Understanding the pH of Ultrapure Water

High purity water system operators frequently inquire about the pH of ultrapure water, a topic that presents unique challenges due to the water’s purity and lack of ion content.

What is pH?

In aqueous solutions, pH represents the negative logarithm of the hydrogen ion concentration, denoted as -Log[H+]. This measurement determines whether a solution is acidic or basic. A pH of 7 indicates neutrality, where the concentrations of hydrogen ions (H+) and hydroxyl ions (OH-) are balanced.

Challenges in Measuring pH of Ultrapure Water

Ultrapure water, with its specific resistance typically exceeding 10 megohms-cm, contains minimal ionic species that can influence pH. Traditional pH measurement techniques using standard electrodes often yield inaccurate readings. One type of pH electrode may indicate a low pH (acidic), around 5.5 – 5.8, while another type may suggest a high pH (basic), approximately 8.3 – 8.8. This discrepancy arises because the electrodes cannot establish a reliable reference point in the absence of ions, particularly chloride ions.

The Role of Specific Resistance

The purity of ultrapure water is better assessed through its specific resistance rather than pH. Water with a specific resistance exceeding 10 megohms-cm lacks sufficient ion content to alter its neutral pH state.

Understanding the Deionization Process

Ultrapure water is produced through a rigorous deionization process involving mixed beds of cation and anion exchange resins. This process effectively removes all impurities, leaving only equal concentrations of hydrogen (H+) and hydroxyl (OH-) ions, resulting in pure H2O.

Factors Affecting pH in Ultrapure Water

Despite its neutral nature, ultrapure water is highly reactive due to its purity. It readily absorbs gases such as carbon dioxide (CO2) from the atmosphere, forming carbonic acid (H2CO3) and leading to a decrease in pH. Even trace amounts of contaminants can influence pH, as there are no buffering salts present to stabilize it.

Solutions for Accurate pH Measurement

To accurately measure the pH of ultrapure water, a small amount of potassium chloride can be added to provide the necessary ions for electrode reference. This addition stabilizes the pH measurement to typically fall between 6.5 and 6.8, regardless of the type of pH electrode used.

Ensuring Consistent Ultrapure Water Quality

The design and maintenance of ultrapure water systems are critical to ensuring a specific resistance greater than 10 megohms-cm. Systems must be designed to prevent any degradation in water quality as deionization resins exhaust, which could otherwise lead to acidic or basic contaminants passing into the final product water.


Understanding the pH of ultrapure water requires acknowledging its unique properties and challenges in measurement. By focusing on specific resistance and employing proper techniques for pH measurement, users can ensure the quality and reliability of their ultrapure water systems for various scientific and industrial applications.