40 % solids.
Quicklime#
Quicklime (CaO) is unslaked lime: slaking releases about 1,140 kJ/kg (490 Btu/lb), and 1 lb equals 1.32 lb of hydrated lime.
Magnesium hydroxide#
Magnesium hydroxide (Mg(OH)2) is a self-buffering 58 to 60 % slurry that reacts slowly and holds sewer pH above 8.0 against hydrogen sulfide.
3. Carbonate alkalis (2)#
Carbonate alkalis add buffer; this group has 2 chemicals: soda ash and sodium bicarbonate.
Soda ash#
Soda ash (Na2CO3) adds carbonate alkalinity and removes noncarbonate hardness in lime-soda softening.
Sodium bicarbonate#
Sodium bicarbonate (NaHCO3) adds alkalinity with only a small pH rise, so it fits water short of alkalinity rather than pH.
4. Ammonia (1)#
Ammonia is the volatile alkali of the group, dosed for chloramination and steam-cycle pH.
Ammonia (anhydrous and aqueous)#
Ammonia (NH3) is fed as anhydrous gas or 10 to 30 % aqua ammonia to form chloramines and to hold boiler feedwater at pH 8.8 to 9.8.
Which pH Adjuster for Which Water? Selection by Goal and Application#
pH adjuster selection depends on 3 questions: which way pH moves, what alkalinity should do, and which system and permit the water serves.
Raise pH, add alkalinity or lower pH#
The goal picks the chemical: hydroxides raise pH, carbonates add alkalinity, mineral acids lower pH and alkalinity, and carbon dioxide lowers pH alone. The 3 goals are listed below.
- Raise pH: caustic soda or hydrated lime.
- Add alkalinity: soda ash or sodium bicarbonate.
- Lower pH: sulfuric or hydrochloric acid, or CO2 to keep alkalinity.
Small systems with pH below 7.2 and alkalinity below 100 mg/L as CaCO3 also use contactors of calcite media at 20 to 40 minutes contact (US EPA OCCT).
By application: drinking water, cooling, membranes, boilers and wastewater#
Each application has a default pH chemical: caustic soda or soda ash for corrosion control, lime and CO2 for softening, sulfuric acid for cooling and RO feed, ammonia for steam cycles. Table 2 gives the key numbers.
| Application | Default pH chemicals | Why | Key number |
|---|---|---|---|
| drinking water treatment chemicals | acid or alkali at rapid mix; caustic soda, lime or soda ash after filtration | coagulation pH window; finished-water pH | SMCL pH 6.5 to 8.5 (40 CFR 143.3) |
| corrosion control treatment | caustic soda, soda ash, sodium bicarbonate, CO2 | lead and copper solubility | pH 8.8 to 10, DIC 5 to 10 mg C/L (US EPA OCCT) |
| lime softening | hydrated lime, quicklime, soda ash, CO2 | precipitate calcium and magnesium, then restabilize | calcium removal at pH 10.3 to 10.6; recarbonate to below pH 8.7 |
| remineralization of desalinated and soft water | lime or calcite with CO2 | rebuild alkalinity and calcium | 0.88 mg/L CO2 per mg/L as CaCO3 with lime, 0.44 with calcite (calc.) |
| chloramination | ammonia (anhydrous or aqua) | form monochloramine | Cl2:NH3-N 4.5:1 to 5:1 |
| cooling tower water treatment | sulfuric acid; hydrochloric acid where CaSO4 limits cycles | lower alkalinity and scaling tendency | 15 cycles: 600 ppm alkalinity without acid, 310 ppm as CaCO3 with acid to pH 8.7 (Veolia) |
| RO pretreatment chemistry | sulfuric acid, hydrochloric acid, CO2 | lower concentrate CaCO3 scaling | seawater RO typically 10 mg/L H2SO4 for feed pH about 7 (DuPont FilmTec manual) |
| all-volatile treatment (AVT) | ammonia | feedwater pH without dissolved solids | feedwater pH 8.8 to 9.8 |
| wastewater pH neutralization | caustic soda, lime, magnesium hydroxide, soda ash; sulfuric acid, CO2 | meet the discharge range | effluent pH 6.0 to 9.0; no discharge below pH 5.0 to a POTW |
| hydrogen sulfide and odor control chemicals | magnesium hydroxide | keep sulfide as non-volatile HS- | collection-system pH above 8.0 |
Every wastewater program is compared on wastewater treatment chemicals.
pH Adjuster Dosage: Alkalinity and Neutralization Equivalents#
pH adjuster doses follow titration or stoichiometry: neutralizing 100 lb (or kg) of sulfuric acid takes 57.2 of quicklime, 81.6 of caustic soda or 108.1 of soda ash (National Lime Association, 2007). Table 3 lists the equivalents.
| Chemical | Alkalinity change per mg/L (mg/L as CaCO3) | DIC added per mg/L (mg C/L) | lb per 100 lb H2SO4 | lb per 100 lb HCl |
|---|---|---|---|---|
| Sulfuric acid | removes 1.02 (calc.) | 0 | n/a | n/a |
| Hydrochloric acid | removes 1.37 (calc.) | 0 | n/a | n/a |
| Carbon dioxide | 0 | +0.27 | n/a | n/a |
| Sodium hydroxide | +1.25 | 0 | 81.6 | 109.7 |
| Hydrated lime | +1.35 | 0 | 75.5 (calc.) | 101.6 (calc.) |
| Quicklime | +1.78 (calc.) | 0 | 57.2 | 76.9 |
| Magnesium hydroxide | +1.72 (calc.) | 0 | n/a | n/a |
| Soda ash | +0.94 | +0.11 | 108.1 | 145.3 |
| Sodium bicarbonate | +0.60 | +0.14 | n/a | n/a |
| Basis and sources | per mg/L or per 100 lb of 100 % chemical | US EPA OCCT (March 2016 update) | National Lime Association (2007) | calc. = from molar masses |
Values refer to 100 % chemical; potable doses also stay within the product MUL (Table 4).
How is a pH adjuster dose turned into a feed rate?#
Worked example: dosing 10 mg/L (ppm) of caustic soda as NaOH into 10,000 m³/d (2.64 MGD) needs 100 kg/d (about 220 lb/day) of NaOH, or about 132 L/d (35 gal/d) of 50 % caustic. Inputs: purity 0.50 at 1.52 g/mL, or 760 g/L (6.34 lb/gal) NaOH; lb/day = mg/L x MGD x 8.34 / purity. The chemical dosage calculator converts mg/L into kg/d, lb/day and pump settings.
How Is pH Adjustment Controlled and Tested?#
pH adjustment is controlled by an online pH loop at each feed point and confirmed by grab samples analyzed within 15 minutes (40 CFR 136.3).
Methods include SM 4500-H+ B and ASTM D1293-18; alkalinity is titrated to pH 4.5.
Scale balance after pH adjustment is checked with the Langelier Saturation Index.
Which Standards and Regulations Apply to pH Adjustment Chemicals?#
pH adjustment chemicals for drinking water are governed in 3 layers: NSF/ANSI/CAN 60 certification, AWWA and EN product standards, and pH limits. Neither mineral acid has an AWWA standard.
Of the 10, only sodium hydroxide and sodium carbonate are listed in 21 CFR 173.310.
Product editions are tracked under AWWA B-series standards.
NSF/ANSI/CAN 60 maximum use levels for pH adjusters#
NSF/ANSI/CAN 60 maximum use levels for pH adjusters are product-specific and stated as product, most commonly 40 mg/L for hydrochloric acid, 50 for sulfuric acid, 100 for caustic soda and 650 for dry hydrated lime (NSF listings, 2026-10-21). Table 4 sets them beside the 2020 edition's typical use levels.
| Chemical | Most common product MUL (mg/L as product) | Strength notes | 2020 edition typical use level (mg/L, not a product limit) |
|---|---|---|---|
| Sulfuric acid | 50 | 93 to 98 % grades; 50 % products 91 to 93; 25 % products 186 | 50 |
| Hydrochloric acid | 40 | 31 to 36 % products 40 to 45; 15 % 80 to 93; 10 % 140; about 13 to 14 as 100 % HCl | 40 |
| Carbon dioxide | 100 | also 200 and 25 | 600 |
| Sodium hydroxide | 100 | 50 % and solid grades; 25 % 200; 20 % 250; about 50 as 100 % NaOH | 100 |
| Hydrated lime | 650 (dry) | slurries mostly 1,000 to 2,030 as product | 650 |
| Quicklime | 500 | also 326 | 500 |
| Magnesium hydroxide | 300 | also 150; magnesium oxide 100 | 150 (magnesium oxide 100) |
| Soda ash | 150 (dry) | dry grades 100 to 200; 10 % solution 1,000 | 100 |
| Sodium bicarbonate | 200 | also 100 | 100 |
| Ammonia | anhydrous 5; aqua 10 | aqua product-specific, 1 to 33 | n/a |
| Note | MULs count the total dose across all application points and change as listings change | NSF listings, 2026-10-21 | NSF/ANSI/CAN 60-2020 Table 5.1 |
Dilute grades carry higher MULs: 25 % caustic at 200 mg/L equals 50 % caustic at 100 mg/L.
MUL rules are explained on NSF/ANSI/CAN 60.
pH limits for drinking water and wastewater discharges#
The US has no enforceable drinking-water pH limit: the secondary standard is 6.5 to 8.5 (40 CFR 143.3), and State-designated corrosion control requires pH 7.0 or above (40 CFR 141.82(f)(2), LCRI). Further limits are listed below.
- LCRI (2024-10-30): compliance from 2027-11-01.
- EU Directive (EU) 2020/2184: indicator pH 6.5 to 9.5.
- WHO: no health-based guideline value.
- 40 CFR 133.102(c): POTW effluent pH 6.0 to 9.0.
- 40 CFR 403.5(b)(2): no discharge to a POTW below pH 5.0.
- 40 CFR 261.22: waste at pH 2 or below, or 12.5 or above, is D002.
Corrosion-control parameters sit in the Lead and Copper Rule.
Complete List of pH Adjustment Chemicals: All 10 in One Table#
The complete list below gives all 10 pH adjustment chemicals with CAS number, group, function, systems, NSF/ANSI/CAN 60 MUL and AWWA and EN standards.
| # | Chemical | CAS | Group | Function | Systems | NSF 60 most common MUL (mg/L as product) | AWWA / EN | 21 CFR 173.310 |
|---|---|---|---|---|---|---|---|---|
| 1 | sulfuric acid | 7664-93-9 | acid | lowers pH and alkalinity; resin regenerant | cooling; RO feed; drinking water; wastewater | 50 | none / EN 899:2022 | not listed |
| 2 | hydrochloric acid | 7647-01-0 | acid | lowers pH; resin regenerant | drinking water; cooling; ion exchange | 40 | none / EN 939:2016 | not listed |
| 3 | carbon dioxide (CO2) | 124-38-9 | acid gas | lowers pH, keeps alkalinity; recarbonation | softening; corrosion control; remineralization | 100 | AWWA B510 / EN 936 | not listed |
| 4 | sodium hydroxide (caustic soda) | 1310-73-2 | hydroxide alkali | raises pH and alkalinity; resin regenerant | drinking water; wastewater; ion exchange | 100 | ANSI/AWWA B501-25 / EN 896 | listed |
| 5 | hydrated lime (calcium hydroxide) | 1305-62-0 | hydroxide alkali | softening; pH; neutralization | drinking water; wastewater; biosolids | 650 (dry) | ANSI/AWWA B202-25 / EN 12518 | not listed |
| 6 | quicklime (calcium oxide) | 1305-78-8 | hydroxide alkali (after slaking) | softening; neutralization | drinking water; wastewater | 500 | ANSI/AWWA B202-25 / EN 12518 | not listed |
| 7 | magnesium hydroxide | 1309-42-8 | hydroxide alkali | self-buffering neutralization; sulfide control | wastewater; collection systems | 300 | none / none identified | not listed |
| 8 | soda ash (sodium carbonate) | 497-19-8 | carbonate alkali | pH and alkalinity; softening | drinking water; boiler pretreatment | 150 (dry) | AWWA B201 / EN 897 | listed (as sodium carbonate) |
| 9 | sodium bicarbonate | 144-55-8 | carbonate alkali | alkalinity, small pH rise | drinking water; wastewater | 200 | none / EN 898 | not listed |
| 10 | ammonia (anhydrous and aqueous) | 7664-41-7; 1336-21-6 (ammonium hydroxide) | volatile alkali | chloramination; AVT pH | drinking water; boiler | anhydrous 5; aqua 10 | ANSI/AWWA B305-22, B306-22 / none | not listed |
Other chemicals are in the water treatment chemical database.
Who Supplies pH Adjustment Chemicals and Programs?#
pH adjustment chemicals are bulk commodities in which water treatment is a small buyer, using less than 1 % of US sulfuric acid and about 7 % of US lime (US EPA, 2018 data).
Distributors are listed under water treatment chemical suppliers.
A program provider runs the pH loop; a supplier delivers to an NSF/ANSI/CAN 60 specification. Providers are compared under water treatment companies.
Are pH Adjustment Chemicals Safe to Handle?#
Sulfuric acid (index 016-020-00-8) and caustic soda (index 011-002-00-6) each carry the full harmonised CLP Annex VI classification Skin Corr. 1A H314; hydrated lime has no Annex VI entry. Diluting 50 % caustic at 43 °C (110 °F) to 20 % heats it to 54 °C (130 °F).
Which pH chemicals must never be mixed?#
Acids must never meet sodium hypochlorite, because the mixture releases chlorine gas: hypochlorite with hydrochloric acid caused 24 % and with sulfuric acid 16 % of 72 unloading accidents from 1953 to 2019.
Every incompatible pair is listed under water treatment chemical compatibility.
How do you lower pH in water treatment?#
Water treatment plants lower pH by dosing sulfuric or hydrochloric acid, which also removes alkalinity, or carbon dioxide, which keeps alkalinity, at a mixed feed point under online pH control.
Does sodium bicarbonate raise or lower pH?#
Sodium bicarbonate raises pH slightly and adds 0.60 mg/L of alkalinity as CaCO3 per mg/L, so treatment plants use it to build alkalinity rather than to shift pH.