V1 INTRODUCTION TO POLAROGRAPHY Last Revised:
May 2014
1. PURPOSE This exercise introduces the basic concepts of polarography. 2. REAGENTS & EQUIPMENT 2.1 KCl (0.1 M) 2.2 Cadmium (1000 mg/L) 2.3 Lead (1000 mg/L) 2.4 Zinc (1000 mg/L) 2.5 Solution containing Cd or Pb. 2.6 Princeton polarography & digital interface 3. PROCEDURE 3A. Evaluation of background solution 3.1
With your teacher’s assistance, familiarise yourself with the basic controls of the polarograph and cell. Adjust the scan controller to the following values: Scan speed: 5 mV/sec Initial potential: ‐0.3V Range: 1.5 V Direction: negative Mode: dc Clock : Off Current range: 5 uA 3.2
Rinse the cell with dilute nitric acid and ultrapure water. 3.3
Add sufficient 0.1 M KCl to the cell to ensure that the electrodes are well covered (approximately 50 mL). 3.4
Raise the mercury reservoir and check that drop formation is satisfactory. 3.5
Check that the current range is appropriate, and adjust if off scale or the oscillations are very small. 3.6
Set the Clock to 1, and record the polarogram. 3.7
Turn the clock off, lower the mercury reservoir, and purge the solution for 5 minutes with nitrogen. 3.8
Record the polarogram of the purged electrolyte, using the same current range. Ensure you record the current ranges for each scan. 3B. Qualitative examination of metal ion polarograms 3.9
Add approximately 1 mL of 1000 mg/L Cd and re‐purge for about 30 seconds. 3.10 Perform a “test scan” – at 50 mV/sec without running the data collection – to check the current range. Adjust as necessary. 3.11 Run the polarogram. 3.12 Add 1 mL (approx.) of 1000 mg/L Pb and Zn. Re‐purge, and check the current scale. 3.13 Record the polarogram. 3.14 Empty the cell contents into the waste beaker and rinse with nitric acid. 3C. Comparison of scan modes & quantitative analysis 3.15 Prepare 100 mL of a 50 mg/L solution of Pb & Cd in 0.1 M KCl. 3.16 Add to the cell and purge for 5 minutes. 3.17 Check the current range and record the polarogram in DC mode. 3.18 Change the scan mode to Differential Pulse. 3.19 Check the current range and record the polarogram. 3.20 Empty the cell contents into the waste beaker and rinse with nitric acid. 3.21 Add about 50 mL of the sample to the cell and purge. 3.22 Check the current range and record the polarogram (differential pulse only). REPORT Calculations 
measure the half‐wave potential and diffusion current for the three metal ions (step 3.13) 
measure the diffusion current for Cd & Pb in each of the scans in Parts 3C 
make the responses for these scans equivalent (for the different current ranges) by multiplying the peak height in mm by the current range in uA 
calculate the increase in sensitivity from DC to differential pulse mode 
identify the analyte present in the sample 
calculate the concentration of the analyte in the sample using the equation below (PH means peak height) Concentration
50x
PHsample
PHstandard
Discussion 
explain the difference between the polarograms for the unpurged and purged KCL solutions in 3A 
compare the measured half‐wave potentials in 3B with literature values 
explain the difference in output between DC and differential pulse mode for the 50 mg/L Pb/Cd 
suggest a reason why the peak heights for the cadmium and lead in the 50 mg/L standard are different 
explain how you identified the analyte in the sample 
describe what you could have done if you had been any doubt regarding the identification Questions 1.
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3.
V1 Explain why the dropping mercury electrode (DME) is used in polarography. Give TWO disadvantages of the use of a DME. You are required to analyse an effluent sample containing 1‐20 mg/L of copper, lead and zinc. Your two options are polarography and flame AAS. Which is the better option? Justify your answer. p2