|Cell constant||Approx. 1 cm-1|
|MicroLab Input||3.5 mm stereo phone plug|
The MicroLab Conductivity Probe, Model 160, can be used to accurately determine the conductance of solutions, and integrated into teaching labs across the chemistry curriculum.
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 Conductivity Probe 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.
Electrodes with alternate cell constants of 0.1/cm and 10/cm are available on special order. The variable cell constant is achieved by altering the electrode surface; 0.1/cm cell constant electrodes have a larger electrode area and are suitable for measuring conductivity of very dilute solutions, and 10/cm cell constant electrodes have a smaller electrode area and are used to measure the salinity of concentrated solutions.
A precise value of the sensor cell constant can be determined experimentally by collecting a calibration curve that compares the measured conductance of a solution (units of microsiemens) to the known conductivity (units of microsiemens/cm). The slope of this curve is the sensor cell constant. Please refer to the calibration video or Conductance Probe Resource for more information.
This plot, taken from the graphics window in the MicroLab software, shows the change in conductivity during a titration of dilute hydrochloric acid with a solution of NaOH. The hydronium ion has a greater conductivity than the other ions, so the solution conductivity decreases during the course of the titration as Na+ ions replace H3O+ ions. Beyond the equivalence point of the titration, the conductivity again increases as excess NaOH is added because the molar conductivity of the OH– is greater than that of the Cl–. The equivalence point occurs at the minimum conductivity value. The absolute slopes are different because the molar conductivity of HCl is almost double that of NaOH.