Electrical Conductivity

Introduction

The electrical conductivities of solutions of strong electrolytes, weak electrolytes, and nonelectrolytes are studied. This experiment is performed as a demonstration when the older and more dangerous conductivity apparatus is used. It may be performed as a student experiment when new apparatus is constructed.

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Procedure
  1. Test the conductivity of each of the following liquids, and record the results in the table: distilled water; toluene; 0.1 M sodium chloride (sodium sulfate, potassium chloride); sugar; acetic acid; and ammonia.
  2. If a high impedance DC voltmeter is available, it can be hooked into the circuit by connecting it across the LED.
  3. Place some 1 M NaCl in the first well of a 12-well strip. Prepare a series of 1 to 5 dilutions from this well. Place 8 drops of distilled water in wells 2 through 12 using a pulled Beral pipet as a dropper. Empty this dropper. Use the Beral pipet dropper to remove 2 drops from well 1 and place in the second well. Mix using the dropper. Remove 2 drops from this well and add to the third well. Mix. Continue in this fashion for all of the wells. Test the conductivity of each well. For each well, plot the meter reading across the LED against the logarithm of the concentration of NaCl.

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Safety

Some of the substances may be toxic. Toluene and ethanol are flammable. Select substances for testing that are not toxic. Avoid ingesting or inhaling chemical substances. Keep flames and other ignition sources 10 feet (3 m) away from the flammable chemicals; provide adequate ventilation.

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TG Background

In order for an electric current to be conducted, some charged mobile carriers such as free electrons (as in a metal) or free ions must be present. A solution which contains ions will conduct electricity. A strong electrolyte contains a large number of ions and will cause the lamp to glow brightly. A weak electrolyte produces fewer ions and will cause the lamp to glow dimly. A non-electrolyte has very few ions; the lamp bulb will not glow.

Strong electrolytes are thought to dissociate completely into ions. Most salts, such as NaCl, and some acids, such as HCl and HNO3, are examples of strong electrolytes. A few salts (CdI2) and a few acids and bases (acetic acid, ammonia) are weak electrolytes. These substances are only partially dissociated into ions when dissolved in water. Non-electrolytes do not dissociate much at all. Ethanol is a non-electrolyte, for example.

To sustain a direct current, some electrode process of oxidation or reduction must occur. An alternating current reduces or avoids this problem. The 9-volt battery makes use of a direct current and, although safe and inexpensive, is limited for this reason.

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TG Disposal

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TG Lab Hints

Construct an apparatus using a 9-volt battery and a blinking LED (light emitting diode).

To build the electrical conductivity apparatus suitable for use by students (Radio Shack¨ catalog numbers are provided):

  1. Make a loop about 3 cm from the end of the ribbon wire(fig. A). Use a razor or sharp knife to remove about 10 mm of insulation from both sides of the loop (figs. B & C). This is done to expose the wires so that they can react with the solutions. Insulation is left between the wires to keep them from touching and prevent a short circuit. Hold the loop in place with tape or with a short piece of tubing (fig. D).
  1. Fasten the resistor and LED to a piece of the perforated board.
  2. Solder the red wire from the battery clip to one end of the resistor. Solder the other end of the resistor to the long lead of the LED.
  3. Solder the short lead of the LED to one wire of the electrode ribbon. Solder the black wire from the battery clip to the other wire in the ribbon/electrode assembly. Place a short length of black rubber tubing over the LED to improve visibility when faint flashes are emitted.
  4. A schematic diagram is shown:

An alternative involves producing an alternating current device (preferred for conductivity measurements) using an inexpensive transformer. To build this device (Radio Shack¨ catalog numbers are provided):

  1. Fasten the transformer to the bottom inside surface of the project case. Drill a 8-mm hole in one side. Insert the power cord. Tie on overhand knot 20 cm from the end of the cord. Strip 1 cm of the insulation from each transformer wire. Strip 1 cm of the insulation from each power cord wire. Use wire connectors to connect the power cord to the 110 volt side of the transformer (2-wire side.)
  2. Of the two lengths of ribbon wire, one will connect to the meter and the other will be used to construct electrodes. Prepare the electrode-end of one of the ribbon wires using the procedure described earlier. Strip 1 cm of the insulation from each of the other ends of that ribbon wire. Use a wire connector to connect one of these ends to the center wire of the low voltage side of the transformer.
  3. Strip 1 cm of the insulation from each of the four ends of the second piece of ribbon wire (the meter wire). On one end, fasten the solderless banana plugs. These will insert into the meter. Use a wire connector to connect one unused end of the meter wire to the one remaining unconnected end of the electrodes. Connect the last unconnected end of the meter wire to one of the two end wires on the low voltage side of the transformer.
  4. One wire from the low voltage side of the transformer will remain unused. Attach a wire connector to this wire, and tape that connector in place.
  5. A schematic of this arrangement is shown below:

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TG Materials

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TG Reference

The modified version of this lesson was developed by:

Steve Wignall
Seward High School
Seward, Nebraska

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