Water is often mistakenly viewed as an excellent electrical conductor because of its connection to electrical hazards, but chemically pure water is actually a very poor carrier of current. Distilled water represents water in its purest form, consisting almost entirely of H2O molecules. The ability of water to conduct electricity is directly tied to dissolved substances, not the water molecules themselves. Lacking these dissolved substances, distilled water functions more like an insulator than a conductor.
Understanding Electrical Conductivity in Liquids
Electrical conductivity is a physical property that measures a material’s ability to allow the flow of an electric current. Unlike metals, where current is carried by the movement of free electrons, electrical conduction in liquids relies on the presence of mobile charged particles called ions. A liquid containing these charge carriers is known as an electrolyte.
When a voltage is applied across a liquid, positively charged ions (cations) move toward the negative electrode, while negatively charged ions (anions) move toward the positive electrode. This directed movement of ions constitutes the electric current. Conductivity is directly proportional to both the concentration of these ions and their speed of movement.
Conductivity is typically expressed in units of Siemens per meter (S/m). For water samples, the unit microsiemens per centimeter (µS/cm) is more common, reflecting the relatively low conductivity compared to solid conductors. This measurement provides a quick assessment of the total concentration of dissolved ionized solids in a liquid.
The Specific Conductivity of Pure Water
The theoretical minimum conductivity value for chemically pure H2O at 25°C is approximately 0.055 µS/cm. This low value confirms that distilled or deionized water is a poor conductor of electricity.
This slight conductivity is not due to impurities but rather the water molecules undergoing a process known as autoionization. In this reaction, a small fraction of water molecules spontaneously dissociate to form hydronium ions (H3O+) and hydroxide ions (OH-). This results in an extremely small concentration of \(1.0 \times 10^{-7}\) moles per liter for each ion type.
These few H3O+ and OH- ions are the only charge carriers present in truly pure water. Because the concentration of these self-generated ions is minute, the electrical current they carry is negligible. The 0.055 µS/cm measurement represents the conductivity limit imposed by water’s own molecular structure.
Factors That Increase Water’s Conductivity
The conductivity of water increases when any foreign substance that can ionize is introduced. Dissolved solids, particularly salts and minerals like sodium chloride (NaCl) or magnesium compounds, are the primary contributors. When these ionic compounds dissolve, they immediately split into positive and negative ions, creating a large pool of mobile charge carriers.
For comparison, while pure water is 0.055 µS/cm, typical tap water ranges from 50 µS/cm to 1,500 µS/cm, and seawater is approximately 50,000 µS/cm. Even dissolved gases, such as carbon dioxide (CO2) from the atmosphere, contribute to conductivity. CO2 dissolves to form carbonic acid, which then dissociates into ions, adding to the total ion concentration.
Temperature is another factor that influences the measurement. Conductivity increases when the water gets warmer because the dissolved ions move more quickly, enhancing their ability to transport charge. Conductivity measurements are often corrected to a standard temperature of 25°C for accurate comparison across samples.
Practical Measurement and Applications
The conductivity of a water sample is measured using a specialized instrument called a conductivity meter, which consists of a probe with two or more electrodes. The meter applies a voltage across the electrodes and measures the resulting electrical current, which is then used to calculate the conductivity value. These measurements are simple, fast, and provide a reliable indicator of water purity.
Maintaining low conductivity is an important requirement across several industries and applications:
- In laboratory settings, ultra-pure water is necessary for sensitive experiments to avoid interference from trace ions.
- In industrial applications, such as boiler feed water for power generation, low conductivity prevents the buildup of scale and corrosion caused by dissolved minerals.
- Electronics manufacturing, particularly microchip production, requires water with minimal conductivity to prevent contamination and corrosion of delicate components.
- Monitoring conductivity serves as a continuous parameter for assessing general water quality and detecting pollution, as a sudden spike can signal the introduction of unwanted dissolved solids or chemicals.