Conductivity of Solutions Apparatus Like many other departments, we have used a 120-V light bulb in series with dipping electrodes to indicate conductivity in solutions. We find this demonstration useful in lower level chemistry courses, but we were concerned with its safety as a laboratory experiment hecause students are exposed to a bare 120-V source. Furthermore, this apparatus does not allow quantification of conductivity. We solved both problems by constructing the device shown in the figure from a 1- X 6- X '/,-in. Plexiglas strip. The electrodes are made by coiling Nichrome wire (about l-mm diameter by 10-em long) around a pencil. They are inserted into a two-screw terminal strip attached with'& machine screws to the Plexiglas (we cut up Radio Shack #274-678 8-screw strips). A pair of LED's are connected with apposite polarities to avoid rectificationand attached to a second twa-screw terminal strip. A copper wire (or more LED's) are inserted as shown, and 18-gaugestranded wires, threaded through twol/s-in. holes for strain relief, are installed and terminated with banana plugs.
erminal strips
nichrome electrode
C copper wire
18 Ga. stranded wire
'
banana plugs
\Vr "st. the device wirha 1 2.V a r sourcesothat the LED'sareunlit with distilled wntrr,dimly lit withtap watw,and hrighrlv lit with I M NaCI. We use an nc milliammeter (Fluke Modrl 7 5 ) in series w ~ t hthe drvice for quantrfimtian, and tdrprnding im the npplird voltage) ue rypically read :300 mA wirh theelectrodes in cunrocr. 00:l mi\ uith disrllkd warer, 0.2 mA with tap water, and tensof milliamperes for salt solutions.
Ed Vllz Kutaown University Kutzlown. PA 19530
550
Journal of Chemical Education
