Electrical conductivity is the ability of a substance to transmit an electric current. While commonly associated with water, pure \(\text{H}_2\text{O}\) is actually a very poor conductor of electricity. Natural bodies of water, such as tap water and oceans, readily conduct a current due entirely to invisible impurities dissolved within the liquid. These dissolved materials, often salts and minerals, are the true carriers of electrical charge, transforming water from an electrical insulator into a conductor.
The Insulating Nature of Pure Water
Pure water, such as highly distilled or deionized water, is an extremely poor conductor, functioning as an electrical insulator. This is because the water molecule, while polar, maintains a covalent bond structure where electrons are shared and not freely moving to carry a current. Electrical conduction requires mobile, charged particles, and pure \(\text{H}_2\text{O}\) contains almost none.
A small number of charged particles are generated through a process called autoionization, where one water molecule transfers a proton to another. This reaction forms a hydronium ion (\(\text{H}_3\text{O}^+\)) and a hydroxide ion (\(\text{OH}^-\)). However, the concentration of these self-generated ions is exceedingly minute, measured at approximately \(\text{1}\text{.0} \times \text{10}^{-7}\) moles per liter at \(25^\circ\text{C}\).
This low ion concentration results in a very high electrical resistance for pure water. The theoretical conductivity of ultrapure water at \(25^\circ\text{C}\) is a mere \(\text{0}\text{.055}\) microsiemens per centimeter (\(\mu\text{S}/\text{cm}\)). For comparison, this is millions of times less conductive than typical tap water or seawater.
The Mechanism of Ionic Conduction
The ability of water to conduct electricity is entirely dependent on the presence of free-moving charged particles, known as ions. When an electric potential is applied across water containing these dissolved substances, the ions act as charge carriers. Positively charged ions, called cations, are drawn toward the negative electrode (cathode), while negatively charged ions, or anions, move toward the positive electrode (anode).
Substances that dissolve in water to create these conductive ions are categorized as electrolytes. For example, table salt (\(\text{NaCl}\)) dissociates completely in water into a sodium cation (\(\text{Na}^+\)) and a chloride anion (\(\text{Cl}^-\)). The migration of these dissolved ions constitutes the electrical current flow through the solution.
The efficiency of this conduction mechanism is influenced by several factors, including the number of ions present and their size and mobility. Smaller ions can move more quickly through the water structure, contributing more to the overall current. The higher the concentration of dissolved ions, the greater the number of available charge carriers, and consequently, the higher the overall conductivity of the water sample.
Common Sources of Dissolved Conductors
The dissolved ions that make water conductive originate from a variety of natural and human-made sources. As water flows over and through the earth, it naturally dissolves minerals from soil and bedrock, a process known as geological dissolution. This action introduces common cations like calcium (\(\text{Ca}^{2+}\)) and magnesium (\(\text{Mg}^{2+}\)), and anions such as carbonate (\(\text{CO}_3^{2-}\)) and sulfate (\(\text{SO}_4^{2-}\)).
Atmospheric absorption is another source, as water in contact with air dissolves carbon dioxide (\(\text{CO}_2\)). This gas reacts with the water to form carbonic acid, which then dissociates, adding conductive hydrogen and bicarbonate ions. Rainwater itself, even in pristine areas, can have a slightly elevated conductivity due to these dissolved atmospheric gases.
Human activities also introduce significant amounts of conductive material into water systems. Agricultural runoff often carries high concentrations of nitrate and phosphate ions from fertilizers, while urban runoff and wastewater can contribute chloride and other salts.
Quantifying Electrical Conductivity
Electrical conductivity (\(\text{EC}\)) is a routinely measured parameter, providing a fast and simple way to quantify the concentration of dissolved ions in water. The standard unit for this measurement is the Siemens per meter (\(\text{S}/\text{m}\)), though micro-Siemens per centimeter (\(\mu\text{S}/\text{cm}\)) and milli-Siemens per centimeter (\(\text{mS}/\text{cm}\)) are more commonly used for environmental and industrial applications. Instruments measure the electrical resistance of the water between two electrodes and then calculate its reciprocal, the conductance.
This measurement is often used to approximate the Total Dissolved Solids (\(\text{TDS}\)) content of water, which is expressed in parts per million (\(\text{ppm}\)). A simple conversion factor can relate the \(\text{EC}\) value to the \(\text{TDS}\) concentration in many water samples. Conductivity monitoring is important in water treatment to ensure purified water quality, with ultrapure water systems aiming for values below \(\text{1}\) \(\mu\text{S}/\text{cm}\).
Environmental scientists use conductivity as an indicator of water quality, where a sudden increase or decrease in a river or stream can signal pollution or a sewage leak. For instance, typical drinking water falls in the range of \(\text{200}\) to \(\text{800}\) \(\mu\text{S}/\text{cm}\), while seawater has a significantly higher value of around \(\text{50}\) \(\text{mS}/\text{cm}\).