Sulfuric acid presents a complex answer to the question of its electrical conductivity, which is neither simply high nor low. The ability of the acid to conduct an electric current is dramatically dependent on its concentration in water, displaying a non-linear relationship. Electrical conductivity, defined as the capacity of a substance to allow the flow of an electric charge, requires the presence of mobile, charged particles known as ions. For sulfuric acid, the number and mobility of these ions change drastically as the amount of water changes, leading to a wide range of conductivity values. A specific concentration of the acid is a highly efficient conductor, while the pure, concentrated form is comparatively less conductive.
The Chemical Basis of Electrical Flow in Acids
Electrical current flows through a liquid acid solution when ions are free to move toward an electrode of the opposite charge. Sulfuric acid (H2SO4) is a strong acid that ionizes significantly when dissolved in water to produce these necessary charge carriers. The primary mechanism for this ionization involves two steps, the first of which is essentially complete in a water solution.
In the first step, sulfuric acid reacts with water to fully yield a hydronium ion (H3O+) and a bisulfate ion (HSO4-). The bisulfate ion can then undergo a second, weaker dissociation, forming another hydronium ion and a sulfate ion (SO42-). The presence of these highly mobile ions determines the overall conductivity of the solution.
What makes strong acids exceptional conductors is the unique way the proton (H+ ion) moves through the water. Instead of the relatively slow physical movement of the entire H3O+ ion, the proton rapidly “hops” between adjacent water molecules in a process known as the Grotthuss mechanism. This rapid proton-switching allows the charge to be transferred through the solution much faster than the physical diffusion of any other ion, giving the solution a very high conductivity.
The High Conductivity of Dilute Sulfuric Acid
Dilute solutions of sulfuric acid demonstrate very high electrical conductivity. This is a direct consequence of the abundance of the water solvent, which facilitates the complete dissociation of the acid. When sufficient water is present, every sulfuric acid molecule can readily break apart to form the maximum number of mobile ions.
Water acts as an effective medium for solvation, stabilizing the resulting H3O+ and HSO4- ions and preventing them from recombining. This process ensures a high concentration of charge carriers, which translates into excellent conductivity. Furthermore, the extensive hydrogen-bonded network of the water molecules provides the necessary pathway for the Grotthuss mechanism to operate efficiently. This combination of a high ion count and the rapid proton-hopping mechanism makes dilute sulfuric acid a powerful electrolyte.
Why Highly Concentrated Sulfuric Acid Exhibits Lower Conductivity
The conductivity trend reverses as the concentration of sulfuric acid approaches its pure state, typically 98% by weight. Highly concentrated sulfuric acid is a poorer electrical conductor than its dilute counterpart. This counter-intuitive drop occurs because the solvent, water, which is necessary for full ionization, is nearly absent.
Without a large quantity of water, the H2SO4 molecules cannot fully dissociate to create a large number of free ions. Instead, the concentrated acid relies on a process called autoionization, where one acid molecule protonates another acid molecule. This self-ionization reaction produces far fewer mobile ions than dissociation in water.
A second major factor inhibiting conductivity in concentrated acid is its high viscosity. Pure sulfuric acid is a thick, oily liquid, and this physical property severely restricts the movement of the few ions that are present. The increased internal friction of the liquid hinders the physical diffusion of the H3SO4+ and HSO4- ions, slowing the charge transfer. The combination of a low number of ions and the hindrance of viscous drag results in a lower overall electrical conductivity.
The Peak Conductivity Point
The relationship between sulfuric acid concentration and electrical conductivity is non-linear, with the conductivity rising from zero in pure water, reaching a peak, and then falling toward the value for pure acid. This peak represents the optimal balance between two opposing factors: the number of free ions and the mobility of those ions. At low concentrations, the number of ions is the limiting factor, and conductivity increases as more acid is added.
The maximum electrical conductivity for an aqueous sulfuric acid solution occurs at a concentration of approximately 30% H2SO4 by weight. At this concentration, there is enough water to ensure maximum dissociation and a high concentration of mobile ions. Simultaneously, the solution is still dilute enough that its viscosity has not increased significantly to impede the movement of the ions. Beyond this 30% peak, the increasing viscosity and the corresponding drop in free ion concentration due to the lack of water cause the conductivity to decrease rapidly toward the lower value of the concentrated acid.