Water is a fundamental substance, universally recognized for its ability to dissolve and transport various materials. Its properties are crucial across many aspects of life, from biological processes to industrial applications. One property often misunderstood, especially concerning highly processed forms, is its pH. This article explores the nature of pH and the factors that influence the pH of purified water.
What pH Measures
The term pH serves as a standardized measure to indicate how acidic or basic an aqueous solution is. This scale ranges from 0 to 14. A pH value of 7.0 signifies a neutral solution. Solutions with a pH below 7.0 are considered acidic, while those above 7.0 are categorized as basic, also known as alkaline.
This measurement is directly related to the concentration of hydrogen ions (H+) within the solution. A higher concentration of hydrogen ions corresponds to a lower pH value and increased acidity. Conversely, a lower concentration of hydrogen ions indicates a higher pH and greater basicity. The pH scale is logarithmic, meaning that each whole number change represents a tenfold difference in hydrogen ion concentration. For example, a solution with a pH of 6 is ten times more acidic than one with a pH of 7.
Types of Purified Water
“Purified water” is a broad category encompassing water treated to remove impurities. Distilled water is produced by boiling water into vapor and then condensing it back into a liquid. This process leaves behind most impurities, such as minerals and non-volatile organic compounds, which do not vaporize with the water.
Deionized (DI) water is water from which mineral ions have been removed through an ion-exchange process. This method typically involves passing water through specialized resins that exchange unwanted ions for hydrogen (H+) and hydroxyl (OH-) ions, which then combine to form water. Reverse osmosis (RO) water is created by forcing water under pressure through a semi-permeable membrane. This membrane allows water molecules to pass through while blocking larger dissolved solids, impurities, and contaminants.
The pH Value of Purified Water
Chemically pure water, consisting solely of H2O molecules at 25°C, theoretically has a neutral pH of 7.0. This ideal state is rarely maintained in practical settings. The moment purified water is exposed to the atmosphere, its pH shifts from this neutral point.
The primary reason for this deviation is the absorption of carbon dioxide (CO2) from the air. When CO2 dissolves in water, it reacts to form carbonic acid (H2CO3). This carbonic acid then dissociates, releasing hydrogen ions (H+), which increases the acidity of the water. As a result, purified water exposed to air commonly has a pH between 5.5 and 7.0.
Factors Affecting Purified Water’s pH
The pH of purified water is susceptible to external influences due to its lack of dissolved minerals, which act as buffering agents. Atmospheric carbon dioxide is the most significant factor; longer exposure to air means more CO2 absorption and a more acidic pH. The longer purified water remains open to the air, the more CO2 it absorbs, leading to a more acidic pH.
Beyond CO2 absorption, other factors can influence pH. Trace impurities, potentially leaching from storage containers or introduced during handling, can subtly alter its chemical balance. Additionally, temperature plays a role; while pure water remains neutral regardless of temperature, its pH value will decrease as temperature increases, although this does not mean it becomes more acidic in the conventional sense. Purified water’s inherent purity makes it exceptionally reactive to its environment, readily interacting with any substances it encounters.
Why Purified Water’s pH Matters
The specific pH of purified water holds considerable importance across a variety of specialized applications. In scientific experiments, maintaining a precise pH is often crucial, as even slight variations can significantly impact chemical reactions or biological processes, potentially compromising the accuracy and reproducibility of results. Laboratories frequently use purified water with tightly controlled pH levels to ensure experimental integrity.
Industrial processes, in electronics manufacturing and pharmaceutical production, rely on specific pH characteristics. In electronics, the purity and pH of water are critical to prevent contamination and ensure the integrity of sensitive components. In pharmaceuticals, water pH affects the stability, solubility, and efficacy of medicinal products. For household uses like aquariums or for specific plant types, controlled pH water is desirable to avoid mineral buildup or provide an optimal environment.