Lab Q

Unveiling the Mystery: Why Isn’t the pH of Purified Water 7?

When it comes to purified water, many of us expect a pH reading of 7.0, which represents neutrality. However, more often than not, there is almost always a steep deviation from this ideal value. In this blog post, we shall dive into the factors behind the unexpected pH variations in purified water and shed light on the importance of accurate pH measurement in water testing. We will also explore the role of cutting-edge ultrapure water technologies and specialized lab equipment in ensuring reliable results.

Understanding pH in Water

To comprehend the pH deviation, let’s first understand what pH signifies. pH is a numerical expression of the concentration of hydrogen ions (H+) in a solution, serving as an indicator of its acidity or alkalinity. The pH scale ranges from 0 to 14, with values below 7 indicating acidity, above 7 indicating alkalinity, and 7 denoting neutrality.

Challenges in Measuring pH of Purified Water

Purified water systems, commonly known as pure water machine play a crucial role in delivering high-quality water. However, achieving a pH reading of 7.0 in purified water can be challenging due to specific resistance and ionic concentration.

Specific Resistance and Its Significance

The specific resistance of water is a measure of its quality. High-quality deionized water exhibits a specific resistance of at least 10 million ohms (megaohm). As the concentration of ions increases, the specific resistance decreases, affecting the pH measurement in deionized water.

The Absence of Chloride Ions: Inaccurate pH readings can occur when measuring the pH of deionized water due to the absence of chloride ions (>10 megaohms). The reference electrode in pH meters relies on chloride ions to establish an accurate reference point for pH measurement. Without this reference, the pH readings obtained are incorrect.

Carbon Dioxide Absorption: Once deionized water is exposed to the atmosphere, it begins to absorb carbon dioxide (CO2). This dissolved CO2 reacts with water, forming carbonic acid (H2CO3). The presence of carbonic acid in water leads to a shift in pH, causing it to deviate from the expected 7.0 and settle within the range of 6.5-6.8.

Ensuring Accurate pH Measurement in Pure Water

There are solutions to overcome the challenges that are associated with purified water. Some of which are as follows:

Overcoming the Reference Electrode Issue: To establish an accurate reference point, it is recommended to add a few crystals of potassium chloride to the deionized water. This addition helps the pH meter reference electrode to set up a suitable reference, resulting in a pH reading within the desired range of 6.5-6.8.

Verification through Specific Resistance: By measuring the specific resistance of deionized water (>10 megaohms), we indirectly confirm its neutrality. If the specific resistance exceeds this threshold, it indicates a lack of excessive hydrogen or hydroxide ions, suggesting a pH of 7.0 or neutral.

The Importance of Laboratory Water Testing

Accurate pH measurement is vital in laboratory settings, especially when performing water testing and analysis. Sophisticated lab equipment specifically designed for water testing ensures precise pH readings, enabling researchers to obtain reliable results and make informed decisions.

Ultrapure Water Technologies and Specialized Equipment: To achieve superior water quality, ultrapure water technologies come into play. These advanced methods combined with pure water filters, effectively remove impurities. This helps ensure that the pH of purified water remains within the desired range. Specialized lab equipment including pH meters and other analytical tools further contribute to accurate water testing and analysis.


Understanding the factors influencing pH readings is crucial. We can certainly achieve accurate pH measurements by addressing the absence of chloride ions and the influence of carbon dioxide, we can achieve more accurate pH measurements in purified water. In laboratory environments, precise pH measurement using specialized equipment is essential for reliable water testing. Embracing ultrapure water technologies and employing appropriate lab equipment enables us to ensure the highest standards of water quality analysis.