Precipitation titration monitored electrochemically

 

Titration of lead with dichromate.

Needed equipment:

mercury dropping electrode
potentiostat (BAS CV27) or a polarographic analyzer
electrochemical cell
reference and auxiliary electrode
nitrogen or argon setup for deoxygenation
pipette, 10 ml
burette, 10 ml
2 volumetric flasks 100 ml
0.05 mol/l potassium dichromate (about 100 ml)
lead nitrate sample (approx. 5 mmol in 100 ml flask, known exactly by the TA)
1 mol/l HCl (about 1 liter for the lab group)
acetic acid/sodium acetate buffer, 0.3 mol/l each (100 ml or more)

Note about the mercury dropping electrode:

Mercury is toxic. Any spill (tiny droplets) needs to be collected or the vapors will contaminate the room for long time.

The capillary is very susceptible to permanent clogging by dried-out salts. When cleaning the electrode, the capillary must be rinsed thoroughly with distilled water and dried while in the down position, with mercury still dropping. For storage, raise the capillary above the reservoir and turn upwards, but only if the tip of the electrode is dry. Aspiration of a solution (even distilled water) in the capillary will result in clogging of the capillary.

The advantage of mercury electrode is its renewed surface and high hydrogen overpotential. Thus, it is possible to study reduction of ions that would be hidden by reduction of water on other electrodes.

 

Procedure:

Pipet a 10 ml aliquot of the sample (after dilution to mark) of lead nitrate into the electrochemical cell and dilute with 15 ml of acetate buffer. Cover the cell with a lid drilled to accommodate the electrodes, a burette and a gas delivery tube. Bubble nitrogen (or argon) through the solution for 5-10 minutes to remove oxygen. Pull the gas tube above the solution and blanket the surface with nitrogen.   Record a polarogram for the lead ions over a potential range from 0.4 to -1.0 vs. SCE. Record the average current at 0.0 and -0.6 V vs. SCE.

Add 1.00 ml of the dichromate solution from a burette, bubble nitrogen for a minute or two through the solution and record current at 0.0 and -0.6 V vs. SCE. Continue this process until 10 ml of dichromate has been added or until the current is excessive, whichever happens first. Discard the solutions in laboratory waste and rinse the beaker and the capillary with 1 mol/l HCl, and then with distilled water. Repeat to duplicate the experiment.

Results:

Plot the i(V+v)/V vs. v curves for both potentials (and for duplicates, if done). i - current, V - volume of the sample, v - added volume. Mark the end point of the titration. Report the amount of lead in millimoles in the volumetric flask (after dilution to mark).

Answer:

  1. Why as acidic buffer desirable for this experiment?

  2. Why is the 1 mol/l HCl used to dissolve any precipitate formed? Give an appropriate equation.

  3. Which of the potentials (0.0 or -0.6 V vs. SCE) would be preferable in routine measurements?

  4. Could any other potential than 0.0 and -0.6 V vs. SCE be used? What would be the advantages from this choice? Would there be any disadvantages? Would a different titration curve shape be obtained?

  5. At the same concentration, which yields the larger diffusion current, lead ions of dichromate ions? Why is this to be expected.

 

(After Sawyer, Heineman and Beebe, Chemistry experiments for instrumental methods, Wiley, New York, 1984.)

This experiment will depend on availability of a mercury dropping electrode.

Originated 20 October 1998 
Last revised: 08 November 2002 10:20
© Petr Vanýsek

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