The ability of a solution to conduct electricity is determined by the concentration and mobility of ions in solution and by the charges carried by those ions. Conductivity is a measure of the number of mobile ions per unit volume in a solution. Aqueous solutions can have a wide range of conductivities, spanning a factor of more than ten million from ultra pure water to concentrated ionic solutions.

At first glance, one might consider this a simple measurement and with the advanced technology of modern cells and meters, it is. Once conductivity probes are calibrated, students simply stir their solution gently, immerse the probe and record the result on a PC. The MicroLab conductivity cell together with a MicroLab FS-522 lab interface is no exception. The real trick with conductivity comes in getting accurate readings, sifting through the bewildering array of units and explaining the vocabulary used to describe conductivity in a cohesive manner to our students.

This may be, in part, due to how circuits are drawn, tracking the ion flow in terms of positive charge; while other concepts concerning electricity, such as electrochemistry and magnetism, are defined with respect to electron flow. The purpose of the following is to make sure that the terms and concepts are as clear as can be for the chemist or chemistry student.

In physics and chemistry, resistance and conductance are both bandied about, and really both are giving information about the same property: the ability of a material to transfer current. Electrical resistance, measured in ohms (Ω),describes the ability of current to pass through a device or solution when a given voltage is applied. Again, this depends on the concentration, mobility and charge of ions present. An ideal conductor has a constant resistance, R, over a wide range of applied voltages. Looking at Ohm's Law:

V=IR 

in which V is a voltage applied across the conductor and I is the current it carries, we see that because the resistance is constant in an ideal conductor, the current is directly proportional to the voltage.  Conventionally, students are primed to think of conductors in terms of resistance, and for materials rather than a solution.

The resistance of a length of copper wire at a particular temperature depends on two things: its length, l, and its thickness or cross section area, A. A wire's resistance increases in proportion to its length (a below) and decreases in proportion to its thickness (b). That is to say, a wire's resistance is an extensive property that depends on size. Measure the resistance of a short piece of wire and it will approach zero. Measure the resistance of a 100 meter length of wire and it will be significant. As you increase the length of the wire, you increase the number of atoms involved in transferring the electron.  

Each interaction steals a bit of energy and converts it to heat (which is why wires with high resistance become hot). Increasing the thickness decreases the resistance since a thicker wire can accommodate more electrons. This is why the power companies use very thick wires in the power grid.

Sometimes, like when discussing properties of solutions, it makes sense to talk about the ability of a material to conduct a current rather than its resistance to current. The conductance, L, of a material or a solution is just the inverse of resistance:

L=1/R = I/V

The units of conductance used to be Ohm spelled backwards (Mho), but conductance is now expressed in Siemens (S), or micro Siemens, uS.

Solutions tend to be characterized by their conductivity, a  measure of the current transferred for a given cross section area of the solution.  By defining conductance in this manner, taking into account length, we find an intensive property of the solution, independent of the amount of solution present and the nature of the cell used to measure the conductance. Conductivity is defined as:

(l/A) x I = kL

The constant k is called the cell constant and takes into account the specific dimensions of the cell used to make the measurement.

Conductivity experiments include characterizing the salinity of environmental samples, finding the total dissolved solids (TDS), and measuring the molar conductivity of ions in solution.  Students can determine ionic charges, the stoichiometry of ionic compounds, the relative strength of acids, bases or salts, or use it to track any titration that consumes or produces ions.

In Physical Chemistry, the sensor can be used to test Kohlrausch's Law or the Onsager equation, to determine limiting molar conductivities of ions, to measure acid or base dissociation constants, or to follow reaction kinetics.