What Is pH in Water?

pH is a measure of the hydrogen ion concentration in water, expressed on a logarithmic scale from 0 to 14. A pH of 7.0 is neutral, values below 7.0 indicate acidic water, and values above 7.0 indicate alkaline (basic) water. Because the scale is logarithmic, each unit represents a tenfold change in hydrogen ion concentration: water at pH 6.0 is ten times more acidic than water at pH 7.0.

Natural water pH is determined primarily by the geological composition of the watershed or aquifer. Water passing through limestone and dolomite formations dissolves calcium and magnesium carbonates, resulting in alkaline pH (typically 7.0-8.5). Water in granite and sandstone formations, or in areas with high organic matter (bogs, peat), tends to be acidic (pH 5.0-6.5).

Factors that lower water pH include acid rain (from sulfur dioxide and nitrogen oxide emissions), decomposition of organic matter producing carbonic and humic acids, acid mine drainage, and volcanic activity. Factors that raise pH include contact with carbonate minerals, algal photosynthesis (which consumes CO2), and lime treatment.

pH is closely linked to alkalinity, which represents the buffering capacity of water against pH changes. Alkalinity is primarily composed of bicarbonate, carbonate, and hydroxide ions. Water with low alkalinity is poorly buffered and susceptible to rapid pH changes.

Effects of pH on Water Quality and Infrastructure

Water pH has profound effects on both water treatment processes and distribution system integrity. The two primary operational concerns are corrosion and scaling.

Low pH (acidic) water is corrosive to metal plumbing materials. It dissolves copper from pipes, causing blue-green staining and elevated copper levels. It accelerates lead leaching from lead service lines, lead solder, and brass fixtures, posing a direct health risk. It corrodes iron and steel pipes, producing rusty water and eventually causing pipe failures. The Langelier Saturation Index (LSI) and other indices are used to predict whether water will be corrosive or scale-forming based on pH, alkalinity, calcium, TDS, and temperature.

High pH (alkaline) water tends to form calcium carbonate scale on pipe surfaces, water heaters, boilers, and RO membranes. While a thin protective scale layer is desirable for corrosion control in distribution systems, excessive scaling restricts flow, reduces heat transfer efficiency, and fouls treatment equipment.

pH also affects the effectiveness of water treatment processes. Chlorine disinfection is most effective at lower pH (below 7.5) because the more potent hypochlorous acid (HOCl) predominates over the less effective hypochlorite ion (OCl-) at lower pH. Coagulation efficiency is pH-dependent. Iron and manganese oxidation requires elevated pH for optimal performance.

pH Guidelines for Drinking Water

The EPA SMCL of 6.5-8.5 is a non-enforceable aesthetic guideline. However, pH control is effectively mandated through the Lead and Copper Rule, which requires corrosion control treatment that typically involves maintaining optimal pH and alkalinity levels.

How to Test pH in Water

pH is measured using electrometric methods (pH meter with glass electrode) per Standard Method 4500-H+B, which provides the most accurate results. Calibration with two or more standard buffer solutions (typically pH 4.0, 7.0, and 10.0) is required before each measurement session.

Field measurements using colorimetric test kits or pH paper strips provide quick screening results but are less precise. For water treatment system design and compliance monitoring, electrometric measurement is the standard method.

pH should be measured as soon as possible after sample collection because dissolved carbon dioxide can escape from the sample, causing pH to rise. Temperature significantly affects pH measurement and must be compensated for, either automatically (using a pH meter with temperature compensation) or manually.

Treatment Methods for pH Adjustment

pH Increase (Acid Neutralization)

For acidic water requiring pH increase, several treatment approaches are available:

• Soda ash (sodium carbonate) injection: Chemical feed systems inject soda ash solution to raise pH and increase alkalinity. This is the most common treatment for acidic well water and is effective for moderate pH adjustment (from 5.5-6.5 up to 7.0-8.0).
• Caustic soda (sodium hydroxide) injection: Sodium hydroxide is used when greater pH increase is needed or when alkalinity addition is not desired. It is more concentrated than soda ash and requires careful dosing.
• Calcite (calcium carbonate) contactors: Calcite filter systems pass acidic water through beds of crusite marble chips, which dissolve to raise pH and add calcium and alkalinity. These systems are passive and require only periodic media replenishment.
• Corosex (magnesium oxide): Used in combination with calcite for very low pH water (below 5.5) where calcite alone cannot raise pH sufficiently.

pH Decrease (Alkalinity Reduction)

For alkaline water requiring pH reduction:

• Acid injection: Sulfuric acid, hydrochloric acid, or carbon dioxide injection lowers pH. CO2 injection is preferred in many applications because it produces carbonic acid, which also adds alkalinity as bicarbonate.
• Reverse osmosis: RO systems reduce alkalinity and TDS, which can lower or adjust pH depending on water chemistry. RO permeate typically has a lower pH than feed water due to the removal of bicarbonate alkalinity.

Frequently Asked Questions

What is the ideal pH for drinking water?

The EPA recommends a pH range of 6.5 to 8.5 for drinking water (Secondary MCL). The WHO does not set a health-based guideline but notes that water in the pH range of 6.5-8.5 is generally suitable for domestic use. Most drinking water treatment plants target a pH of 7.0-8.0, which balances corrosion control with disinfection effectiveness.

Why does low pH water cause pipe corrosion?

Low pH (acidic) water is corrosive because it contains excess hydrogen ions that dissolve metals from pipes and fixtures. In copper plumbing, low pH water causes blue-green staining and elevated copper levels. In lead service lines, low pH accelerates lead leaching into drinking water. Corrosion is influenced by pH together with alkalinity, dissolved oxygen, temperature, and TDS.

What is the relationship between pH and water treatment?

pH affects virtually every water treatment process. Chlorine disinfection is most effective below pH 7.5. Coagulation with aluminum sulfate works best between pH 6.0 and 7.0. Iron and manganese oxidation requires elevated pH. RO membrane performance and scaling potential are pH-dependent. Lime softening operates at pH 10-11. Proper pH control is essential for optimizing any water treatment system.

Need a pH Adjustment Solution?

ForeverPure provides pH adjustment systems for commercial and industrial water treatment, including chemical feed systems, calcite contactors, acid injection equipment, and integrated corrosion control solutions. Our engineering team designs pH control systems based on your water chemistry, target pH, and application requirements.

Contact ForeverPure for a customized pH adjustment solution.