Electrical conductivity is the ability of a substance to transmit an electric current, which in liquid solutions relies on the presence and movement of charged particles. Concentration refers to the amount of solute, such as a salt or acid, dissolved within a specific volume of a solvent. The relationship between these two factors is a complex curve: conductivity initially rises significantly with concentration before eventually reaching a maximum and potentially falling. Understanding this behavior requires examining how charged particles facilitate the flow of electricity.
How Ions Carry Electrical Current
Unlike metallic wires, which conduct electricity through the movement of electrons, solutions carry current through the migration of mobile ions. Substances that dissociate into ions when dissolved, such as salts, acids, or bases, are known as electrolytes. The solvent, typically water, plays a crucial role by surrounding the solute particles and pulling them apart into individual, charged ions.
When a voltage is applied across the solution using two electrodes, the ions begin to move directionally. Positively charged ions, called cations, are drawn toward the negative electrode, while negatively charged ions, or anions, move toward the positive electrode. This coordinated migration of charged particles constitutes the electric current within the solution.
The current’s magnitude depends directly on the total number of ions available to carry the charge and how quickly they can move. Each ion acts as a distinct charge carrier, contributing to the overall transfer of electrical energy.
The Initial Increase in Solution Conductivity
At low to moderate concentrations, the relationship between concentration and conductivity is nearly linear. Doubling the amount of dissolved substance roughly doubles the solution’s conductivity because of a simple increase in the density of charge carriers. As more electrolyte is dissolved, more ions are released into the solution, providing more pathways for the current to flow.
In this dilute regime, the ions are widely separated, allowing them to move with minimal interference. The conductivity increases because the total number of mobile ions per unit volume is rising.
The type of electrolyte significantly influences the rate of this initial increase. Strong electrolytes, such as sodium chloride, dissociate almost completely into ions, resulting in a large increase in conductivity for every unit of solute added. Conversely, weak electrolytes, like acetic acid, only partially dissociate, meaning the effective number of charge carriers is much lower than the total concentration of the dissolved substance.
Why Conductivity Plateaus at High Concentration
As the concentration continues to rise, the linear relationship breaks down, and the increase in conductivity slows, eventually reaching a maximum value. This non-linear behavior occurs because the environment changes significantly when ions become densely packed. At high concentrations, the movement of individual ions is hampered by the presence of all the other charged particles.
Interionic Attraction
One major limiting factor is interionic attraction, described through the concept of an ionic atmosphere. Each ion becomes surrounded by a “cloud” of oppositely charged ions due to electrostatic forces. This atmosphere creates a drag that constantly pulls on the central ion and slows its movement toward the electrode. The atmosphere must constantly reform as the ion moves, further impeding the ion’s speed.
Viscosity
Viscosity also limits conductivity at high concentrations. Adding large amounts of solute increases the internal friction of the solution, making the liquid physically thicker. This increased viscosity acts as a resistance, slowing the physical migration of all ions and reducing the overall conductivity.
Ion Pairing
A chemical limitation known as ion pairing becomes more likely in highly concentrated solutions. When ions are forced into close proximity, oppositely charged ions may temporarily associate to form a neutral species. These ion pairs possess a net charge of zero and therefore do not contribute to the flow of electric current. The formation of these neutral species removes charge carriers from the solution, causing the conductivity to plateau or even decrease.