CHEMISTRY 425
Instrumental Methods of Analysis

POLAROGRAPHIC DETERMINATION OF CADMIUM AND LEAD

MATERIALS:

Polarograph PAR 367

X-Y recorder, digital voltmeter

Polarographic cell with DME and SCE

Equivalent network (dummy cell)

Volumetric flasks, six 50 ml

Pipets, 10 ml

25 ml

Stock solutions:

1.00mol/l KCl plus 3 mmol/l HCl (base electrolyte)

1.000 mmol/l Cd(NO₃)₂ in base electrolyte

1.000 mmol/l Pb(NO₃)₂ in base electrolyte

0.2% Triton X-100 (surfactant)

Unknown sample:

Pb(II) and Cd(II) in a volumetric flask. Dilute with base electrolyte to mark and mix well.

PROCEDURE:

i) Calibrate the X-Y recorder. The current flowing through the cell is converted into potential which is output into the recorder. Therefore the deflection of current scale of the recorder is a function of the polarograph and recorder sensitivity. Replace the polarographic cell with an equivalent circuit and scan potential from 0 to -1 V. Set sensitivity such as to get approximately diagonal line. The slope is equal to the known resistor between working and reference electrode. Determine sensitivity of both X and Y scales.

You need to do this part since the absolute value is necessary for the diffusion constant calculations. Do not rely on values obtained by other groups as the internal sensitivity of instruments may be changed on purpose.

ii) Prepare the following standard solutions in 50 ml volumetric flasks using the base electrolyte for dilution:

# mmol/l Cd(II) mmol/l Pb(II)

1 0.500 none

2 none 0.500

3 0.500 0.200

4 0.200 0.500

5 none none

iii) Secure a polarogram (0 to -1 V vs. SCE) for each of the standard solutions and for your unknown solution. For each sample, remember to 1) maintain a constant mercury height; 2) clean the DME with dilute nitric acid; 3) add several drops of surfactant; 4) deareate with nitrogen for about 3 minutes; 5) pull the nitrogen capillary above the solution and let the gas flow during the measurement.

iv) Also, measure the drop time. Since it is determined by the mechanical knocker, it should be uniform throughout the experiment.

v) For solution #5 measure the drop weight. It should be done for both Cd(II) and Pb(II) half-wave potentials.

DATA TREATMENT:

(Note: Measurements of current can be made either as maximum or average value. The appropriate set of equations has to be used for each case.)

Express sensitivity of X axis in V/length and Y axis in µA/length.

For samples #1 and #2, plot log [I/(I_d-I)] vs. -E. Evaluate n and E_1/2 for Cd(II) and for Pb(II).

Using the data obtained with the standard solutions, plot, for each of the two metal ions, I_d vs. concentration. Determine the concentration (in mmol/l) of Cd(II) and Pb(II) in your unknown sample.

Calculate diffusion current constants I_d/cm^2/3t^1/6, for cadmium and lead. Estimate the value of diffusion coefficient D for each species.

NOTES:

Do not remove the capillary from the cell assembly during the session, let the mercury flow. After completion of all measurements, rinse the capillary thoroughly with water and DRY it before placing to the upward position. Any water sucked into the capillary will make it useless for further experiments. Fill the cell with base electrolyte after finishing, to prevent the SCE from drying out.

Mercury is poisonous and the biggest hazard comes from chronic inhalation of vapors. Make certain that even small droplets are not spilled.

The following settings should not be changed. Check them in case of difficulty first: Rear switch on the polarograph OFF, switches on the knocker box: 3-el. and Noise OFF. The three Y scale filter switches on the X-Y recorder should be depressed. Lit red LED Potential Overload indicates incomplete electric circuit, probably disconnected reference electrode.

 PVTJG

Copied from a previous handout: 24 August 1998 
Last revised: 27 February 2008 16:42
© Petr Vanýsek
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