EXPERIMENT 6

QUANTITATIVE DETERMINATION OF COPPER BY COULOMETRY

 

Objective: To quantitatively determine the concentration of copper in an unknown solution.

 

Introduction:

An acidic, copper containing solution can be electrolyzed, causing the copper to deposit onto the cathode of the cell.  The overall reaction is

Cu2+ + H2O ® Cu(s) + ½ O2(g) + 2H+

The half reactions are as follows:

Cu2+ + 2e- Û Cu(s)

½ O2(g) + 2H++ 2e- Û H2O

 

Using this idea, the amount of copper in a solution can be determined by placing two platinum electrodes into an acidified copper solution and applying an electrical current across it.  The cathode can be weighed before the experiment and after the experiment to determine how much copper has plated onto it.  This weight difference should be equal to the amount of copper that was in the solution.

 

Procedure: 

1)      Cleaning the electrode – Handle the gauze electrode with a Kim-wipe or tissue and dip the

electrode into a hot 8 M HNO3 solution to remove any previous deposits.  Wash the electrode with distilled water and then with ethanol.  Place the electrode into a 100°C oven for about 5 minutes and then allow it to cool to room temperature.  Weigh the electrode accurately to obtain the initial weight. 

 

2)      Preparation of the solution – Place EXACTLY 50.00 mL of the copper sulfate solution into a

250 mL beaker and add to it 3 mL of 98% sulfuric acid and 2 mL of 8M HNO3.  To measure the copper sulfate solution be sure to use a volumetric pipet. 

 

3)      Electrolytical cell setup – Position the gauze electrode (cathode) so that the gauze portion is

not completely submerged into the copper solution.  Position the anode so that it is not in contact with the cathode.  Switch on the electrical current and adjust to 2 amperes and also record the voltage reading from the meter.  In addition, record the initial starting time after switching on the power.

 

4)      When the blue color of the copper solution has disappeared COMPLETELY, add some

distilled water to submerge the unexposed portion of the cathode.  Wait several minutes and check to see if additional deposition of copper has occurred on the fresh portion of the cathode.  If more copper has deposited, wait an additional 10 minutes and check again.  If no more copper has deposited then raise the electrode assembly and shut off the current. 

 

5)      Gently wash the electrode with water and ethanol and place the electrode into the oven for

five minutes.

 

6)      After cooling to about room temperature, reweigh the electrode to find the final mass.  The

difference in final and initial weights is approximately the amount of copper that was in the 50.00 mL of solution.

 

DATA SHEET

QUANTITATIVE DETERMINATION OF COPPER BY COULOMETRY

 

Please be sure to include proper unit labels!

 

                                                                        1                                  2

Electrical current used:                                                                         

 

Voltage Used                                                                                                 

 

Volume of solution used:                                                                       

 

Approximate time for hydrolysis:                                                                 

 

Mass of Pt gauze electrode

prior to electrolysis:                                                                           

 

Mass of Pt gauze electrode

            after electrolysis                                                                            

 

Mass of Cu in the

unknown solution                                                                             

 

Concentration of CuSO4                                                                                

In the Solution

           

Standard Deviation of the two runs:                                                  

 

 

Questions:

 

1)      How do you know whether the electrolysis is complete?

 

2)      Why is the electrogravimetric technique considered to be very accurate?

 

3)      What is the major drawback of the electrogravimetric device you used?

 

4) Be sure to include a  sample calculation for the concentration of CuSO4.

 

 

 

 

 

 

Notebook:                                                                                          Final Grade: