Pure water is expected to have a neutral pH of 7.0. However, laboratory measurements of deionized (DI) water often show an acidic value, typically between 5.5 and 6.5. This lower reading is not a failure of the purification process but results from a rapid and unavoidable chemical interaction with the atmosphere. This scientific paradox is resolved by understanding the chemical definition of neutrality and the specific vulnerability of highly purified water.
Defining the Ideal: What pH 7 Really Means
The concept of a neutral pH of 7.0 is based on the theoretical behavior of absolutely pure water at a standard temperature of 25 degrees Celsius. The pH scale is a logarithmic measure of the concentration of hydrogen ions (H+) in a solution. In pure water, the molecules constantly undergo auto-ionization, where two water molecules react to produce equal concentrations of hydrogen ions (H+) and hydroxide ions (OH-). This equilibrium reaction, \(H_2O \rightleftharpoons H^+ + OH^-\), ensures the concentrations are exactly equal. At 25 degrees Celsius, this concentration is \(1.0 \times 10^{-7}\) moles per liter for both ions. Since pH is the negative logarithm of the hydrogen ion concentration, a concentration of \(10^{-7}\) M translates precisely to a pH of 7.0.
The Reality of Deionized Water Purity
Deionized (DI) water is created by passing source water through ion exchange resins that effectively remove nearly all dissolved mineral salts and charged particles. The success of the deionization process is typically measured by the water’s electrical conductivity, which reaches extremely low levels due to the lack of dissolved ions. This makes DI water highly desirable for applications requiring minimal ionic interference, like in laboratories and electronics manufacturing.
However, the deionization process only removes ionic contaminants. It does not remove dissolved gases or non-ionic organic compounds. Crucially, the absence of dissolved ions means DI water lacks buffering capacity—the presence of minerals that normally stabilize water’s pH. This lack of buffering makes the water extremely susceptible to chemical changes from its immediate environment.
The Atmospheric Culprit: Carbon Dioxide Absorption
The primary reason deionized water does not measure at pH 7.0 is its immediate and unavoidable exposure to the atmosphere. Air contains approximately 420 parts per million of carbon dioxide (\(CO_2\)), which is readily soluble in water. As soon as DI water is dispensed into a container or exposed to the ambient environment, it begins to absorb this atmospheric \(CO_2\) until it reaches a state of equilibrium with the surrounding air.
This absorption process is governed by Henry’s Law, which states that the amount of gas dissolved in a liquid is proportional to the partial pressure of that gas above the liquid. Because the DI water initially contains very little dissolved \(CO_2\), the significant partial pressure of \(CO_2\) in the atmosphere drives the gas into the water rapidly. This means the measured sample of deionized water is no longer pure \(H_2O\), but rather a dilute solution of dissolved atmospheric gas.
The Acidic Shift: How Carbon Dioxide Lowers pH
The dissolved carbon dioxide gas immediately reacts with the water molecules to form carbonic acid. The reaction is represented as \(CO_2\text{(aq)} + H_2O\text{(l)} \rightleftharpoons H_2CO_3\text{(aq)}\). Carbonic acid (\(H_2CO_3\)) is a weak acid, and a portion of it quickly ionizes, releasing free hydrogen ions (H+) into the water. This increase in H+ concentration is the direct cause of the pH drop. The ionization reaction is \(H_2CO_3\text{(aq)} \rightleftharpoons H^+\text{(aq)} + HCO_3^-\text{(aq)}\).
High-quality deionized water typically stabilizes at an acidic pH between 5.5 and 6.5 when measured under normal laboratory conditions. Because the water lacks buffering agents, the addition of even minute amounts of carbonic acid causes a substantial shift in the pH reading. Consequently, the measured pH is the measurement of a very dilute carbonic acid solution, not a true reflection of the water’s ionic purity.