Measuring Conductivity

The purpose of this experiment is to build a conductivity tester, to use it to test the conductivity of several substances, and to draw conclusions about the presence of ions in these substances.

Electrolytes are substances which dissolve  in water to produce a solution which conducts an electrical current.  Such substances produce ions when dissolved in water, and it is the ions which carry the current through the solution. Nonelectrolytes are substance whose aqueous solutions do not contain ions and hence do not conduct an electrical current.  Electrolytes are classified as either  strong electrolytes or weak electrolytes.  Strong electrolytes when dissolved in water react completely to produce ions.  For example, when NaCl is dissolved in water the reaction is

NaCl(s) Na+(aq) + Cl-(aq)

All of the NaCl(s) reacts to give ions.  There are no dissolved NaCl molecules present in the solution.  Solutions of strong electrolytes are good conductors of electricity because they contain a relatively high concentration of ions. 

Weak electrolytes are only partially converted to ions in aqueous solution.  Acetic acid, CH3CO2H, the active ingredient of vinegar, is a weak electrolyte. In water it undergoes the reaction

CH3CO2H(l) CH3CO2 -(aq)  + H+(aq)

Only about 1% of the dissolved acetic acid molecules are converted to ions.  The rest remain unchanged.  This is in contrast to  NaCl which is completely converted to ions in water.   Thus, a solution of a weak electrolyte contains a relatively low concentration of ions and its conductivity would be expected to be low. 

Nonelectrolytes do not conduct an electrical current because they do not produce ions when dissolved in water.  An example is sucrose or table sugar, C12H22O11.  A solution of sucrose contains molecules with the formula C12H22O11 , but no ions.

 

Procedure

Constructing the conductivity tester:

The parts  and tools required are:
1  35 mm film canister
1  9 volt battery
1  1000-ohm  1/4 watt resistor
2   alligator clips
1   battery snap connector
1   light-emitting diode (LED)
     electrical wire
     solder (0.032'' dia., 60% Sn, 40% Pb, rosin core)
1  candle
1  nail   
    Pliers
    wire stripper and cutter

Refer to Figure 1 for a circuit diagram of the conductivity tester.   The complete tester is shown in Figure 2.

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Figure 1

 

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Figure 2.

Carefully inspect Figure 1 before beginning to assemble the various parts. Note that one of the wires of the LED is longer than the other.  The longer one must be connected to the resistor which, in turn,  is connected to the positive terminal of the battery. 

In order to produce a reliable and sturdy circuit the wires must be soldered together. Solder is an alloy of tin and lead with a low melting point( We will use a candle to melt it).  The procedure for soldering  is as follows.   Remove about 1/2   '' of insulation from the ends of the two wires to be soldered together,and twist them together.  Hold the twisted portion over the flame of a candle.  Do not place the wire in the flame.  This will cause soot to deposit on the wire and the solder will not stick to it.  Hold the wire above the flame to heat it.  touch the solder to the hot wire.  If the wire is hot enough the solder will melt and flow between the strands in the wire. Remove the wire from the heat and let it cool. In soldering, be careful not to apply heat to either the resistor or the LED. When soldering a wire to an alligator clip, first attach it to the clip by crimping the tabs on the clip over the wire, and then soldering.

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Practice soldering scraps of wire together before attempting to assemble the conductivity tester. 

Before assembling the tester it will also be necessary to make holes in the canister cap as shown in Figure 2.  This can be done by holding a nail in the Bunsen burner flame with a pair of pliers until it is hot, and then using it to melt a hole through the cap.

Once the conductivity tester is completed, test it by touching the two alligator clips together.  The LED should glow.  If it doesn't, check with the instructor.  You are now ready to test the conductivity of some solutions.  Do not place the alligator clips directly into the solution to be tested.   This will result in the eventual corrosion of the clips.  Instead, insert a partially straightened paper clip into each alligator clip, and place the paper clips into the solution.  See Figure 1.

Testing Conductivity

Test the conductivity of the solutions below.   Use a 50 or 30 mL beaker.  Fasten the paper clips to opposite sides of the beaker with the alligator clips. All solutions  should be prepared with deionized water. Record whether or not the LED glows, and whether the glow is bright, moderately bright or dim.  Rinse, the paper clips and beaker with deionized water between trials.

1.  Deionized water.

2.  Tap water.

3. A 5% NaCl solution prepared by dissolving 5 g of salt in 100 mL of deionized water. Save this solution for other experiments.

4.  Vinegar, which is a 5% solution of acetic acid.  Can you detect a difference between the conductivity of vinegar and the 5% NaCl solution?

5.  A 5% sugar solution.

6.  Household ammonia.

7.  Coca Cola. Look at the list of ingredients on the label.  Which ones do you think contribute to the conductivity? 

8. mineral water.  Again, which ingredients contribute to the conductivity?

9.  Prepare a 0.5% solution of NaCl by mixing 10 mL of your 5% solution with 90 mL of deionized water.  Prepare a 0.5% solution of acetic acid in the same way, but starting with  vinegar.   Measure the conductivities of the two solutions.  Can you detect any differences?

10.  How many grains of salt must be added to 100 mL of deioinized water to produce a noticeable glow in the LED? What percentage of salt is this?  This is a measure of the sensitivity of your tester.

11.  Assuming that the conductivity of tap water is due entirely to NaCl, devise and perform an experiment to determine the percentage of dissolved salt in tap water.

 

Conclusion

Answer the all of the questions in the procedure and the questions below.

1.  Which sustances below contain ions and which do not:  deionized water, tap water, aqueous NaCl, vinegar, Coca Cola, mineral water, household ammonia, sugar water?

2.  What is your estimated percentage of salt in tap water?