The acidity or alkalinity of a liquid solution is determined by pH, which expresses the hydrogen ion concentration. The pH scale is logarithmic, meaning a single whole number change represents a tenfold difference in the concentration of hydrogen ions (H\(^{+}\)). Electrical conductivity measures a solution’s capacity to transmit an electric current. This ability depends entirely on the presence of mobile, charged particles called ions dissolved within the liquid. A higher concentration of these free ions allows the current to flow more readily, resulting in a higher conductivity reading.
The Ionic Basis of Electrical Flow
The charged particles that define pH, specifically hydrogen ions (H\(^{+}\)) and hydroxide ions (OH\(^{-}\)), are highly effective at facilitating electrical flow. These ions possess an unusually high mobility compared to other common ions, such as sodium (Na\(^{+}\)) or chloride (Cl\(^{-}\)). Their speed is attributed to a unique transport mechanism called the Grotthuss mechanism, often described as proton hopping.
The Grotthuss mechanism allows a proton (H\(^{+}\)) to move through a chain of water molecules without the need for the entire hydronium ion (H\(_{3}\)O\(^{+}\)) to physically diffuse through the liquid. Instead, the proton forms a covalent bond with a neighboring water molecule, creating a new hydronium ion, while simultaneously breaking a bond to release a different proton further down the line. Hydroxide ions (OH\(^{-}\)) facilitate a similar hopping mechanism, though in the opposite direction, allowing both the acid- and base-determining ions to move at exceptional speeds.
This proton hopping dramatically increases the efficiency of charge transfer. Small changes in the concentration of H\(^{+}\) or OH\(^{-}\) can therefore cause a noticeable shift in the solution’s conductivity. The measured conductivity value is a composite of the contributions from all mobile ions present. Because of their speed, H\(^{+}\) and OH\(^{-}\) ions contribute disproportionately to the total conductivity, even at low concentrations, compared to slower, larger ions from dissolved salts.
Impact of Electrolyte Strength on Ion Generation
The relationship between pH and conductivity is not a simple linear one because the total number of free ions available is determined by the strength of the dissolved substance. Substances that dissolve in water to produce ions are called electrolytes, and they are categorized as either strong or weak. Strong electrolytes, such as strong acids or bases, dissociate completely into their constituent ions when dissolved. This complete separation generates a maximum number of free, mobile charge carriers, resulting in high electrical conductivity.
Conversely, weak electrolytes, such as acetic acid or ammonia, only partially dissociate in solution. A significant portion of the molecules remains intact and electrically neutral, failing to contribute to the current flow. For instance, a 0.1 M strong acid solution fully dissociates, releasing 0.1 M of H\(^{+}\) ions, while a 0.1 M weak acid solution might only release a small fraction of H\(^{+}\) ions.
This difference in dissociation means that two different solutions can have the exact same pH but vastly different conductivities. A strong acid solution and a weak acid solution may both be adjusted to a pH of 3, meaning they have the same final concentration of H\(^{+}\) ions. However, the strong acid solution will have a higher overall conductivity because its non-pH-determining counterions (like chloride) are also fully dissociated and contributing to charge transfer. The weak acid solution, conversely, has fewer free counterions because most original molecules remain undissociated.
Therefore, conductivity is directly proportional to the total number and mobility of all free ions, not just the concentration of H\(^{+}\) or OH\(^{-}\) indicated by the pH measurement. The strength of the electrolyte determines the extent of ion generation, making it a key factor in understanding the solution’s ability to conduct electricity.
Other Variables Influencing Solution Conductivity
While the concentration and mobility of H\(^{+}\) and OH\(^{-}\) ions are significant factors, several other independent variables affect a solution’s overall conductivity. One primary factor is the total concentration of all dissolved solids (TDS), which includes any background ions or salts that are pH-neutral. These non-pH-active salts, such as sodium chloride or calcium sulfate, fully dissociate and contribute positive and negative ions, raising the measured conductivity even if the pH value remains unchanged.
Temperature exerts a strong influence on conductivity because it directly relates to the energy and movement of the ions. As the temperature increases, ions move faster, reducing the resistance they encounter. This increased mobility results in a higher conductivity reading. Because of this dependency, specialized conductivity meters often employ temperature compensation to standardize readings to a reference temperature, typically 25 degrees Celsius.
The specific chemical identity of the ions also plays a role due to variations in ionic mobility and size. Different ions carry charge with varying efficiencies. For instance, the smaller lithium ion (Li\(^{+}\)) is actually slower than the larger potassium ion (K\(^{+}\)) in solution because lithium attracts a larger shell of surrounding water molecules, effectively increasing its hydrodynamic size. This difference in intrinsic ion mobility means that two solutions with the same total ion concentration can still exhibit different conductivity values based on the specific mixture of dissolved chemical species.